EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Addicted2AnalogTek on May 03, 2015, 02:21:33 pm
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Hello everyone!
I am new on the block and am already finding this forum and all of the people here very helpful and nice, so thank you all for that, and thank you Dave! :-+
I recently picked up a Tek 468 that was said to "not read input". After a thorough cleaning and some parts reseating I was able to bring it to life.
There is still at least one issue, though, other than the likely need for calibration. (I am going to get more into checking how off it really is shortly.)
Channel 2 has a "jitter problem". Anyone have any ideas? I'm thinking it's in the vertical preamp since CH1 is perfectly fine.
Also, with x10 probes on the cal loop, when I decrease the v/div setting the trace gets fuzzy. My guess is that this is a cal issue, but I'm definitely not certain.
If anyone has a Tek 468 Volume 2(?) manual on paper, would you be willing to scan the CH2 vertical preamp schematic and send it to me?
Thank you!!
Regards, Brian
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Page 496 of the service manual on K04bb:
http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_468_Digital_Storage_Oscilloscope (http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_468_Digital_Storage_Oscilloscope)
It's a pretty decent scan, actually.
I've fixed a couple of these; they're interesting scopes. I've collected a couple of service notes; PM me your email and I'll send them to you. These scopes are getting prone to ROM rot, although I haven't seen it on the ones that I had.
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I actually found that the manuals online have decent schematics (especially EBAMAN) but the diagrams for board layout aren't very good. They're still discernible, though. I'll check the K04bb and ElektroTanka 468 ones to see if they are any better than EBAMAN's.
-I noticed that the 468 actually has a "Jitter adjustment" - not sure if it's exactly the adjustment/fix I am looking for, but worth investigating.
-Channel 2 still has the jitter when I run my input from a FG, so I know that it's the CH2 Vertical amplifier.
-However, there is some high freq noise coming off of the calibrator, because I have to have the 20Mhz BW limit turned on to get a sharp trace. Otherwise the trace looks "doubled" - the horizontal parts of the sq wave (sorry-forgot what the specific name for them are....) have their double directly over them. They also look a bit fuzzy. The calibrator is also about 585Hz low.
-On my 465 there was about 4.5V ripple on the +15V rail. The +55V rail was over by 145mV (well within spec), but when I adjusted it to exactly +55V, the ripple on the other rails completely went away. This was the first step in the calibration section of the manual, and of course I'm going to check the rails and adjust before I do anything further. I felt I should get a good idea of any issues before making any changes, replacements, or adjustments - but after a thorough cleaning.
Other than these minor issues and bad cal, this 468 is great! super sharp traces, physically great shape, and I believe it was previously owned by GE. There definitely have been repairs done. several tantalum caps are newer, a couple ICs are also newer, and there is evidence of leads being desoldered for testing. Overall, quality repair work and top quality replacement parts.
I'm definitely interested in those notes, and thank you! I'm sure they will be quite helpful.
+ Could the "jitter" issue be something as simple as a bad connection inside of the position knob?? It does seem very loose compared to CH1, and esp compared to the 465. It seems that these Allen Bradley sealed pots aren't near the quality of the Bourns pots w/ the removable back cover.
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The word "jitter", without further context or qualifying words, refers to time-based jitter. Are you seeing horizontal jitter only on Ch2? That would be strange (but not impossible).
I do not have a 468, but my reading of the service manual says that the jitter adjustment refers to horizontal jitter correction when using storage mode. I wouldn't mess with it.
Assuming you mean intermittent amplitude in Ch2, I would start by applying the same signal to both Ch1 and Ch2 and then compare signal levels at the same points between the two channels. You can start with the output of the attenuator boards and work your way through the preamps (diagrams 1 and 2) to the vertical switching board (diagram 3).
There are many test points marked along the way with example waveforms in the service manual. But don't feel constrained to look at only those.
The two channels are switched together at Q323 and Q324 on diagram 3, so it shouldn't be any further than that since you don't see a problem on Ch1. If there's a bad pot, such as position, this method should pinpoint it.
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The word "jitter", without further context or qualifying words, refers to time-based jitter. Are you seeing horizontal jitter only on Ch2? That would be strange (but not impossible).
I do not have a 468, but my reading of the service manual says that the jitter adjustment refers to horizontal jitter correction when using storage mode. I wouldn't mess with it.
Sorry - What I am referring to is vertical jitter only on channel 2. The amplitude of the applied waveforms seems to stay stable, but the entire waveform jitters.
Assuming you mean intermittent amplitude in Ch2, I would start by applying the same signal to both Ch1 and Ch2 and then compare signal levels at the same points between the two channels. You can start with the output of the attenuator boards and work your way through the preamps (diagrams 1 and 2) to the vertical switching board (diagram 3).
I don't think I should consider it entirely intermittent. The waveform is always displayed, and the "jitter" is always there. It intermittently gets worse, though.
There are many test points marked along the way with example waveforms in the service manual. But don't feel constrained to look at only those.
The two channels are switched together at Q323 and Q324 on diagram 3, so it shouldn't be any further than that since you don't see a problem on Ch1. If there's a bad pot, such as position, this method should pinpoint it.
Sounds like I am on the right track, since that is exactly what I have been doing thus far, and I also came to the same conclusion about the section that the issue is likely in.
There is also some HF noise that may or may not be coming from the bad component. I guess I will just have to see if it goes away when I find and replace (or fix) the bad part. I dripped some deoxit into the adjustment pots before bed last night, so we will see if anything has changed.
Thanks for the help!
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You can also search the BAMMA website for the schematic for this scope.
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I made a video of some odd happenings at the front end of the vertical preamp section. I think the overall fuzzyness and possibly even the HF noise are due to my cables and that they are unterminated (as recommended in the manual for this part of the testing). I am about to order some new cables, but in the meantime I am going to compare terminated/unterminated connections to see what changes.
RG58 or RG316??? The longest I am going to get is 3', so the max attenuation won't have all that much of a difference. The actual difference is about .29dB more attenuation for the 3 foot length of RG316. Anyone feel like making a recommendation?
https://www.youtube.com/watch?v=zOc2fA6fs24 (https://www.youtube.com/watch?v=zOc2fA6fs24)
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RG58 or RG316??? The longest I am going to get is 3', so the max attenuation won't have all that much of a difference. The actual difference is about .29dB more attenuation for the 3 foot length of RG316. Anyone feel like making a recommendation?
Unless you are doing real fast stuff RG58 C/U is quite adequate.
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I made a video of some odd happenings at the front end of the vertical preamp section. I think the overall fuzzyness and possibly even the HF noise are due to my cables and that they are unterminated (as recommended in the manual for this part of the testing). I am about to order some new cables, but in the meantime I am going to compare terminated/unterminated connections to see what changes.
I watched your video.
You need to use a ground clip on your probe. You're injecting tons of RF noise into both scopes.
As far as the intermittent position on Ch2, take a look at Q348 for SN < B039999, or Q348, U341A/B, Q350 for SN >= B040000.
What SN is your scope?
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I made a video of some odd happenings at the front end of the vertical preamp section. I think the overall fuzzyness and possibly even the HF noise are due to my cables and that they are unterminated (as recommended in the manual for this part of the testing). I am about to order some new cables, but in the meantime I am going to compare terminated/unterminated connections to see what changes.
I watched your video.
You need to use a ground clip on your probe. You're injecting tons of RF noise into both scopes.
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^^^ what he said.
You can't use a probe that isn't grounded. Otherwise the only common reference is the earth ground connection that both scopes have. Consider that even when using the inches-long ground clip, you can introduce inductance that throws off the high frequency performance quite badly, and provide a loop antenna to pick up RF garbage. With two 6-foot power cords and a couple more feet of conductor inside the two chassis, you created the world's longest and worst ground clip. Just make sure that you connect the ground clip to a ground, not some other voltage, or you may let the magic smoke out of the probe or one or both scopes.
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I made a video of some odd happenings at the front end of the vertical preamp section. I think the overall fuzzyness and possibly even the HF noise are due to my cables and that they are unterminated (as recommended in the manual for this part of the testing). I am about to order some new cables, but in the meantime I am going to compare terminated/unterminated connections to see what changes.
I watched your video.
You need to use a ground clip on your probe. You're injecting tons of RF noise into both scopes.
As far as the intermittent position on Ch2, take a look at Q348 for SN < B039999, or Q348, U341A/B, Q350 for SN >= B040000.
What SN is your scope?
Thanks for the pointers. I will do that.
My 468 has Option 5 installed (in case it isn't noticeable in the video).
SN is 706133.
Even though both scopes and the FG are plugged into the same power strip I should still use a ground lead? I didn't know that this would be an issue since they're both connected with very low impedance via the power source... I removed the ground("reference") clip to make sure that I didn't acidentally touch anything other than a grounding connection or the frame.
The cables I am using right now are LMR240 with "LMR240 compression BNC connectors" which I'm fairly certain are actually for RG59, and may even be 75ohm rated instead of 50ohm as they were specified. So unfortunately, I cannot do a real PROPER troubleshooting with these. I am ordering some RG58C/U cables today and I'm just going to stick these cables in a box for later when I find a deal on some REAL 50ohm LMR240 clamp connectors.
So it is possible that the HF noise issue is an artifact of these cables/connectors. I hope it is anyway.
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Even though both scopes and the FG are plugged into the same power strip I should still use a ground lead? I didn't know that this would be an issue since they're both connected with very low impedance via the power source... I removed the ground("reference") clip to make sure that I didn't acidentally touch anything other than a grounding connection or the frame.
You must connect the probe's ground.
The two devices may have a relatively low DC resistance connection to ground, but what about the AC impedance at 10's of MHz? Even the ground clip isn't ideal and that is why spring clips like these are preferable whenever possible:
(http://www.cliftonlaboratories.com/userimages/Update202.jpg)
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You need to use the shortest ground clip possible. In general, the longer the ground wire, the more crap you'll see on your screen. I have a probe with a 12 inch ground wire and while it makes it convenient to probe around, it's useless if I'm not looking for a DC level. The difference between that and the more normal 4 or 5 inch lead is huge. The shorter leads are OK for most common situations.
Here's a short ap note on probes:
http://www.syscompdesign.com/assets/Images/AppNotes/probes.pdf (http://www.syscompdesign.com/assets/Images/AppNotes/probes.pdf)
Page 5 talks about grounding. There's a lot of engineering in probes; it's not just a piece of coax with a pointy end on it.
Here's a Tek booklet on probes. They talk about grounding on page 46:
http://www.ece.vt.edu/cel/docs/TekProbeCircuits.pdf (http://www.ece.vt.edu/cel/docs/TekProbeCircuits.pdf)
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LMR240 cable is 50 ohm cable, and at the frequencies we're talking about here you won't see any difference with a 50ohm vs. a 75ohm BNC connector on the ends of it.
The grounding path between all the equipment may be low impedance at DC, but as the frequency increases the impedance increases. Your ground becomes not much of a ground and more of an antenna, as macboy said. Plus you're also picking up whatever signals the other equipment have coupled into the grounding path. The shorter the grounding lead, the better.
If you want to see the noise, just touch the probe tip to the DUT ground (device under test) and trigger on it. You'll see it's a mess.
As long as you know you have a common ground between all the equipment, it's ok to connect the ground clip to a known ground on the DUT. If you're in doubt, use a DMM to measure AC and DC voltage between the scope's ground clip and what you think should be ground on the DUT. It should be zero but you might read a little noise (for the same reasons above).
If the DUT is floating this doesn't apply, since when you connect the probe ground clip you are providing the ground.
SN 706133 sounds like a really high serial number. For Tektronix it usually starts with a letter too. The only reason I'm asking is because I want to know which schematics to look at. If you've already deduced which schematics apply by looking at the board, please let us know.
You should be able to troubleshoot the vertical position intermittent with what you have. For old or new models, it looks like Q348 could be involved since it provides the current to move the position. Take a look at its pins to see if the voltage is bouncing along with the jumps in vertical position.
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SN 706133 sounds like a really high serial number. For Tektronix it usually starts with a letter too. The only reason I'm asking is because I want to know which schematics to look at. If you've already deduced which schematics apply by looking at the board, please let us know.
The serial number does not start with "B", oddly enough. It is only " 706133 ". Also interesting, but the SN is stamped into a single plate that is inlayed into a detent in the face, unlike my 465 which seems to have individual digit "stickers" set into the detent. I have no idea if that means anything.
CHANNEL 1
TP5 & TP6 are 8VDC
TP9 is -32mVDC / 240mV p-p
CHANNEL 2
Test point 14 and 15 are 8.1VDC
Test point 17 is -28.5mVDC / 220mV p-p
[TP5, TP6 & TP14, TP15 are both supposed to be 9.5VDC / 50mV p-p]
[TP9 & TP17 is supposed to be 0VDC / 125mV p-p]
The waveforms of all of the test points are correct in relation to my input waveform.
So far I have found nothing different from channel to channel that would indicate why CH2 has issues. I'm starting to wonder if it may be directly at the input?
Is it safe to have the scope running with the attenuator covers off? It appears that they are used as grounding, so I figured I would ask before just doing it.
I did notice that the CH2 input connector has been slightly bent, and then bent back into position. Visually, nothing else seemed off - but as I've been told - the issue might not be visible until highly magnified.
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U479 (Hybrid IC vertical output amplifier channel 2) has 5mVDC on pin 1,14, 13, 16 under the proper test conditions,
and under the same conditions for Channel 1 there is only .4mVDC on the corresponding pins for the CH1 IC.
Is there a possibility that this is the problem? maybe the resistor inside of it is bad?
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SN 706133 sounds like a really high serial number. For Tektronix it usually starts with a letter too. The only reason I'm asking is because I want to know which schematics to look at. If you've already deduced which schematics apply by looking at the board, please let us know.
The serial number does not start with "B", oddly enough. It is only " 706133 ". Also interesting, but the SN is stamped into a single plate that is inlayed into a detent in the face, unlike my 465 which seems to have individual digit "stickers" set into the detent. I have no idea if that means anything.
It means that your scope was not built in Beaverton, Oregon USA. Tektronix had several manufacturing facilities in Europe, Heerenveen, The Netherlands and Guernsey, England at least and one or two more that I can't remember. My 465 was made in Heerenveen and its serial nr also starts with a 7.
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SN 706133 sounds like a really high serial number. For Tektronix it usually starts with a letter too. The only reason I'm asking is because I want to know which schematics to look at. If you've already deduced which schematics apply by looking at the board, please let us know.
The serial number does not start with "B", oddly enough. It is only " 706133 ". Also interesting, but the SN is stamped into a single plate that is inlayed into a detent in the face, unlike my 465 which seems to have individual digit "stickers" set into the detent. I have no idea if that means anything.
It means that your scope was not built in Beaverton, Oregon USA. Tektronix had several manufacturing facilities in Europe, Heerenveen, The Netherlands and Guernsey, England at least and one or two more that I can't remember. My 465 was made in Heerenveen and its serial nr also starts with a 7.
Ah I see... the letter(s) represent the plant/city it was made in. Now I just need to find out what plant used no letter. Thank you for sharing the knowledge!
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SN 706133 sounds like a really high serial number. For Tektronix it usually starts with a letter too. The only reason I'm asking is because I want to know which schematics to look at. If you've already deduced which schematics apply by looking at the board, please let us know.
The serial number does not start with "B", oddly enough. It is only " 706133 ". Also interesting, but the SN is stamped into a single plate that is inlayed into a detent in the face, unlike my 465 which seems to have individual digit "stickers" set into the detent. I have no idea if that means anything.
It means that your scope was not built in Beaverton, Oregon USA. Tektronix had several manufacturing facilities in Europe, Heerenveen, The Netherlands and Guernsey, England at least and one or two more that I can't remember. My 465 was made in Heerenveen and its serial nr also starts with a 7.
Ah I see... the letter(s) represent the plant/city it was made in. Now I just need to find out what plant used no letter. Thank you for sharing the knowledge!
I remember from a discussion on the yahoo TekScopes group that you shouldn't attach an awful lot of meaning to Tektronix serial numbers. Supposedly they're all over the place. However, as far as I know, only Beaverton built units have a serial number starting with a letter (B, obviously). None of the others do. Writing this from memory, wrapping myself in disclaimers. :blah: >:D
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CHANNEL 1
TP5 & TP6 are 8VDC
TP9 is -32mVDC / 240mV p-p
CHANNEL 2
Test point 14 and 15 are 8.1VDC
Test point 17 is -28.5mVDC / 220mV p-p
[TP5, TP6 & TP14, TP15 are both supposed to be 9.5VDC / 50mV p-p]
[TP9 & TP17 is supposed to be 0VDC / 125mV p-p]
The waveforms of all of the test points are correct in relation to my input waveform.
So far I have found nothing different from channel to channel that would indicate why CH2 has issues. I'm starting to wonder if it may be directly at the input?
Is it safe to have the scope running with the attenuator covers off? It appears that they are used as grounding, so I figured I would ask before just doing it.
I did notice that the CH2 input connector has been slightly bent, and then bent back into position. Visually, nothing else seemed off - but as I've been told - the issue might not be visible until highly magnified.
(One side comment: Please, if you're posting a pile of new information, don't edit your old posts to add it. Make a new post. Many people, myself included, use "Show new replies" and it doesn't include things that have been edited. Plus the discussion can get out of time order and may be confusing for someone reading the thread for the first time.)
The actual origin of the issue might need higher sensitivity (magnification) to see it, but where it actually starts manifesting itself, or immediately after, should be in the same proportion to what you see on the screen. In other words, it should be obvious.
The difference in the absolute voltage measurements may or may not be related to the problem. It may just be normal differences in bias voltages, or could be related to the ground position of the trace on the screen. Unless you can see them jumping too, I wouldn't pursue those differences at the moment.
I suggest the goal is to find where the signal is jumping at the same time as it is on the screen, and then work backwards from there.
Did you not want to look at Q348?
How about Q228/Q222, which drives the diode switch for Ch2? I'd be surprised if you didn't see the problem there. If present, you could use that as your starting point.
And on the serial number question, please confirm which schematics match your unit.
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The actual origin of the issue might need higher sensitivity (magnification) to see it, but where it actually starts manifesting itself, or immediately after, should be in the same proportion to what you see on the screen. In other words, it should be obvious.
The difference in the absolute voltage measurements may or may not be related to the problem. It may just be normal differences in bias voltages, or could be related to the ground position of the trace on the screen. Unless you can see them jumping too, I wouldn't pursue those differences at the moment.
Yes, the trace jumps around vertically with a bit of noise when it is in the ground position. I will do some fiddling and document exactly what settings it jumps on and what ones it does not. Are you suggesting that I may be looking for some "noise" via probing with my 465, possibly in 10x mag? of course I would be adjusting the volt/div setting until I have some amplitude and adjusting the timebase until I have a visible trace, if any. And of course this would be done mostly in AC input coupling so I am only seeing the noise that is riding on top of the signal.
I suggest the goal is to find where the signal is jumping at the same time as it is on the screen, and then work backwards from there.
Did you not want to look at Q348?
I have been looking at Q348/Q350 and comparing to Q151/Q153, but thus far I have found nothing suspicious. That doesn't mean I am done testing that area yet, as I am sure I will end up running thru the entire board until I find the problem.
How about Q228/Q222, which drives the diode switch for Ch2? I'd be surprised if you didn't see the problem there. If present, you could use that as your starting point.
And on the serial number question, please confirm which schematics match your unit.
Funny you mention Q228/Q222 - I was poking around there last night and am picking up there right now. The only thing I found with the DMM last night when comparing Q448/Q447 and that vicinity to Q222 and Q228. With that, the only thing I have found is that the junction between CR338/CR335 is -4.7V when it is supposed to be -4.1V. TP26 measured at -4.8V.
I have more numbers on the schematics, but I need to do a little more figuring instead of just dumping all the numbers at you with no kind of organization.
I am about to track where else that extra -0.6V travels to/originates from, so hopefully I am on the right track!
Yes, we are all looking at the correct schematics. My unit matches the higher range serial number category of schematics, and so do most of the voltages on everything except for a large part of channel 2!
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Yes, the trace jumps around vertically with a bit of noise when it is in the ground position. I will do some fiddling and document exactly what settings it jumps on and what ones it does not. Are you suggesting that I may be looking for some "noise" via probing with my 465, possibly in 10x mag? of course I would be adjusting the volt/div setting until I have some amplitude and adjusting the timebase until I have a visible trace, if any. And of course this would be done mostly in AC input coupling so I am only seeing the noise that is riding on top of the signal.
Yes, I'm suggesting probing with your 465 trying to find erratic changes in voltage that match the jumps in the vertical position. "10x mag" meaning 10x more sensitivity? Yes, you could do that too, but I don't think it's necessary. If you can see the test input signal, the erratic jumps would be in the same proportion once you've found where the problem is being introduced.
Funny you mention Q228/Q222 - I was poking around there last night and am picking up there right now. The only thing I found with the DMM last night when comparing Q448/Q447 and that vicinity to Q222 and Q228. With that, the only thing I have found is that the junction between CR338/CR335 is -4.7V when it is supposed to be -4.1V. TP26 measured at -4.8V.
I have more numbers on the schematics, but I need to do a little more figuring instead of just dumping all the numbers at you with no kind of organization.
I am about to track where else that extra -0.6V travels to/originates from, so hopefully I am on the right track!
Hmmm... There is no junction between CR338/CR335 on the schematic I'm looking at. It's dated "Rev Jun 1983" at the bottom. Did you mean CR339/CR335?
That's interesting on TP26, but I wouldn't be measuring that signal with a DMM. The signal controls when Ch2 is being switched into the vertical amplifier for the CRT. So, depending on what channels you are displaying, it could be an actively changing signal. Who knows how the DMM is interpreting that.
What does TP26 and TP21 look like on your 465? How does that compare to TP25 and TP20, which are the two corresponding TPs for Ch1? Take readings in DC mode so you know the absolute voltage.
Is Ch2 still erratic when Ch1 is off? If so, try looking at TP26 and TP21 with only Ch2 on, and then TP25 and TP20 with only Ch1 on.
It's possible that Q419 or the output from U409 is sick (only one possibility of many at this point).
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Hmmm... There is no junction between CR338/CR335 on the schematic I'm looking at. It's dated "Rev Jun 1983" at the bottom. Did you mean CR339/CR335?
Yes, that is what I meant. Sorry, blurry text on the schematics.
What does TP26 and TP21 look like on your 465? How does that compare to TP25 and TP20, which are the two corresponding TPs for Ch1? Take readings in DC mode so you know the absolute voltage.
Is Ch2 still erratic when Ch1 is off? If so, try looking at TP26 and TP21 with only Ch2 on, and then TP25 and TP20 with only Ch1 on.
CH2 is erratic no matter what the settings are. As long as it is on, it is erratic.
I didn't find anything unusual measuring those test points in DC input coupling mode, but I started fiddling around and found what I may be looking for.
I am attaching several pictures. Their titles describe what they are, and the test parameters are as follows:
465:
Vertical: 5mV/div, AC Coupling, either Channel 1 setting or Channel 2 setting (as specified in pic titles)
Horizontal: A
Timebase: .2uS/Div
Trigger: AC, NORM, Ch 1 or CH2 (corresponding to input channel)
458:
Vertical: 5mV/div, DC Coupling, Channel 1, Channel 2, or ADD ( As specified in the pic titles)
Horizontal: A
Timebase: 1mS/Div
Trigger: AC, NORM, NORM
Input: 1khz Positive Going Square Wave, 20mV p-p from Wavetek 145. Function Out direct to tee, then to each channel input. No terminations.
Here is the link to the pictures:
http://s1081.photobucket.com/user/Brian_Bloom/library/Tek%20468%20CH2%20Vertical%20Troubleshooting%20-%20Part%201 (http://s1081.photobucket.com/user/Brian_Bloom/library/Tek%20468%20CH2%20Vertical%20Troubleshooting%20-%20Part%201)
Let me know if I forgot anything. Now that I have finally found something I can track (thank you!) I am going to continue the routine that I have going with comparing channel to channel, settings to settings, until I have the entire area mapped out and have found the troublemaker. So far, it seems that this noise corresponds to the higher voltage I had found. Time to see if that trend continues...
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I had a dirty AC/Gnd/Dc switch on one 468 I worked on. That changed the attenuation by 30%. I don't remember now, but it might also have been noisy.
Tie the inputs of the 2 channels together after the attenuators and display both at once and see if there's any difference. That will tell you if it's in the attenuator or not.
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I had a dirty AC/Gnd/Dc switch on one 468 I worked on. That changed the attenuation by 30%. I don't remember now, but it might also have been noisy.
Tie the inputs of the 2 channels together after the attenuators and display both at once and see if there's any difference. That will tell you if it's in the attenuator or not.
I vigorously cleaned everything, especially the finger switches. I have swapped the attenuators, measured the attenuators, measured the input components from the input and after the attenuators.... it's not those or the switches. Thank you, though. That is actually how I fixed my 465, and how I got a trace to display at all on this 468. I kinda wish it were that simple, but at the same time I kinda don't because this is a much needed learning experience for me. I need to learn how to properly troubleshoot scopes and other gear, and also to fully understand what it is I am looking at and should be expecting from various circuits and devices.
I am in the process of adding a few more pictures as I take them. On screen comparisons of the waveforms from comparable test points and "test points". Once I have some of my own "test points", I will mark them on the schematic and diagram and upload those as well.
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I finally found something that causes my trace on the 465 to go erratic in sync with channel 2!!! The only way I can trace it is using A Gate and B gate. What's actually weird about how I found this....
I was removing my probe from TP25 and noticed that when I get the tip close to certain components, my 465 will trigger and display a stable trace.. so I took the grabber off and started putting the tip to the cases of the transistors, ICs, caps, etc. I found that even without anything displayed on the 468, I get enough RF off of Q512 to trigger the 465 steadily. With CH1, CH2, and CHOP selected on the 468, the entire corner of the vertical preamp goes hot with RF. The strongest is off of C504's side towards R504, and next in line is Q419, Q418, and Q512. I get a little off of U508 also. If this is anything of interest, let me know and I will make a video of that as well.
Here's the link to the one I just made:
https://www.youtube.com/watch?v=CTU7be0H94w (https://www.youtube.com/watch?v=CTU7be0H94w)
Oh, and the area that goes hot with RF is from R420 to U408 and that entire width down to the edge (The IC side of the board)
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Sorry, I kinda had a bit of a mental moment. B gate is for the B trigger which is why I had no trigger at all there, since I didn't switch it on. :palm: I don't think that there is any reason to do that, either.
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I would keep moving forward through the vertical circuitry until you find the erratic jumping. I'm not ruling out Q512 and friends, but it's not what I would suspect first because it's not carrying the vertical signal.
On the screen shots of TP20, TP21, and TP26, you have the sensitivity way up on the 465. I'd expect to see a little bit of noise in there, probably from the digital circuitry in the 468.
Here's something to try if you want: Set the 468 to display Ch2 only and Alt (no Ch1 or Add). No input signal to either channel.
The manual is not explicit, but I would expect TP25, TP26, and TP27 to be quiescent. Check them with your 465. If there's a steady signal at these TP's, measure the voltage with your DMM. It should be around -4.1V for TP25 and TP27, and around -2.0V for TP26.
If all these voltages are ok, it means that only Ch2 is selected and being sent to the vertical of the CRT (which would be correct), and Ch1 and trigger view are off.
Now apply a test square wave to Ch2 and take a look at TP29 and TP30. These test points are the selected vertical signal. I would expect the erratic jumping to show up here as well as the test square wave. If you can't get the test square wave here, something is not set up right.
If you can see the test signal and it's not being erratic, it says the the whole Ch2 front end is probably ok and there's something more insidious and bizarre happening further down the vertical path.
It's also possible that your 465 is set up to trigger in such a way that you're never seeing the erratic signal. Make sure you can see several divisions of the test signal and trigger in the mid-point vertical voltage of it.
Here's something else you could try: You could set your 465 to free-run by setting the trigger input to EXT, AUTO, and set the trigger level to the extreme left or right position. The sweep speed is not particularly important; 1ms/div is fine. When you probe signals they will appear as a band across the screen (or a line if there's no signal), but you will see any very low frequency intermittent behavior as the entire band jumping. Think of it as an extremely fast min/max voltmeter.
And one other assumption I've been making: You're not using the 468 in storage mode, right?
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I would keep moving forward through the vertical circuitry until you find the erratic jumping. I'm not ruling out Q512 and friends, but it's not what I would suspect first because it's not carrying the vertical signal.
On the screen shots of TP20, TP21, and TP26, you have the sensitivity way up on the 465. I'd expect to see a little bit of noise in there, probably from the digital circuitry in the 468.
Yes, I had the sensitivity way up on purpose in order to see beyond the standard waveform (as specified in the schematics section - test waveforms). I have already went thru and checked the voltages and waveforms for all of the test points on the vertical board with the correct settings. There were a few test points in section 3 (logic) that I am not sure if I checked properly, but I am fairly certain that I did and the waveforms were incorrect, since I was able to get the correct waveforms with the specified settings on other test points in the same group.
Here are the full results:
http://s1081.photobucket.com/user/Brian_Bloom/library/TEK%20468%20SECTION%201%202%203%20TEST%20POINT%20DATA (http://s1081.photobucket.com/user/Brian_Bloom/library/TEK%20468%20SECTION%201%202%203%20TEST%20POINT%20DATA)
In my pictures, when the settings are different from the standard settings mentioned in the manual, it is because there is nothing out of the ordinary to see until the settings I have used in the pictures have been reached.
Here's something to try if you want: Set the 468 to display Ch2 only and Alt (no Ch1 or Add). No input signal to either channel.
The manual is not explicit, but I would expect TP25, TP26, and TP27 to be quiescent. Check them with your 465. If there's a steady signal at these TP's, measure the voltage with your DMM. It should be around -4.1V for TP25 and TP27, and around -2.0V for TP26.
If all these voltages are ok, it means that only Ch2 is selected and being sent to the vertical of the CRT (which would be correct), and Ch1 and trigger view are off.
Now apply a test square wave to Ch2 and take a look at TP29 and TP30. These test points are the selected vertical signal. I would expect the erratic jumping to show up here as well as the test square wave. If you can't get the test square wave here, something is not set up right.
If you can see the test signal and it's not being erratic, it says the the whole Ch2 front end is probably ok and there's something more insidious and bizarre happening further down the vertical path.
It's also possible that your 465 is set up to trigger in such a way that you're never seeing the erratic signal. Make sure you can see several divisions of the test signal and trigger in the mid-point vertical voltage of it.
Here's something else you could try: You could set your 465 to free-run by setting the trigger input to EXT, AUTO, and set the trigger level to the extreme left or right position. The sweep speed is not particularly important; 1ms/div is fine. When you probe signals they will appear as a band across the screen (or a line if there's no signal), but you will see any very low frequency intermittent behavior as the entire band jumping. Think of it as an extremely fast min/max voltmeter.
I already went thru all of the testing procedure to check the waveforms on the test points for section 1, 2, and 3, but I will check again doing specifically what you suggested to see if anything new turns up. You may be able to get an idea of the issue(s) from the test point data I linked to above.
And one other assumption I've been making: You're not using the 468 in storage mode, right?
Yes of course. I am running ALL testing with storage mode off, as it has nothing to do with the problem at hand. Most of my testing up until now, unless I have specified otherwise, has been done using the settings mentioned at the beginning of the schematics/diagrams section of the manual & the additional settings specified at the beginning of each section. When I do not see the correct outcome of the testing with the correct settings, I modify my 465's settings until I see something, and modify them more to see if I can see anything else. It seems that my trigger level can either be extremely sensitive or extremely hard to obtain a trigger. I'm not sure why that is, but I know it is from the input, not the 465 itself.
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I'm pondering going over all of the test points/testing procedure. Maybe if I did that, we could examine all of the results and see what the 'off waveforms/voltages" have in common. Thoughts?
One thing I find interesting that I came across last night is that when I have the 465 triggering off of A gate, and the 468 on B trigger, the 465 has a stable trigger with no jitter no matter what I do with the settings of the 468 - even when nothing is being displayed. As long as I have input going in from the FG I have a stable trigger on the 465. Mean anything?
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found something very strange.....
This is with the scope set to Ground, CH1/CH2/CHOP. It only does it when CHOP is selected. AUTO, NORM, diff v/div, timebase, etc etc. does not matter. still does it. Any horizontal display mode... still does it. I never noticed until now because I never bothered to set it into the uS timebase settings. I'm very curious to find out where this is coming from.....
Going to stick my probe to the A gate and B gate and see if it happens to be the trigger, and also going to put it to A vert out as well, even though I don't think that it will output anything when the scope is in GROUND input coupling mode. I guess we will see, though.
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found something very strange.....
Does it change in amplitude with different input attenuator settings?
It's a square wave, but at what frequency?
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I decided that it would be a good idea to make a video of this. Due to the fact that when I have "HF REJECT" selected on either A or B trigger, I'm guessing that whatever this is is also affecting the trigger circuitry as well. Either there are multiple problems, it's a CH2 vertical problem, or it's a CH2 trigger problem. I'm betting it's mostly CH2 trigger, and likely an issue on CH2 vertical as well.
https://www.youtube.com/watch?v=x26EaIqhpM8 (https://www.youtube.com/watch?v=x26EaIqhpM8)
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found something very strange.....
Does it change in amplitude with different input attenuator settings?
It's a square wave, but at what frequency?
about 540khz.
No it does not change in amplitude.
remember that those are the baselines of two channels, not one waveform. are you still sure it's a square wave? I'm going to eat and then start probing around the A and B gate, vertical out, and the trigger board area. I'm going to check the ground planes for sure..
*** In the video I meant to say that channel 1 and channel 2 are 180 degrees out of phase from each other, not 90. ***
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This is what A gate and B gate look like on the 465. I have the probe tips stuck into the BNC connector and the ground/reference clipped to the frame.
A gate is the top, and it's baseline is the bottom of the waveform. It is 540mV p-p and on 20mV/div
B gate is the bottom, and it's baseline is way off screen. What you are seeing is at +60mV, and the setting is 5mV/div.
Timebase is set at 50uS.
I am only able to get a trigger off of A gate. B gate gives me nothing at all to catch a trigger on. When I set it to auto, both traces go sloppy as expected.
Ahhh I screwed up my math on that last frequency calculation from the picture/video. That, I am fairly certain, is approx 550khz.
The frequency of A gate is approx 37khz.
( 14.7 / .0004 ) or 14.7 cycles in 8 divisions, divided by 50uS x 8 divisions - 400uS or 0.0004
** when CH2 on the 468 jitters towards the top of the screen, the frequency decreases on A gate's trace (465). When it goes down, the frequency increases.
There seems to be no particular set amount that it increases/decreases, but it seems like it can be as much as +/- 3khz approx.
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The frequency of A gate is approx 37khz.
( 14.7 / .0004 ) or 14.7 cycles in 8 divisions, divided by 50uS x 8 divisions - 400uS or 0.0004
Aha, typically SMPS frequencies. ;)
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The frequency of A gate is approx 37khz.
( 14.7 / .0004 ) or 14.7 cycles in 8 divisions, divided by 50uS x 8 divisions - 400uS or 0.0004
Aha, typically SMPS frequencies. ;)
re-read the bottom part of my last post. I thought I had added that little bit of info before anyone really read it, but apparently not. I was trying not to fill a whole page up with my posts/edits. haha I won't edit them at all anymore, even if it's only a minute later.
So you're suggesting that there may be an issue in the power distribution to the trigger board? and maybe a bad reverse-biased diode somewhere in one of the rails in the CH2 trigger area? or something of that nature?
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well, the trigger board or the vertical preamp board? I shall start probing power rails to see if I can confirm and trace!
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AHHH CRAP!!! It's the little mistakes!!
A gate probe is on 10X!!! therefore I am getting just a wee bit over the specified/correct voltage of 5.5V!!!!! ok....
That means A gate is working properly, but is affected by the noise.
B gate is not working properly - at all. the probe is in fact on 1X there, showing only 60mV and definitely not putting out any specific waveform, let alone a square wave.
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I can see the 550khz riding on the A gate and B gate waveforms. B gate has small spikes where the rising and falling portions of the square wave should be.
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The frequency of A gate is approx 37khz.
( 14.7 / .0004 ) or 14.7 cycles in 8 divisions, divided by 50uS x 8 divisions - 400uS or 0.0004
Aha, typically SMPS frequencies. ;)
re-read the bottom part of my last post. I thought I had added that little bit of info before anyone really read it, but apparently not. I was trying not to fill a whole page up with my posts/edits. haha I won't edit them at all anymore, even if it's only a minute later.
So you're suggesting that there may be an issue in the power distribution to the trigger board? and maybe a bad reverse-biased diode somewhere in one of the rails in the CH2 trigger area? or something of that nature?
Edits for spelling or meaning are fine, most can follow a post, but always good to tidy things up. ;)
Consider this: Any PSU under stress will produce increased ripple and EMI while trying to stay in spec. (voltage). In that case it will be operating OUTSIDE the designers design and possibly producing all sorts of rubbish that permeates throughout the DUT.
Now how many SMPS's are in the 468? Don't forget the CRT HV supply. ;)
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Thoughts:
A capacitor that is on the fritz.
It is only affecting CH2, but also affecting the triggering and A/B gate.... hmmm
I wish it would just blow out and show me the magic smoke so I don't have to keep digging!! haha
Well crapola. I don't have a HV probe for the DMM or the scope.
Resistor divider, maybe?? The best I've got in the megohms range are 3w, though.. hmm.
I guess it is finally time to order a couple HV probes. I wanted to get some anyway. A good 100X scope probe and dmm probe...
What would u recommend for those? I shouldn't have passed up the set of tek 6007, 6008, 6009's without tips for $20..... :palm:
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I decided that it would be a good idea to make a video of this. Due to the fact that when I have "HF REJECT" selected on either A or B trigger, I'm guessing that whatever this is is also affecting the trigger circuitry as well. Either there are multiple problems, it's a CH2 vertical problem, or it's a CH2 trigger problem. I'm betting it's mostly CH2 trigger, and likely an issue on CH2 vertical as well.
https://www.youtube.com/watch?v=x26EaIqhpM8 (https://www.youtube.com/watch?v=x26EaIqhpM8)
It looks to me like you're triggering on the chop clock. A tiny amount of it must be leaking into the trigger circuitry. It's difficult to trigger on it because it's very weak. Ideally, it shouldn't be there at all, but nothing's perfect.
These aren't the droids you're looking for...
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I decided that it would be a good idea to make a video of this. Due to the fact that when I have "HF REJECT" selected on either A or B trigger, I'm guessing that whatever this is is also affecting the trigger circuitry as well. Either there are multiple problems, it's a CH2 vertical problem, or it's a CH2 trigger problem. I'm betting it's mostly CH2 trigger, and likely an issue on CH2 vertical as well.
https://www.youtube.com/watch?v=x26EaIqhpM8 (https://www.youtube.com/watch?v=x26EaIqhpM8)
It looks to me like you're triggering on the chop clock. A tiny amount of it must be leaking into the trigger circuitry. It's difficult to trigger on it because it's very weak. Ideally, it shouldn't be there at all, but nothing's perfect.
These aren't the droids you're looking for...
What do you mean? that triggering issue in the video is not related to the jumpy CH2 trace? I've started looking into the power supply, and before I even started probing I noticed that all but the -8V bridge rectifier were quite hot. almost too hot to hold onto for more than a few seconds, so I shut it off and let it cool. it was running for about 2 hours, maybe more. I felt around a bit after shutting it off and nothing else seemed to be hot. the two big resistors were warm but nothing major.
so while letting it cool, I took the HV shield off, and while doing so I noticed that C603's lead (towards HV side) has the discoloration look to it from excessive heat, and that side of the cap doesn't look too healthy. looks like maybe some electrolyte leaked out... so I am pulling one lead and measuring it's capacitance and resistance. Those 3uf 150V caps are $2-5 a piece, so if I have to replace it I will order a few Spragues. I have a 4.7uf cap that I could try temporarily if it is bad so I can keep moving with the testing and find any other problems. If not, oh well... on to the next step.
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Well, more interesting stuff.....
I replaced that capacitor, and doing so made all of the AC ripple that was measurable with the DMM's go away... however.... now that all that ripple is gone and all but the +110V rail is tight to perfect, all the rectifiers except for the +110 get HOT HOT HOT.. I had the scope on for about 4 min and they got so hot that I could only keep my fingers on them for about 4 seconds. That's when I decided to pull out the freeze spray.
The 110V rail is the ONLY one that is out of spec.... or even off by more than 2mV. It is high 1.8V.
When I freeze the -8V and +15V bridge rectifiers with my freeze spray, the +110V rail goes down to 110.1V, and then as they warm back up, it rises back to 111.3V until the freeze spray has worn off completely....
The trace also stabilizes a good bit when I freeze those two rectifiers.
I am going to do this again tomorrow and check waveforms on the PSU test points while I freeze/let thaw. Before that, I will get out the other 3 DMMs and hook them up to rails so I can record/watch the results of the big freeze.
I think I vaguely remember reading something about someone having a similar rectifier problem, now that I think about it.
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You'd have to ask yourself just WHY do they get so hot?
THEY ARE CONDUCTING TOO MUCH CURRENT!
Why?
Well, you've had her going for a while then problems emerge....gotta be heat related.
Suspect more faulty caps. :rant:
So where to from here?
How are you determining/measureing PSU ripple?
I hope not with a DMM, this is fine for a quick check, but because of their limited frequency response, you must use a scope, ESPECIALLY for SMPS.
Having a look at a 465 SM, there is a procedue to isolate where loading of the PSU occurs.
Page 243 of 291 viewed in Adobe reader.
Also there is a fault finding flow chart
Sure swap out the bridges, but check for a faulty diode in them too.
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What do you mean? that triggering issue in the video is not related to the jumpy CH2 trace? I've started looking into the power supply, and before I even started probing I noticed that all but the -8V bridge rectifier were quite hot.
What I'm saying is I wouldn't give it a very high likelihood that it's related to the root cause. I don't think it's worth tracking it down when you have an unexplained, major symptom of the vertical jumps staring at you.
The power supply needs to be verified. That would be a good next step (actually should always be the first step).
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If everything checks out with the power supply, you could take a look at the final vertical stage that's driving the vertical plates on the CRT. The jumpy vertical will be visible there. Guaranteed.
Take a look at TP38 and TP39.
The problem is happening in an earlier stage, but hopefully looking at it on the plates will give you an idea what to search for and allow you to experiment with how to correctly set up your triggering on the 465.
Once you can confidently observe the jumping, then go backwards through the vertical signal path to find where it starts.
Keep in mind that the 468 is running fine with the exception of the jumpy Ch2. If something was wrong with the power supply it would likely be affecting more than just Ch2. Unless one of voltages or ripple is way out of spec, don't let it drag you off on a tangent.
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You'd have to ask yourself just WHY do they get so hot?
THEY ARE CONDUCTING TOO MUCH CURRENT!
Why?
Well, you've had her going for a while then problems emerge....gotta be heat related.
Suspect more faulty caps. :rant:
So where to from here?
How are you determining/measureing PSU ripple?
I hope not with a DMM, this is fine for a quick check, but because of their limited frequency response, you must use a scope, ESPECIALLY for SMPS.
Having a look at a 465 SM, there is a procedue to isolate where loading of the PSU occurs.
Page 243 of 291 viewed in Adobe reader.
Also there is a fault finding flow chart
Sure swap out the bridges, but check for a faulty diode in them too.
It was 3 am when I started it back up and did the freeze thing, so yes, I did use the DMM just for quick check of ripple. I definitely intend on checking with the scope today. After getting some rest, reading these posts, and thinking about it a little, of course MarkL is correct. If the problem was specifically in the PSU, it would affect more than channel two. So what is happening is the CH2 issue is pulling so much current off the +15 and -8 rails that it's really heating those two bridge's up a lot. It's pulling extra voltage off the +110, but the current is likely normal. Thus when I cool the rectifiers and get them back into normal spec, it's likely limiting the current that can pass thru them and stabilizing CH2 and the +110V rail a bit temporarily.
Do you mean the 468 SM? Volume 1 I assume..... I will have a look at it.
I'm thinking it may be a bad cap or resistor that's allowing more than it should be allowing to pass thru into the ground plane..
What I'm saying is I wouldn't give it a very high likelihood that it's related to the root cause. I don't think it's worth tracking it down when you have an unexplained, major symptom of the vertical jumps staring at you.
The power supply needs to be verified. That would be a good next step (actually should always be the first step).
The power supply was the first thing that I checked, but I overlooked actually checking it with the scope. I checked the rail voltages and ac ripple with a DMM, adjusted +55 to exact, and moved on.. I should have been a bit more thorough there. Now I know. The reason I adjusted the +55 is because when I was working on my 465, I had a lot of ripple on the rails - and when I adjusted the +55V all the ripple went away. So I thought it would be worth a shot here.
If everything checks out with the power supply, you could take a look at the final vertical stage that's driving the vertical plates on the CRT. The jumpy vertical will be visible there. Guaranteed.
Take a look at TP38 and TP39.
The problem is happening in an earlier stage, but hopefully looking at it on the plates will give you an idea what to search for and allow you to experiment with how to correctly set up your triggering on the 465.
Once you can confidently observe the jumping, then go backwards through the vertical signal path to find where it starts.
Keep in mind that the 468 is running fine with the exception of the jumpy Ch2. If something was wrong with the power supply it would likely be affecting more than just Ch2. Unless one of voltages or ripple is way out of spec, don't let it drag you off on a tangent.
The vertical output amp is part of my planned check-out for the power supply section. That and the CRT circuit. It's accessible and I figure I might as well just check it while I'm checking the PSU section anyway.
I will certainly go thru your suggested checks.
My thoughts so far are:
1- It's pulling excess current off of +15, -8, and possibly some from +5
2- It only affects CH2 vertical
3- It affects the trigger circuitry - specifically A and B gate
4- It has caused B gate's waveform to collapse, but only causes A gate to 'jitter'
The B gate problem could be unrelated, and probably is, so I am excluding it from my current task. I will deal with repairing it once CH2 is stable.
Good point MarkL - I need to focus on finding the CH2 'jitter' problem and solving it. I now know it's likely pulling more current than it should be. There's nothing wrong with the PSU. If there was, a lot more issues would be obvious than just CH2 and the B gate collapsed output. Like I mentioned, I am excluding B gate issue for now since it is most likely unrelated.
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Well, I've been looking at replacements for that capacitor, and being a Sprague 109D type 150V cap, they are ridiculously expensive. I found some comparable ones for $2 each, but the ones I would like to use are $14 each. NUTS!
Also, upon realizing it is a wet tantalum, I cannot safely leave that regular electrolytic cap in place of it, so I am going to probably reinstall the original until my replacement arrives in a few days. The difference in type of capacitor may be why the rectifiers got sooo much hotter, and that the one I have in there is 2.4uf, while the original is rated 3uf and it was about 2.8uf and losing capacitance as it gets worse.
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Do you mean the 468 SM? Volume 1 I assume..... I will have a look at it.
No, 465 SM.
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Anyone see any problem with using a 3.6uf cap to replace the one that's got issues? The only Sprague 109D I am able to find is 3.6uf 150V. I can find other ones that are 3uf 125V, but thats bout it.
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Anyone see any problem with using a 3.6uf cap to replace the one that's got issues? The only Sprague 109D I am able to find is 3.6uf 150V. I can find other ones that are 3uf 125V, but thats bout it.
If it's just acting as a filter, close enough. Standard tolerance is 20%, so Tek can't be expecting an exact value anyway.
Sprague tantalum division now owned by Vishay. Seems that the higher voltages are discontinued. You're lucky to have found a 150V one.
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Good, I'm glad I was able to find them and that I ordered extra.
Ordered 10:
SPRAGUE 30D305F150CB3A AXIAL 3UF 150V -9.5%
Ordered 2:
Sprague 109D365X0125C2 3.6uF 125V 20%
The 109D's are the correct size, physically, and the 30D's are quite a bit larger. Don't know if I will be able to fit it correctly, but at least I'll have them.
The power supply test points are way off voltage spec and waveform, and p-p........... so I am going to start isolating boards until it gets back up. Unless it's actually the power supply, then I'll find that out last. First thing I disconnected was the vertical preamp, and it changed nothing. The grounding of the entire unit has loads of noise that fluctuates from 20-60mV p-p. Im unable to even guess as to a frequency, and from what I see, it looks like I have multiple near-shorts into ground. I have no idea how the scope even functions the way it does at this point. The trigger board seemed to have been giving off a little bit higher amplitude noise, but that doesn't tell me much since I haven't confirmed... and running around poking the grounds isn't going to get me anywhere either! hahaha
One thing weird I noticed.... TP 159 drops to -1V and takes a min or so to climb back to zero once I power off the scope.
-8V and 5V rails discharge immediately upon powering down.
55V and 15V drop like a capacitor connected to a dmm, as I would expect.
My assumption is that isolating boards is probably going to be the best way for me to go at this point, and then once I have the board or boards isolated, I can pull one lead of each cap and check them. if I don't find the rotten egg(s) right away, I will re-connect it and probe away. But I think that there are at least a couple issues to deal with that are causing the PSU problems, and I have a feeling that is why I was getting nowhere just probing test points and attempting to trace pulsating waveforms and the like (there are multiple). What do you think? (I think I should order that assortment of tantalum capacitors now....... )
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Oh, and Volume 1 of the 468 manuals has the flow charts and power supply isolation instructions. Starts at page 354 for the main power supply.
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Oh, and Volume 1 of the 468 manuals has the flow charts and power supply isolation instructions. Starts at page 354 for the main power supply.
:-+
I can pull one lead of each cap and check them. if I don't find the rotten egg(s) right away, I will re-connect it and probe away.
Away from the PSU ESR is less of a concern, but measured to labeled value is important. Any cap more than 5-10% from labeled value should be given "flying lessons". ;D
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I finished the checks for the main power supply.. along with getting shocked by the 6kv CRT capacitor. WHEWWWW. I'm still alive, though, and so are both scopes.
I white-outed the parts of the waveforms that didn't closely match, and doodled in my own scriblets.
Sooooo the +110V/+55V section has issues, especially since 110V rail has the jitters in sync with CH2... not sure how it doesn't affect CH1 but we shall find that out soon enough.
I think that the +110V is peeing in the other rails Cheerios.
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Well, removing the trigger board cleaned up the waveforms on 158 and 159 a bit, but the voltages were still off.
Pulling one end of both CR727 & R633 unloaded the PSU rails for the most part, but now 159 is 5V high, while 55V is on point.
Fun stuff. This may take a while. I may pull most of the boards off and repair each section/board one at a time until it's all done. The obvious, noticeable problem of the CH2 jitter seems to be one of several problems I need/want to deal with. If I only fix that one and nothing else, I'd likely be doing this again in the near future. Better to get it all over with now, right?
TauTech - you are correct. There is something rotten in the CRT circuitry. Glad I'm getting to it before it kills my CRT.
The trigger board definitely has some surprises for me, and so does whatever is down the like from CR727 and R633 which I will now trace... well, once I finish the isolation procedures until I have a correctly functioning power supply.
Thanks for the help Tautech and MarkL. I will keep on updating the thread with data and info as things progress and bad parts are singled out. I'm anxious for that final pic of some parts laying on the table, with the caption " The Bastards". haha
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I white-outed the parts of the waveforms that didn't closely match, and doodled in my own scriblets.
Sooooo the +110V/+55V section has issues, especially since 110V rail has the jitters in sync with CH2... not sure how it doesn't affect CH1 but we shall find that out soon enough.
I think that the +110V is peeing in the other rails Cheerios.
The causal effect may be the other way around. Here's a plausible explanation:
The +110V unregulated supply is used to create the +110V regulated supply. And the +110V regulated supply is used by the vertical deflection amp to drive the plates on the CRT. When Ch2 changes position vertically on the CRT, the vertical amp is going to change its current draw to do it. This will cause the unregulated +110V supply to fluctuate at the same time.
Now I do think from your waveform drawings and voltage readings that the unregulated +55V supply (TP159) might be loaded too much. The unregulated +110V (TP148) supply is probably ok. The fact that it's low and has a similar waveform is a side effect of TP159, namely TP158 equals TP159 plus the other half of the transformer winding that's common to both voltages.
It could also be the load is ok and filter cap C829 is in bad shape.
How is your +55V regulated (TP637) voltage and ripple? It's important becasue all the other regulated voltages are derived from it.
I still don't think it's the power supply, but I would check the above before moving on.
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the ESR of C603 is 6.3ohms. Glad I ordered newww ones. I think I may as well check the esr of all the big filter caps so I can make sure at least that part of the scope is good. I went thru the parts lists and wrote down all the tantalum cap values... I think I will be ordering enough to replace every one of them and then have some extra.
Oh, I also ordered a Fluke 6kv dmm probe last night. Wanted to get a 40 but I think I can live without being able to measure the CRT itself for now. If it works, it works. I still have several hundred dollars worth of gear to pick up to do calibrations, so $120 for a probe is out of the question. I'd rather use that for the PG506 & TG501.
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The causal effect may be the other way around. Here's a plausible explanation:
The +110V unregulated supply is used to create the +110V regulated supply. And the +110V regulated supply is used by the vertical deflection amp to drive the plates on the CRT. When Ch2 changes position vertically on the CRT, the vertical amp is going to change its current draw to do it. This will cause the unregulated +110V supply to fluctuate at the same time.
Thus far I'm not getting any results that show there is anything wrong with any of the vertical boards. When they are all disconnected from all of the rails, the jitter and loading issues are still present.
Now I do think from your waveform drawings and voltage readings that the unregulated +55V supply (TP159) might be loaded too much. The unregulated +110V (TP148) supply is probably ok. The fact that it's low and has a similar waveform is a side effect of TP159, namely TP158 equals TP159 plus the other half of the transformer winding that's common to both voltages.
It could also be the load is ok and filter cap C829 is in bad shape.
How is your +55V regulated (TP637) voltage and ripple? It's important becasue all the other regulated voltages are derived from it.
I am in the process of isolating the +55V completely right now. The only thing still connected to the +110V is the CRT, and I am under the impression that the CRT circuit is my main source of problems. The A & B sweep gen appears to be what is causing the large majority of the loading. Once I fully isolated it, the loading almost completely went away and the BR's no longer heated up.
However, now that I have the trigger board off, all the knobs on the bottom of the panel disconnected, A & B sweep isolated, and all the verticals isolated:
Regulated supplies: 121.5V; 54.99V; 14.99V; 6.78V, -8.02V ( so the 110V and 5V DC voltages are off)
All of the waveforms of the regulated supplies are a lot better, all being correct except for TP158, TP162, TP163 - they look like TP162 from the drawings I last uploaded.
Unregulated Supplies: 162.8V; 80.1V: 24.74V; 10.35V; 6.131V
R830, R728 both are getting fairly hot after a few minutes powered on.
R400, R834 both getting slightly warm.
Q838 seems OK temp wise, but I'm starting to think that either it's bad, or Q740 is bad. The 110V was not folded when I checked before isolating anything, but It's worth checking again now. The Vbe of Q740 was .5V, so I think it's likely good, but Q838 may be bad.
I have re-removed C603 and temporarily replaced it with a measured 2.4uF electrolytic, and I am now about to power on and see what changed now that I took that bastard out again. I had no idea that the ESR was that high!! the C measured at 2.6uF - it's a 20%, but I should have realized that Tek isn't going to use parts that are ACTUALLY anywhere near their rated tolerance limit. It is about 13% off, which should have been a red flag. Oh, the disadvantages to working on stuff late at night when I'm tired...
I still don't think it's the power supply, but I would check the above before moving on.
We will be finding out if there is anything wrong with the power supply very soon. My guess is that yes, there are power supply issues. If one cap is bad, then there are more bad caps and maybe bad transistors as well - such as Q838. Fun stuff.
It's a great learning opportunity for me, since I am rebuilding this scope from the supply up, and then am going to also calibrate it myself.
I have a STP 2145 A Double OCXO on the way that I am designing and building a board for, and then an enclosure and LiIon battery system so that it stays on 24/7, and I am about to order a few voltage standard ICs as well. Then to the cal gear. I'll have the engineering dept of the local college help me out with calibrations.
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the ESR of C603 is 6.3ohms. Glad I ordered newww ones.
...
Looking at the Vishay datasheet for the 3.6uF @ 125V tantalum (109D365X0125C0) in the current product line, the max ESR is 15.0 ohms @ 120Hz.
C603 is 3.0uF @ 150V (109D305X0150C2). So, it's max ESR will be even more than the one above because of 1) less capacitance, and 2) higher voltage. But it's not.
Unless the capacitance is off, or there's goo coming out the side, I think your C603 is fine.
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theres crust on one end. Looks like it's dried goo. That is what caused me to pull it in the first place, and then I put it back in because it measured within tolerance. Took it back out because I wanted to make sure it wasn't part of the problem. It is but only a little bit.
With the parts I already specified being isolated, here are the reg and unreg voltages as just measured, and the ripple.
The change in measurements between last night and now is due to the isolation of the Calibrator only..
Regulated:
(+110V) 121.4V 15mV ripple
(+55V) 55V <1mV ripple, but has "noise" above 20mhz
(+15V) 14.99V <1mV ripple, same noise
(+5V) 6.174V 40mV ripple
(-8V) -8.02V <1mV ripple, same noise
Unregulated:
(+110V) 162V TP158: .6V p-p, Waveform same as on scan posted previously. Unstable (vertical+horizontal jitter)
(+55V) 80V TP159: 400mV p-p Unstable (jitter)
TP160: .8V p-p correct waveform - stable
(+15V) 24.8V TP161: +80mV -70mV Correct waveform + slight amount of vertical jitter
(+5V) 10.3V TP162: -80mV +125mV Bad waveform (same as TP158 scan posted previously)
(-8) 6.13V -100mV +130mV Waveform OK but also has vertical jitter.
So now I am going to isolate the horizontal amplifier from the +110 and +55 and see what that does, then isolate it entirely. Next in line will be the CRT. The PSU is going to end up being 100% isolated so I can repair that, and then add the CRT, Horizontal, Sweep, Vertical, Trigger, etc etc. It's interesting that the scope still kinda worked before. It was WAYYYYY out of cal... the calibrator was off, B intensity was bad, B gate was flatlined, A gate's intensity output was bad....... plus the CH2 vertical/trigger problem. If I would have paid any more than I did for this scope, I'd be pretty mad right now.
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Ah, yes, datasheets. :palm: I was going to do that earlier, then I got carried away thinking a bit too far ahead of myself. Trying to do too many things at once.
That 3.6uf is probably close to the one that is in there now, since they are both 109D's, but the original is 3uf. Probably near the same ESR tolerance, though.
I cannot find the chart or info for what the correct voltage ranges are for the unregulated power supplies. I know that the expected voltages are on the waveform sheet and the schematic, but that doesn't tell me anything about what is completely unacceptable for troubleshooting. Should I just assume that they should all be within about 6-7%???
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Pages 6-8 and 6-9 in Vol 1 under calibration has the LV power supply tolerances and ripple. For ripple, 110 is 20 mv, for 55 it's 4 mv and the others are 2mv. For voltages 55 is +/- 0.5%, 110 is +3 / -2% and the others are 1.5%
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Pages 6-8 and 6-9 in Vol 1 under calibration has the LV power supply tolerances and ripple. For ripple, 110 is 20 mv, for 55 it's 4 mv and the others are 2mv. For voltages 55 is +/- 0.5%, 110 is +3 / -2% and the others are 1.5%
I'm talking about the unregulated supplies and what I should expect when I am removing power from various sections.
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Apply the KISS principle, it's just a linear PSU is it not?
http://www.geofex.com/Article_Folders/Power-supplies/plus-9v1.gif (http://www.geofex.com/Article_Folders/Power-supplies/plus-9v1.gif)
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Pages 6-8 and 6-9 in Vol 1 under calibration has the LV power supply tolerances and ripple. For ripple, 110 is 20 mv, for 55 it's 4 mv and the others are 2mv. For voltages 55 is +/- 0.5%, 110 is +3 / -2% and the others are 1.5%
and for the 468 the regulated PSU tolerances are on page 4-31 table 4-10 and 4-11.
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Apply the KISS principle, it's just a linear PSU is it not?
http://www.geofex.com/Article_Folders/Power-supplies/plus-9v1.gif (http://www.geofex.com/Article_Folders/Power-supplies/plus-9v1.gif)
so what are you saying? that the waveform irregularities are from a reverse-biased diode that's gone bad somewhere? I like the KISS principle, it helps, and it's the reason why I have put everything back together since I wasn't getting very far, and now the only thing off is the trigger board. I am running some repetitive testing as it heats up to see how the rails change from cold to hot, etc etc. Right now all of the unregs are showing instability, varying in voltage in a 1-3V range, some as they heat up and some back and forth. and varying ripple as well. I'm just going to follow the flow charts and isolation procedures, but as I isolate something and nothing changes, I am going to hook it back up. Instead of isolating one voltage rail of any specific area, I am going to isolate it entirely, though. no more of this one V of one board here, another here, etc etc. It gets me nowhere at all since I have so many rails that are having trouble. As soon as I remove the main board to isolate the CRT, I am going to pull the filter caps and check the esr. I am considering doing it in-circuit, but I'm not really sure about that.....
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As soon as I remove the main board to isolate the CRT, I am going to pull the filter caps and check the esr. I am considering doing it in-circuit, but I'm not really sure about that.....
For a linear PSU, I'd concentrate on "measured" value more so than ESR. Cross check against a similar value new cap and you'll likely find ESR values in the single digit Ohms range. In a linear application (working correctly) smoothing caps don't need to work as hard, generally the values are much greater to accomodate/manage the lower frequency ripple.
As I indicated a few posts back, overloaded linear PSU's will display much more ripple when operated outside "design".
Regulation "should" reduce ripple to intended levels IF the PSU is not overloaded.
And YES, you'll need to lift a leg.
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I cannot find the chart or info for what the correct voltage ranges are for the unregulated power supplies. I know that the expected voltages are on the waveform sheet and the schematic, but that doesn't tell me anything about what is completely unacceptable for troubleshooting. Should I just assume that they should all be within about 6-7%???
The unregulated voltages in a linear supply are directly related to the incoming AC line voltage, and the load being placed on 1) each of the individual voltages and 2) all of them as a whole (meaning a large load on one can drag them all down).
Tektronix specifies the measurements made with AC line voltage at 115VAC, and probably assumes a well behaved sine wave. Hopefully they've thought of and specified all the other conditions that affect the measurement too, but I wouldn't bet my life savings on it. These rails are called "unregulated" for a reason - they're not guaranteed to be steady or accurate.
If you have some unregulated loads disconnected it will have a large impact on the voltage reading and waveform shape. Or, as I explained before, if you have changing loads on an unregulated supply, it can and probably will fluctuate. It's completely normal.
With everything connected and under normal load, I would personally not get stressed out if the unregulated voltages were off by up to 10%. At 15% I would take note and if other symptoms pointed to a possible power supply issue I would start looking. At 20% nothing else would get done until I found why.
Linear or switched, the specific design of the supply, how the unregulated voltages are used, and the AC input range specification can sway exactly how much slack I'd allow.
If you're seeing that they're all low by 6-7%, I'd move on. For this scope it's right on lower bounds of the AC input range of 108VAC (115VAC nominal). If you have low AC line voltage, there's a switch that allows for 90VAC to 110VAC operation. But I wouldn't flip it because it's going to create more heat dissipation, and you've already noted that things are running suspiciously too hot already.
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It's similar to a faulty diode.. since it is a N-P-N junction, but Q828 is toast. I'm checking my lists to see if I have a temp replacement on hand, but I am ordering some motos to replace it and have a spare. I know that there has to be more going on here, esp in the 5V section, so I would like to diagnose that as well so I can maybe do the whole combined shipping thing.... and not have to wait forever for one replacement to come every couple of days!!
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I cannot find the chart or info for what the correct voltage ranges are for the unregulated power supplies. I know that the expected voltages are on the waveform sheet and the schematic, but that doesn't tell me anything about what is completely unacceptable for troubleshooting. Should I just assume that they should all be within about 6-7%???
The unregulated voltages in a linear supply are directly related to the incoming AC line voltage, and the load being placed on 1) each of the individual voltages and 2) all of them as a whole (meaning a large load on one can drag them all down).
Tektronix specifies the measurements made with AC line voltage at 115VAC, and probably assumes a well behaved sine wave. Hopefully they've thought of and specified all the other conditions that affect the measurement too, but I wouldn't bet my life savings on it. These rails are called "unregulated" for a reason - they're not guaranteed to be steady or accurate.
If you have some unregulated loads disconnected it will have a large impact on the voltage reading and waveform shape. Or, as I explained before, if you have changing loads on an unregulated supply, it can and probably will fluctuate. It's completely normal.
With everything connected and under normal load, I would personally not get stressed out if the unregulated voltages were off by up to 10%. At 15% I would take note and if other symptoms pointed to a possible power supply issue I would start looking. At 20% nothing else would get done until I found why.
Linear or switched, the specific design of the supply, how the unregulated voltages are used, and the AC input range specification can sway exactly how much slack I'd allow.
If you're seeing that they're all low by 6-7%, I'd move on. For this scope it's right on lower bounds of the AC input range of 108VAC (115VAC nominal). If you have low AC line voltage, there's a switch that allows for 90VAC to 110VAC operation. But I wouldn't flip it because it's going to create more heat dissipation, and you've already noted that things are running suspiciously too hot already.
that sounds like a reasonable set of guidelines for dealing with unreg rails. I have been measuring my line voltage here and it is fairly stable at 116VAC. sometimes it gets up to 118VAC, but it's never been over that.
I found a variac on craigslist that is $10, but an hour away. I need to go and get it..... it's only rated 150W and up to 150V, so at the 132max that the manual specifies, I would only have about 1.14A rated current, but it's an older one (not too old) so it can prob be pushed a little for a short time if need be. I'm thinking it's worth the gas, drive, and ten bucks. I have hooked my clamp meter to modified cables to the 568, and thus far even with the problems it draws less than 1A.
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I found a variac on craigslist that is $10, but an hour away. I need to go and get it.....
Never found the need for one.....just use a dim bulb tester
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I made an octopus out of an old train transfo. :-+ Now I can do in circuit testing..... and CR600 is bad and several others seem to also be bad, but I won't know until I pull a lead up and check again. CR729, CR728, CR815...........................
I made sure that this octopus DOES in fact work when I measure definitely good components, and I have found plenty of components in the scope that are good acc to the octo. I know that I will have to check the schematics to see if there are any components that would interfere with the octo, such as C603 interfering with CR729.... but I know what a 3uf cap looks like, and it didn't look like that.
I just wanted to put an update on here.
Another thing that seems strange to me is that now that the trigger board is the only thing removed, all the loading on all the regulated rails is gone except for the -8. hmm
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Another thing that seems strange to me is that now that the trigger board is the only thing removed, all the loading on all the regulated rails is gone except for the -8. hmm
Don't forget the "trap for newbies", installing polarised components incorrectly on a negative rail.
Very easily done, especially without an overlay.
Pay attention!
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Another thing that seems strange to me is that now that the trigger board is the only thing removed, all the loading on all the regulated rails is gone except for the -8. hmm
Don't forget the "trap for newbies", installing polarised components incorrectly on a negative rail.
Very easily done, especially without an overlay.
Pay attention!
I havent yet removed or installed anything on the -8 rail. I just got done checking all the components in the 110 reg, and VR825 is bad. It is conducting like a regular diode instead of a zener. there is no way I would have known that without this octo setup I made last night. See any reason why I cant power up to check the other rails with the 110 reg out of comission??
Also, that dim bulb tester you mentioned, that is for short circuit testing only, correct? What I was getting at is actually controlling the input voltage to the scope. Plus I will need one when I start doing the cal on these scopes.
***If It's not a prob to power up without 110, which I don't think it is, I know I need to disconnect CR840 sp no power feeds those transistors***
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Forgot to mention, I did use the octo to check parts I have, and checked some 100V zeners in scope and out to make sure that my rig is able to show me the output to the scope correctly. I did manage to figure out that the voltage and current to the CUT needs to be raised sometimes. Such as Q828 - I had to turn the transfo up to about 12V to get it to show me the "Z" and "long L" shapes. (my transfo does 0-21V, and I have a 15K R + 10K pot , so I have a pretty good V and I range. :-+ )
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Well, I feel like a complete noob idiot. I did not have enough voltage going into the zener to properly test it, and then said that I knew I had to crank up the voltage for some components! shortly after I realized that the zener I had checked that was supposed to be 100V but was actually a 22V. So I wired up a transfo out of a high end stereo amplifier, and now I have 56V, 98V, and 115V. I also realized that I can use it a 2 output isolation transo of sorts, and use it to give me a steady 115V.
But anyway, I did find a bad part in the 110V regulated section.... a resistor... R741. a lead fell off when I it to isolate Q828. I replaced it with 2 10ohm comp resistors for now. Hasn't made all that much difference, though. Only thing that has improved is the amplitude of the waveforms on the unregs. everything else is the same.
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I found some bad CRT caps. and a bad cap on the trigger board. Of course there is a whole lot more checking to do... I get my 6kv probe tomorrow, too!
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Let me know if there are any particular voltages, or resistances you would like to confirm. I have a running 468 I can check.
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Ok, thank you. I am just now getting ready to do a little more checking around. There are some caps in the CRT, Trigger, Sweep, and vertical section that are on their way out, plus at least one diode somewhere that's clipping my waveforms. Once I have a few things sorted out on it I will definitely be taking you up on that offer. :-+
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What do metallized poly film capacitors look like on the scope when measuring the ESR (w2aew style) ???? all 3 of these MKT1816's look like this. I don't know if this is some sort of artifact of being low capacitance/high voltage, or if they are bad. According to my scope and the ESR calculation, these capacitors have an ESR of 0 because I am putting in 1V and getting back 1V.
in the pics, the scope is on DC input, .2V/div. In AC mode, it's centered but the same. My input from FG is 200khz 1V positive going sq wave.
I don't have any similar caps to compare to. I have another MKT but it is 1.5uf 100V.
These caps are C218, C220, C225 of the CRT circuit. They are 6.8nF 5kv.
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and I have no idea where to find out what the ESR of the caps in this scope is supposed to be. none of the datasheets I have found specify. very irritating.
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ended up finally finding some info about ESR:
http://patriotgaming.com/manuals/gaming_monitors/sencore_instruments/LC103/TT104-4416.pdf (http://patriotgaming.com/manuals/gaming_monitors/sencore_instruments/LC103/TT104-4416.pdf)
https://en.wikipedia.org/wiki/Equivalent_series_resistance#Typical_values_of_ESR_for_capacitors_.5B3.5D (https://en.wikipedia.org/wiki/Equivalent_series_resistance#Typical_values_of_ESR_for_capacitors_.5B3.5D)
http://preher-tv.blogspot.com/2011/03/typical-electrolytic-capacitor-esr.html (http://preher-tv.blogspot.com/2011/03/typical-electrolytic-capacitor-esr.html)
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I doubt the values are critical, you could just replace for piece of mind.
http://www.aliexpress.com/store/product/10pcs-20KV-10nF-0-01uF-103-High-Voltage-Ceramic-Capacitor/929745_1382467275.html (http://www.aliexpress.com/store/product/10pcs-20KV-10nF-0-01uF-103-High-Voltage-Ceramic-Capacitor/929745_1382467275.html)
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yeah they just go to the Z axis ground plane. but replace metal-poly with ceramic? I know that those are what is used in the 465's for the same purpose, or at least in mine.
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I also forgot to mention that I figured out why I am seeing what I am on the scope for ESR. Under about .5uf, the readings using this method become unreliable (on the equation side). I have probed a bunch of various low value caps, and it seems if they are good, there will be a smooth curve on the + and - charge cycles of the waveform. at those values, it pretty much acts as a cap in circuit and smooths the charge cycles of the waveform. So if the cap is bad, and it would probably have to be pretty bad to do this, but they are either choppy or just square. I guess indicating a short or near-short. So more or less, under .5uf or so, my ESR contraption is useless.
Oh yeah, and the lower the value, the smaller the curved portion of the +charge and "- charge" cycles are. It probably doesn't look the same for polarized caps, though.
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Persistance pays off!!! I'm starting to find bad components in the 110V circuitry. The astigmatism pot is bad. It's a 100k pot and it goes from .6ohm to 30kohm and back down to 15k ohm. C142 was also dead. I was only able to tell by comparing the ESR trace to other .01uf ceramic caps to figure out it had a high ESR. (Also, increasing the frequency helps with seeing if very small value capacitors are bad. I doubt the measurement is accurate, but comparing them to new ones of the same value is what I did. )
C303 of the CRT circuit measured 16ohms ESR.
Another thing is that there is a dead short between the Z axis ground plane and the main ground plane.
The resistance measured between the +110V (at TP642) and ground is 19.56Kohm - which doesn't seem like anything, but.. I measure the exact same resistance between the +110V and -8V, and also the +110V and +5V. but not between the +5 and -8.
-8V reg and -8V unreg are a dead short from the filter cap + to to TP740.
110V reg to 110V unreg reads 1.46Mohm and climbing, like my meter is charging a cap.
+15V reg to unreg does the same thing, except it starts at 0 and rapidly climbs. It's up to 5kohm within 4 or 5 seconds.
the other rails seem fine when measured from reg to unreg. stable readings, no complete shorts.
I've already checked every component in the 110 V reg section, so there are no issues there. I'm about to check Q401 and it's buddies in the -8, and all the diodes in the +55. here very shortly, whether I find issues with it and do some temp fixes til I get proper parts - or not, I am going to stick it all back together and power up to see where I'm at. I've got my HV probe now, so I can measure the CRT, but my Fluke is wayy off and needs to be cal'd so unfortunately I will need to use the RS.
One thing I thought of that I find strange, is that when I had the Vertical Amp section isolated, the only voltages that weren't affected are the ones that go into it - 55, 15, and -8. I thought that meant that there is nothing wrong there, but it can't be coincidence that all the voltages but those 3 changed..
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I've got my HV probe now, so I can measure the CRT, but my Fluke is wayy off and needs to be cal'd so unfortunately I will need to use the RS.
The only thing you need to check is the ~2kV CRT HV.
You don't really even need to bother until you're ready to go through the cal process.
You have a display so the PDA multiplier is working, no need to measure that.
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Welll I found at least one part that is causing my power supply mahem. I had tracked my problem down to the region on the output pin of U712A and determined that either U712, C609, R608, or VR604 was bad. The 15V rail is fine, so that made me think that U712 is likely good. C609 didn't seem like it was a likely candidate, since the voltage at pin 1 of U712 is low, not high. R608 would have been hot or noticeably cracked (when pushing on it with pencil eraser).. so I decided to pull the zener and test it with my PSU and a resistor. It doesn't start clipping until 48V, so there is the 5V that I am low on pin 1.
I noticed from some pics of a 468 main board from ebay that the CRT caps I have are not original. The only one that is in there and still original is the .0047uf 6kv, which I found an exact NOS replacement for for $4. However I'm sure that is not the main issue in the CRT circuit that is dragging down my 110V unreg.
I decided to map out what components get unusually warm, hot, and scalding on the main board, trigger, and vertical preamp. Mapping that out on the circuits is where I am now, but one thing I noticed is that Q530 is likely bad. It appears to have been replaced previously, also, since it is the 2n2484 and not the ST1046. When I touch the can, all hell breaks loose with the CRT. That transistor is right there smack in the first part of the CRT circuit at the 110V unreg feed, and the 5Vreg feeds into it also. So now I am going to put the bad zener back in so I can keep going until the replacement gets here, and check the voltages around Q530 to confirm. Fortunately enough, the voltage on all 3 pins is labeled on the schematic! :-+
As for the other issues... I noticed that quite a number of components get really hot in the Horizontal Amp circuit as well. I wish I had a 43V zener on hand to stick in there for now so I could figure out what issues that solves, and work on the ones that aren't caused by the bad waveforms that the zener issue caused. I'm thinking that fix will cool down all the bridge rectifiers and power supply rail components - and numerous other things, too.
I modified my UPS so that I can run my 465 off the battery, so now I can reference on stuff other than ground!!! YAAYYYY! That should help quite a bit!
I have a decent list of parts to get, and some I already ordered (109D's, 30D's, 500D's, Z5U ceramics..). So far my list contains solid tantalums, some of the common diodes, zeners, 2n5551's... does anyone have any suggestions for spare parts I should have on hand for this scope and whatever one I end up with next?
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Other than getting a few more of each component because they are usually cheaper in a lot of 5 or so and never too expensive to have anyway, the one transistor that seems to be common for CRT output stages is BF259. If not spec'ed for it they are generally an excellent substitution, often better than the OEM device.
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yeah it specifies ST1046 and the recommended substitution is 2n2484. so you're saying I should replace the pair of them with BF259's?
I'm sorting thru my zeners and I have a 36V that clips at 41. That 43V specifies 5%.... 5% under 43 is 40.85V. do you think I should try it if I can't find a closer one? or just put the old one back and wait?
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yeah it specifies ST1046 and the recommended substitution is 2n2484. so you're saying I should replace the pair of them with BF259's?
Carefull study of datasheets for any substitution requirements is the only way.
I'm not saying BF259 is a "general" output stage replacement, it's the highest voltage device in that family and IME has been a good substitute IF REQUIRED.
These metal can transistors are quite common in output stages in many CRO's and BF259 is often a perfect "drop in".
I generally just check a BJT with a cheap DMM Hfe tester and ensure it falls within typical gain specs for that device. Sometimes they are used as push-pull, so check both devices have guite similar gain.
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Now that I understand opamps a bit better, I understand what that bad diode has been doing to my power supplies... the output voltage of U712A was getting too close to the 9V on the input pins, so it was clipping the waveform. That replacement should help clear up the waveform itself, but I'm not so sure it will stabilize the jitter. My guess is that is being caused by the excess current that is being drawn which is destabilizing the opamps and transistors. (am I close, or correct?)
Until my zeners and 2N2484's arrive, all I can do is try to figure out what's causing the channel 2 vertical instability and continue mapping out the overheating components in the schematics to figure out where the caps that are trying to fry my scope are. I'm considering just replacing all the tants just to be sure. I already listed all that are in the 468.
(((I am just going to stick with the original match components for now [2N2484] until everything's fixed, then I may try the BF's. It was cheaper for me to get 10 mil spec 2N's than it was to get 5 ST BF's anyway.)))
This may be a little premature, but I'm wondering what a good alternative to the type 106 would be. Basically I am wondering if I could get a FG with near 1ns rise-time and high amplitude that would be more worth the money than just buying the 106 for $120.00. My Wavetek 166 only does around 7ns. The PG506's are insanely expensive so I bought the 067-0502-01, and a type 191. still need the TMG and a fast rise/high amplitude sq gen.
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For plate output transistors, I found devices used in OEM sometime have little voltage headroom, more so in designs that operate at ~150V. It seems the 468 has only 55V for the output stages so to use a 60V device should be OK but it is too close to it's Vce max for my likeing.
For Calibration checks you can get away with quite little gear, Dave's got a good vid on a Cal check:
http://www.eevblog.com/2013/08/03/eevblog-502-19-hameg-analog-oscilloscope/ (http://www.eevblog.com/2013/08/03/eevblog-502-19-hameg-analog-oscilloscope/)
Something like that can be quite adequate for other than precise use when you would buy a new unit or get it professionally Cal'ed, wouldn't you?
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Good point. I'll order a few of the BF529's to try. they may be a better choice for while I am troubleshooting and getting everything back in order, and then when everything is back where it should be I could put the 2N's in if I want. I'll just need to double check the circuit and make sure that allowing more voltage thru those won't allow something else that isn't so easily replaced (or multiple parts) to be blown.
Of course a professional NIST traceable cal is best for precision. I do want to get my scopes back into calibration fairly well, but mainly I want to get the gear and do the cals myself for the knowledge and experience. Also, I could offset the cost later on by calibrating other peoples gear or by in-house renting them. I'm lucky enough to know some people in the engineering dept of the local college where I could go and check the cal on all my gear for free.
I bought an STP2145A double OCXO that I am starting to design a board/LiIon constant supply/enclosure for soon, which I will use as my freq standard. I'll probably try to score an old 11 or 12 digit counter to mod for it. Something like that I could easily take there and calibrate.
Other than all that, I just want the old Tek analog gear! ;D I intend for my hobby to be buying, restoring, and calibrating old analog scopes and gear, mainly Tek. Once I get a good bit better at it, I may even do some to sell to offset the cost - If I can convince myself to part with them! haha
I'm thinking the filter cap on my -8V is bad. I need to pull it and check. When I have several boards isolated from it, the reg output is OK, but the unreg jumps to over 200%. When nothing is isolated, the unreg is 1V high. Also, Q401's emitter is 1.7V instead of 1V with nothing isolated (didn't measure it with boards isolated, unfortunately.) Either Q401, Q402, or CR304 is my guess for the culprit. I'll be powering it up with the bad zener and doing a bit of digging to see what I can come up with soon. I have 30 values of zener diodes, but nothing close enough to 43V. Nothing at all between 36V and 50V. figures, right?
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I may have just found the source of the excessive current problem...... I really got to slowly and closely inspecting parts... mainly looking for zeners with burnt leads, and boy did I find a bunch. :o
So I decided to look at capacitor ESR... a few of the tantalums are.. ehhhhh... which could be part of the problem. But those little bullet "electrolytic" capacitors that are mostly on the trigger board... there are several of the same value that do not have anywhere near the same ESR. From what I gather, 1uf 35V capacitors should have an ESR of about 4ohms. most have 5ohms (acc to my device) and some have about 1ohm. I'm about to pull some leads and take some better measurements of ESR, capacitance, and check resistance across them to see if any are shorting.
However there are zeners on several boards with burnt leads, so I'll be doing a lot of capacitor checking for now. Some of the zeners must be ok or I don't think the scope would be working, but the longer I have it on, the worse they are going to get, so I'm not going to power it back on until I've found some caps to replace and get the current draw down a bit.
Have any other ideas for why the zeners are getting so hot that their leads turn black?
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I was mistaken about some of the capacitor values, which makes the dramatic difference in ESR understandable since some are 2.7uf and some are 1uf, but I am still fairly certain that several of them are bad. I actually think that there are only two good ones on the whole board. They are the only ones that give me a "flat" response on the peaks of the waveforms. the rest of them are ramps. I think that these ones are originals. There is no variation in any of them at all, not even in the text. I know that several tantalums have been replaced, a couple of the main filter caps, and all the electrolytics were replaced with ITT's at some point. Makes me kinda wonder why the one in the CRT circuit was soo bad (15ohms). That area shouldn't have been getting so hot that it killed that one cap..
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I accidentally shorted the B-E of 740, so now I'm scrooged until Mouser gets me my new ones. :-[
Installing that new 43V zener diode worked wonders for the 110V and 55V waveforms, and of course brought the voltage on TP158 up 5V. Also allowed the 5V and -8V rectifiers to run a little cooler, but I'm still waiting on the new caps so I can lower the current draw. It's so high that it's killing the 43V zener again. The new one started at 14.8V and dropped to 13.5V fairly quickly. I didn't get all that far testing stuff once I killed Q740 when I was checking it's Vbe. Oh well...
The most expensive capacitor in the entire scope needs to be replaced! but fortunately it is in the storage PSU so It's not crucial to get the main fixes done. It's C162, and it's a 3 lead VPR cap. It's just a filter cap, so when the time comes I,m just going to stick in a 390uf 16v radial, and I guess I could just cold solder a lead to the top of the can? Unless I find out that it's a bad idea before hand.
at this point, I'm just worried about getting it running properly. Im not going to buy top notch parts, or the manual specified -expensive- ones anymore, unless I can get them cheap.
I also noticed one more BIG problem....... Once the scope warms up, once every several minutes, it will "almost" power down. Everything goes to shut off, and then it goes back to normal. The trace never disappears or anything, but the rail voltages drop by a volt or so and then jump back up.
Overcurrent protection of the 55V rail, maybe? (that's why I was checking Vbe) Q740/Q828 provide 110Vreg, but also provides 55V for the non-inverting input to U712 for the 55V rail. So if it is overcurrent protection, it would have to be affecting one of those two rails in order to cause a "shutdown" like that, right?
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Speaking of C162, what is this third lead for ?
Are these two caps ? Or one of the lead is doubled ?
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Speaking of C162, what is this third lead for ?
Are these two caps ? Or one of the lead is doubled ?
It's one single cap, and it appears to be just a radial cap with a 3rd lead soldered to the top of the can.
I'm not really sure what the 3rd lead is for, but I do know that the can of a capacitor can also act as a plate and store charge. On the schematic, it only shows a standard capacitor. I haven't taken the board off to look at the traces, but my guess is that it's just extra grounding for added stability.
I just know that these caps are very hard to find replacements for unless specially ordered, and are very expensive. I'm hoping that I can replace it with a regular radial cap with a 3rd lead soldered to the top of the can.
Here's a link to the VPR capacitor catalog info.
http://www.cde.com/resources/catalogs/VPR.pdf (http://www.cde.com/resources/catalogs/VPR.pdf)
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I have seen the catalog, but could not understand really what was the meaning of this pin.
I am asking that because I might be very soon confronted to the same problem.
I just got a tek 468 from my favorite dumpster and it is right now in my trunk.
I have not done anything on it, and not powered it yet. All that I should start to do this week end.
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When I get everything working in the non storage and start poking around replacing caps in the storage area, I will post some info about those caps and their connections, and also a link to the replacements.
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The third lead on those capacitors doesn't appear to connect to anything. It's a 4 layer board so it's a bit hard to tell, but I stuck a high-powered light to the board and there doesn't appear to be any traces connecting to it on any of the layers. I checked around a bit with an ohm-meter as well and just got the typical 5Mohm countdown from the capacitor charging to one of it's other leads. So it really doesn't seem all that necessary to even have the 3rd lead, although I expect it was used for a reason..... If I can't find out for certain what it's purpose is and if I can omit it, I suppose I will try and find a suitable 3-lead replacement, or tack a lead to the can of a radial, since that seems to be exactly what it is. I actually think that is what was done when this capacitor was previously replaced, since it's a 672D and not a Mallory VPR like the 540uf on the board.
I think I'll just avoid using storage mode until I replace it since its reading over 43 ohms ESR. I'm about done replacing all the tantalums and smaller electros now, so hopefully it has made a dramatic difference in the current consumption. I figure that the enamel on the transformer wires would have been fried off if it would have gotten any hotter...
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I'm finally getting somewhere with this now... I think I may have tracked down the source of the CH2 vertical position instability issue.
There are several problems with this scope, so it's not an ideal first troubleshoot/repair piece of gear, but it is one heck of a learning experience!!
Here is a short video that shows a sample of the signal I get from pin 1, 2, and 3 of U341A and the B and E of Q350. It gets a whole lot worse than what my scope screen shows.
I traced it all the way back to here from the Vertical Output Amp, and this is where it is the worst. I don't see any difference between CH1 and CH2 beyond this point, but the signal back farther is so small that I can hardly view it at 5mv/div.
https://www.youtube.com/watch?v=9eSi20WsQ6g (https://www.youtube.com/watch?v=9eSi20WsQ6g)
Anyone want to take a guess for which one is the rotten egg? Is it U341A, Q350, or neither??
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Is the jitter on the base of Q350, that will confirm Opamp is the culprit.
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Is the jitter on the base of Q350, that will confirm Opamp is the culprit.
yeah the jitter is on the base and the emitter - and both inputs and the output of U341A. I really thought that I had it nailed, but I replaced the opamp and it's still there. It seemed a tiny bit less pronounced, but it's definitely not by any means gone.
There is a lot of HF noise throughout all of the vertical section on both channels, so if it is a small oscillation in the signal that is being amplified by Q350/U341A, it's going to be nearly impossible to track unless I can solve the noise problem that covers all the vertical.
After failing to fix it by replacing the opamp, I went back directly to the input directly behind the attenuators to see if I could find ANYTHING different between CH1 and CH2, and there is a bit of a voltage difference there as well as throughout all the vertical boards. But as for AC signal, there isn't anything noticeably different from CH1 to CH2. The voltage difference starts directly after the 6.2V Zener at VR388, so I put new ones on both channels - and no difference again.
All of the vertical stuff has HF oscillations, the chop clock has oscillations, the CRT circuit has issues (maybe Q109?) and who knows what else is wrong. I'm starting to think I should just keep this as a parts scope and move on...
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Oh, and I forgot to mention - I started pulling up leads and checking the resistors around U341... R442 increases in resistance when I heat it up with the iron. Isn't that the opposite of what is supposed to happen? I checked a couple other new ones I have, and they drop when I heat them.
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well I fixed it. The turd was the .1uf 50V MLCC cap on the non-inverting input of U341A. It fell apart when I was pulling the lead up to check it. The lead fell off. I stuck it back in to test it anyway and it was over 50%. (156nF) . So one problem down, and who knows how many more to go!
In hidsight : When the opamp swap didn't fix it, it had to be something between the position pot and the non inverting input, since there was that noise on it, and no way for it to get from the inverting input or output back to that pin. So it had to be either the pot, those resistors, or that cap. I really didn't expect to have any ceramic caps that are bad, but now I know that when they go bad, they really go!
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well I fixed it. The turd was .....
:-DD
:-+
Yes with older gear expect any component, tests and faultfinding will point you to the suspect area then the rest is up to you.
Thanks for sharing.....everybody learns. :-+
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I'm finally getting somewhere with this now... I think I may have tracked down the source of the CH2 vertical position instability issue.
...
This op-amp area was the first suspect in post #8.
https://www.eevblog.com/forum/repair/tektronix-468-repairrefurb/msg667054/#msg667054 (https://www.eevblog.com/forum/repair/tektronix-468-repairrefurb/msg667054/#msg667054)
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I'm finally getting somewhere with this now... I think I may have tracked down the source of the CH2 vertical position instability issue.
...
This op-amp area was the first suspect in post #8.
https://www.eevblog.com/forum/repair/tektronix-468-repairrefurb/msg667054/#msg667054 (https://www.eevblog.com/forum/repair/tektronix-468-repairrefurb/msg667054/#msg667054)
Yes it sure was - The reason why I missed it is because I was following the manuals testing procedures. I only had input into channel 1, I was looking mostly at the waveforms as shown in the diagrams, and looking for inconsistencies in those waveforms. It took a little while before I realized that the manual and it's test procedures are only useful to a certain extent. From there you are on your own to figure out (if you don't already know) what to do in order to track down the specific type of issue you are looking for.
I also was in my infant stage of knowledge in using a scope. Didn't even know how to use the delaying time base.
I'm a first timer on a steep learning curve who is also fairly new to circuitry in general. This whole scope repair experience is what I am using to learn how to do all this, and also as an aid in learning how circuits/components function.
This is by no means over, though. There are still several more problems and PSU loading going on. I'm fairly certain that Q109 and at least one of the .0012uf caps need to be replaced, and a 22Mohm resistor that measures 25Mohm. ( waiting for delivery ). There is some issue with the signal going to the trigger gen also. When I did some checks on A and B trigger, the only way it triggers correctly ( from a - point to a + point on the level ) is if I have it on HF reject. It mostly has issues when CHOP is selected, so I know there's something up with or interfering with the chopped clock. Something on the vertical preamp board is dumping oscillations into the -8 rail. Who knows what's related to what here, and only 1 way to find out... and ONE AT A TIME this time!! Those are just the things that I've noticed so far.
A portion of the oscillations I was seeing were from the Wavetek 145, so I switched to the Tek CFG253 for now. I poked some capacitors in the 145 and picked a handful of questionable electro's to replace, and if it works - great. if not - I'll deal with that later. I'm not spending more than an hour on it until I either finish or give up on the scope. My 166 needs help, too. Positive going waveforms have a neg offset, so I'm not using that. Too bad I bought everything except for a 106!! oh well. At least I've learned a bit and am back on the right track - and I definitely appreciate all of the help and pointers!
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found the source of the additional noise on CH2. There is also a little on CH1, and I'm betting that replacing the R-C behind the attenuators on both channels solves that. C3042 is a bit rough and causing some oscillations. I'm going to replace the R3042's as well since the cap is soldered directly against the body - It might not last being heated up 4 times.
I figure I should work from the input signal back so I don't get any more false ideas about what parts are bad. I'll get my signal sorted so that my waveforms and voltages match the manual spec and work it like that.
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Both R3042's were bad - wayyyy out of spec!
Oddly enough, the caps are fine as far as I can tell. They are 1nf +100-10%. Replacing them anyway.
Chan 1: R3042 = 731K ohm C3042 = 1147pf
Chan 2: R3042 = 736Kohm C3042 = 1125pf
The resistors are supposed to be 470Kohm 5%, so that means they are both out over 30%!
I thought that maybe there was something I overlooked regarding the values (since it's a Heerenveen scope Opt 5), but no.... the color coding is correct. Yellow Violet Yellow.
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OLD metal film resistors?
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actually no. they are 1/8W comps. The 30ohm ones behind them are films but they are fine. the parts list says those are supposed to be comps as well, so I added that to the list. For now I have 1/4w films and 102K 2kv caps in as temp replacements so I can keep moving. I used ones that are within 2% of spec.
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I'm close to finding what is pulling down the rails on the vertical board. it is something in between test point 7 and 9 on CH1. it may not be the only thing, but it's definitely part of it. there is 100mv extra on the TP9 waveform and 13mv DC noise where it shouldn't be. Also the -8V rail is 50mv lower there than anywhere else.
See any reason why I shouldn't use all comp resistors in my repairs except for where they are wire wound?
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For those in the signal path I would probably keep to spec ( :-// ) all others metal film other than WW of course.
Others might like to chip in with advice on this matter.....
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Ive found quite a few metal film caps that must be replacements since the manual says they should be comp resistors. I didn't know if comps would be more reliable or not. While looking up info on the CRT circuit I had read a comment someone left stating that said person should replace any bad caps in the "Focus" section of the CRT with composition resistors regardless of what the manual says - so I thought maybe that means they are considered more reliable in gear such as this and figured I'd ask. One can never go wrong sticking exactly to spec, though, so if I don't find or get any definite feedback confirming my thought, I'll just stick with spec.
It seems that these axial MLCC caps are going to be a major culprit of many of my problems. I've found 3 on channel 1 alone that are way out of spec. I'm assuming their ESR is quite high as well (I can't get reasonable results using scope/FG below about 1uf). C177 in the NORM trigger section (diagram 3, 1-L ) is the most recent. 0.1uf reading 0.13uf. And I've got the hang of finding the ones directly off the rails (most are) using just a DMM. Actually harder to find some of them with the scope since I can hardly see the noise riding the waveforms. If I'm looking for a bad cap off the 5V rail, I just go thru and find where that rail is the lowest, then activate that circuit and find exactly what cap has the lowest voltage at it's 5V rail side. etc etc.
Seems there's a trend of 0.1uf's that are bad. Maybe my scope got lower quality batch.
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Remember those in the Focus section are normally dividers subjected to HV, often high Ohmic values and IME fail or drift in value......they are subjected to some stress.
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good point. I did measure some comps that have drifted (such as the 22Mohm that is now 25Mohm). I don't want to continue to pay multiple shipping charges since I now know that I am going to need a variety of parts (of which I will definitely be buying a bit extra!!!) so I am replacing what I have replacements for or that are bad enough for me to make do with what I've got. Noting them and adding them to the order list.
I did notice that using metal film instead of comp for those 470K inputs isn't going to be working out. The signal is clean before those, but directly following them the peaks are slanted. Maybe it's the higher voltage caps, the resistors, or the combo, but I can tell that the RC is causing overshoot on the rises and falls. I just hope that the caps I find for replacements work well, otherwise I will be trying again since it's a critical part. If the input signal is wrong, everything is going to be wrong.
It's incredible how many of those little MLCC's are bad.. I replaced all of the tantalums - dipped, 173Ds, 150Ds, and all of the small electros, so I won't have those to deal with. Following all that, I noticed a tremendous drop in the amount of heat on the transfo and rectifiers - and also various components throughout.
I guess there is a reason why almost every single resistor that the manual specifies as comp have been replaced with metal film???? It appears that someone has systematically went thru and replaced them all at once.. the soldering is professional, so I guess I should keep to that and where there is currently a film resistor, and it needs to be replaced, replace it with another film?
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It's incredible how many of those little MLCC's are bad.. I replaced all of the tantalums - dipped, 173Ds, 150Ds, and all of the small electros, so I won't have those to deal with. Following all that, I noticed a tremendous drop in the amount of heat on the transfo and rectifiers - and also various components throughout.
:-+
I guess there is a reason why almost every single resistor that the manual specifies as comp have been replaced with metal film???? It appears that someone has systematically went thru and replaced them all at once.. the soldering is professional, so I guess I should keep to that and where there is currently a film resistor, and it needs to be replaced, replace it with another film?
I'll consult my Dr. EE mate on this matter a little later tonight and report back.
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I guess there is a reason why almost every single resistor that the manual specifies as comp have been replaced with metal film???? It appears that someone has systematically went thru and replaced them all at once.. the soldering is professional, so I guess I should keep to that and where there is currently a film resistor, and it needs to be replaced, replace it with another film?
I'll consult my Dr. EE mate on this matter a little later tonight and report back.
Well HOLD UP!
As I suspected and has been confirmed there is EXTREMELY good reason why composite resistors are to be used.
Inductance, or more correctly lack of.
For any in the signal path, inductance will impact on frequency response. :--
So, to repair to spec......best you keep to spec. ;)
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I guess there is a reason why almost every single resistor that the manual specifies as comp have been replaced with metal film???? It appears that someone has systematically went thru and replaced them all at once.. the soldering is professional, so I guess I should keep to that and where there is currently a film resistor, and it needs to be replaced, replace it with another film?
I'll consult my Dr. EE mate on this matter a little later tonight and report back.
Well HOLD UP!
As I suspected and has been confirmed there is EXTREMELY good reason why composite resistors are to be used.
Inductance, or more correctly lack of.
For any in the signal path, inductance will impact on frequency response. :--
So, to repair to spec......best you keep to spec. ;)
Ahhh yess.... that definitely explains what I am seeing behind the R3042's! INDUCTANCE! That's what is causing the "overshoot" on rise and falls! Like I've said, I always prefer to stick to spec.... so now I'm going to need to buy a stockpile of comp resistors to fix what some dumdum did to this scope!!!!! Oh well, at least I'll finally have some new comps, and 1/4w resistors in general.
I'm having a very hard time locating exact replacements for those C3042's. They are 0.001uf 200V +100-0% Ceramics. I was thinking about using Orange Drops to replace it - but aren't those a type of stacked foil film/polypropelyne? So I better stick to ceramics only for signal, I think anyway.
I am not sure if film foil caps are all inductive or just some, but I found these as possible replacements. Let me know what you think.
http://www.digikey.com/product-detail/en/WMC2D1K-F/338-3227-ND/1930379 (http://www.digikey.com/product-detail/en/WMC2D1K-F/338-3227-ND/1930379)
Datasheet: http://www.digikey.com/product-detail/en/WMC2D1K-F/338-3227-ND/1930379 (http://www.digikey.com/product-detail/en/WMC2D1K-F/338-3227-ND/1930379)
---It says those are specifically for high pulse applications, which leads me to believe they might give me a sharp, clean signal. Non-inductive of course.
http://www.digikey.com/product-search/en?pv16=2&FV=fff40002%2Cfff8000b%2C340040%2C38000e%2C1140050&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=50 (http://www.digikey.com/product-search/en?pv16=2&FV=fff40002%2Cfff8000b%2C340040%2C38000e%2C1140050&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=50)
---The mil spec MLCC option
http://www.ebay.com/itm/LOT-24-NEW-SPRAGUE-192P102X9200-CAPACITOR-200V-0-001UF-D478252-/381077387116?pt=LH_DefaultDomain_0&hash=item58b9fb776c (http://www.ebay.com/itm/LOT-24-NEW-SPRAGUE-192P102X9200-CAPACITOR-200V-0-001UF-D478252-/381077387116?pt=LH_DefaultDomain_0&hash=item58b9fb776c)
---The Sprague 192P option
And of course there's the Orange drop option.
From the info I have found from part number searches, it appears that the originals are single layer ceramics, of which I cannot find. I have found some for $18 a piece off ebay that are single layer.... No way I'm paying $36 for two ceramic caps!!!
I wouldn't be expending all this effort and asking all these questions about these if they weren't critical parts.....
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even though the searches for the manufacturers part number turned up info stating that they are single layer type, they physically look like MLCCs. I found a deal on AVX mil-spec molded ones so I bought those - along with all the other MLCCs that I know I need so far. I'm about to place an order for all of the comps that are directly in the signal path from input to vert out and to trigger. That'll be enough for now, and if I need more for the horizontal, I will place an order for those then.
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Composite resistors will almost always drift higher with age, and as you have found out, you need low inductance film resistors ( the standard one is a spiral cut, so you need a more expensive straight cut one) or use SMD chip resistors ( the largest ones you can get in the value) and solder wire leads on them, as they are trimmed using a low inductance L cut. Only usable where power dissipation and voltage rating is not going to be a problem.
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That's nice to know. I didn't realize that there was so much to choosing resistors. I did know that there are application specific types, but didn't really think too much into it. Thanks for the info.
Basically what I am getting is that for the critical circuits (most likely the ones I need to replace, of course) it will be easier to find the best type/build resistor in SMD - and probably less expensive than trying to find thru hole ones like that. Correct?
I've already spent quite a bit on parts and now realize that I could easily be spending over another $100 ( I always buy quite a bit extra of each for later ), so for now I think I am going to stick with manual spec. However I will definitely be researching a bit more into this so I make sure to get the best replacements for any future repairs. Plus I have already ordered a bunch of AB CC resistors. I'll be going thru my 465 after this and sharpening up all the circuits + calibrating, and for that I will DEFINITELY be getting either SMD or straight cut films for the signal path and CRT and any others in need of replacement.
Again - thanks for the info. :-+
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Found this:
http://www.eetimes.com/document.asp?doc_id=1278251 (http://www.eetimes.com/document.asp?doc_id=1278251)
and this:
http://www.electronics-tutorials.ws/resistor/res_1.html (http://www.electronics-tutorials.ws/resistor/res_1.html)
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every single bad axial mlcc appears to have just turned into a crumbly clay type material around the lead that goes to ground. every one thus far has had a lead come off, and every time it's got a lead to ground that is the one that comes off. I'll bet that it's the lead on the negative voltage rail side or lower voltage side on all the other ones.
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I found this listing which might be very useful for not just myself but anyone doing repairs and needing high quality oddball value resistors. The delivery time isn't great but it is compared to other things from hong kong.
http://www.ebay.com/itm/30x-Vishay-Dale-Resistor-1-4W-0-25W-1-Non-inductance-Customerized-Values-/221010258491?pt=LH_DefaultDomain_0&hash=item33753cea3b (http://www.ebay.com/itm/30x-Vishay-Dale-Resistor-1-4W-0-25W-1-Non-inductance-Customerized-Values-/221010258491?pt=LH_DefaultDomain_0&hash=item33753cea3b)
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I'm just going to replace every one of these axial ceramic caps. too many are bad for me to sit here and poke around. I'll be here for a year and I'll end up replacing 90% of them anyway. Fortunately it seems like just about all of the transistors and ICs are functional, so that's a plus. There are some diodes and obviously resistors that are bad as well, but I don't even think that q109 is bad. I'm fairly certain that the noise around it is being caused by bad stressed out off value resistors and bad capacitors. More specifically that 47uf 35V 30D type that I haven't replaced yet....
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Composite resistors will almost always drift higher with age, and as you have found out, you need low inductance film resistors ( the standard one is a spiral cut, so you need a more expensive straight cut one) or use SMD chip resistors ( the largest ones you can get in the value) and solder wire leads on them, as they are trimmed using a low inductance L cut. Only usable where power dissipation and voltage rating is not going to be a problem.
Well since I am replacing all of the caps - and just bought probably 1500 or more NP0 and X7R MLCC's, I might as well buy the good kind of resistors for at least the first stages of the input signal-- where it will really count. What type would likely be best? Maybe the Ohmite non-inductive type? for the first 4 or so that aren't board mounted, I don't want to go with chip resistors and have all that extra lead and solder joints like that. for the rest, I am going to end up getting a big assortment of the biggest SMD resistors I can find.
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A bit off topic, but check out this bad capacitor in this Tek Type 184 Time-Mark Gen I got for cheap...
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I'm a bit confused here........
On several of these MLCC's the schematic says one value but the parts list says another. For example: A11 Diagram 3 Section 2-D, the schematic says ".01uf" and the parts list says "0.1uf". The installed part is a .01uf, so then that makes me believe that the schematics are correct and the parts list is filled with errors (since there are at least 8 0.01uf caps that the parts list says are 0.1uf caps).
My thoughts are to replace any/all parts that are going to be replaced with the values that were previously installed even if the parts list - or possibly even the schematic - says otherwise. Unless there is some obvious discrepancy where I can tell that someone just "made do" with a close value or something, and in that scenario I will use what the schematic specifies.
Does anyone disagree with this?
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I have a 184 with the same bad capacitor. It is not obvious at all from the schematic what would fry it like that.
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Have a hunt for other SM versions, just in case those errors have been corrected. :-//
I'd also assess where/how the 103/104 MLCC's are used, coupling/de-coupling etc......just for a sanity check.
When you think you have it sorted, make a list of changes(if any) for future reference. :-+
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If I assume that the parts on the boards are all correct, then there are errors in both the schematics and the parts list. Some are just incorrect in the parts list, some are just incorrect on the schematics, and some on both.
However, on the trigger board (where I am working now), C134, C330, and C333 have 0.01uf caps installed. My scope is an Option 5, and on the schematic it has 0.1uf marked for those, and the parts list also has 0.1uf marked. They are all decoupling caps. My guess is that whoever did the last change-out of all of these caps (probably sometime back in the 80's) they used a manual that was not for the Option 5 model, and installed 0.01uf instead of 0.1uf. Since both the parts list and manual have 0.1uf specified, I am going to switch them to that value. I think it's highly unlikely that both the parts list and schematics are both going to be wrong.
For any such future discrepancies like this, if both the list and schematic have the same value marked and it is different from the installed part, I am going to change the value. If I cannot decide, I will refer to other revisions of the manual. As far as I know, the schematics I am using are the most recent. They are a "MAR 1981 Revision" set.
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I have a 184 with the same bad capacitor. It is not obvious at all from the schematic what would fry it like that.
If you repair yours before I repair mine, please let me know if there are any other bad parts on the 1Sec trigger selector circuit. I always thoroughly clean any piece of gear before I do anything else, so I am in the process of doing that right now. (during breaks from replacing capacitors)
I don't think mine has had any repairs done to it for the last 3 decades, or even used for that matter. The oxidation on the switch contacts didn't even have any scratches in it from being pressed, so I'm a bit scared to power it on without checking everything first. I'd like a variac for this but a dim bulb tester will have to do.
Does yours also have the Bulova 10MHZ ocxo in it?
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It seems like all the manuals I can find online are the have the same revision schematics in them. I think what I will do is just take note of the ones I have changed the values of in case I need to change them back.
I'm really not sure why they are different values from what is stated in the schematics and parts list. maybe it was a bit of a tweak by Tek service guys? I can't see it being a "I didn't have the right part" fix since it's more than just one or two on a board, and it seems to be specific sections that have been changed. for instance, the ones on the trigger board that I mentioned before - those are specifically on the "Option 5 trigger switches and trigger pickoff" schematic that specifies the changes/additions to the circuits. Only those had different values installed.
Maybe I'll just stick with the installed values. :-// I can't decide. If it was a tweak by Tek, I want to keep it that way. If some doof did it on accident, I want to change it back. .... who the heck knows.
Oh yeah, I want to add that 80% of the MLCCs that I've pulled thus far have fallen apart at the lead on one end, so I think I've made the right decision. I'm sinking way more money into this scope than I could ever hope to get back, but it's worth it for the knowledge - and I'll have a perfect 468 for decades to come!!! :-+
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If they're decoupling just install 104's and be done with it.
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Gotcha. will do. Thanks
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Even this 184 appears to have some corrosion buildup on the neg side of that same capacitor. Obviously it's in much better condition than mine is, but I guess that one has been used a lot less?
http://www.ebay.com/itm/Tektronix-184-Time-Mark-Generator-Great-Condition-/251978155600?pt=LH_DefaultDomain_0&hash=item3aab118650 (http://www.ebay.com/itm/Tektronix-184-Time-Mark-Generator-Great-Condition-/251978155600?pt=LH_DefaultDomain_0&hash=item3aab118650)
The third picture is the one I am referring to.
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I'm inclined to think that damage is from aging/corrosion failure. There's always a time distribution of failures, so perhaps that's one of the lucky ones.
The inside of test equipment is more related to the environment it's been sitting in than how much it's used. If you have a lot of fine dust in the air, it will filter through any holes over time.
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some boards have those glass capacitors that look like SMD caps that are formed into a glass package. Anyone have any thoughts on whether I should replace or leave them?
Basically what I'm interested in finding out is if those are a lot more reliable than these MLCC caps I am installing.
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I think I may have figured out the trick to finding out what part is correct..... It appears that all of the actual part numbers in the list are correct, even though the descriptions and/or schematic may have the wrong values. Most of the time the schematic is correct, though. It's the descriptions of the parts in the list that are off.
For example, on A-29 (TV sync separator) C401 is .01uf on the schematic. That is the installed part value. The parts list description for that part is "..... 0.1UF, 20%, 50V" but the part number is "MA201C103KAA" - stating that the part is a 103, or 0.01uf.
I went thru and checked the same thing for caps on boards I've already done, and it's the same there as well. So for future reference when repairing an old Tek scope, check the part number to verify the value, not the description!
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For example, on A-29 (TV sync separator) C401 is .01uf on the schematic. That is the installed part value. The parts list description for that part is "..... 0.1UF, 20%, 50V" but the part number is "MA201C103KAA" - stating that the part is a 103, or 0.01uf.
That seems to confirm a SM error.
Not uncommon. :palm:
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well, I'm glad I found the "MANUAL CHANGE INFORMATION" In the back of SM Vol2!!!!! Especially since the parts lists and schematic haven't been revised with the changes!!! Things could have gotten a little ugly if I had changed the values back to the old and actually powered it up without knowing any better... I don't even know if my scopes been updated with those changes or not, though, so I guess I have some checking to do. It'll be easy since it's entirely in pieces....
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Well, I'm thoroughly confused. My 468 has a Heerenveen Holland part number, but it doesn't have the parts to back that up. The board model numbers don't match up, and some of them have part numbers that aren't even in the manual!! :wtf:
What the hell is going on here?!? I think I picked the wrong scope as a first rebuild! hahaha Oh well, more to learn... I'll get it back to mint one way or another!!
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This scope has not been updated according to the Change Information that is at the end of the Volume 2 manual. Do you think that I should perform the changes/additions after I have gotten it running properly again? I mean those changes must have been implemented for a reason, right?
According to the model number of my scope and the board model numbers, most of the changes do apply to my scope.
I'll be getting the rest of the caps and resistors I need tomorrow so I will have it back together and running shortly after that so I can figure out what else I need to get. There are still a few caps I want to replace that I haven't ordered yet, but they are easily accessible ones (CRT and Horizontal amp circuit). In my mission to replace all of the small ceramics, I found quite a number of them that were cracked, and several that were very easily broken off. Most of the 50V ones have been replaced with 100V mil-spec ones, so they should last quite some time.
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One would have to consider why Tek made the changes first. Reliability? Availability? Better specs?
Or general improvement in the scopes operation/stability?
Many part these days are better quality than those from back then, but wholesale changes especially in the signal path are to be avoided IMHO. Full and precise calibration of a scope is not a simple task, even less so due to the precision equipment required to do the job properly.
Have a good look at the chapter on calibration and see how much of the equipment required you have or have access to.
Sure the Cal will have drifted due to the scope's age and repairs you have made but there are good basic checks that can be done to ensure reasonable accuracy.
Dave did a good vid on basic checks of a Hameg, #502
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I've been trying to figure out why those changes were made. I don't really know where to look or who to ask specifically, but I'm sure I will find someone who knows or find the info at some point. Considering the changes I am most concerned with - The zener change directly at the input - my guess is that it is a operation/stability type change. It's definitely not availability though.
The 6.2V zener off of the +15V to Q285 gets replaced with a 270ohm resistor, and an 8.2V zener gets installed parallel to C186 with cathode to Q285. Anode and one side of C186 go to ground. It sounds to me like that type of a change would affect the performance & stability. Maybe they initially had not done long term testing on Q285 and decided a change in the input voltage would enhance performance? I don't really know - I haven't gotten much into those aspects of circuit performance and semiconductor operating performance parameters yet.
The only reason I am really going at it with the signal path parts is because the waveforms and voltages in the signal path are/were way off. at some points almost 25% off spec. Otherwise I would have avoided replacing all of these capacitors and these few resistors. My thoughts are that if I know that the freshly installed parts are good, I will have a better basis to work from for fixing the issues it has. Every few minutes I was finding a bad cap, and it just seemed more logical to me to replace them all since they are all quite old, the scope has been used a heck of a lot, and they aren't all that expensive. (Plus I needed to get some for other projects anyway.)
I have been collecting the cal gear and have most of it so far. only things I lack are the 106 square wave gen, RC normalizer, and 10X attenuators. other than that I'm good to go. (aside from the couple caps in the time mark gen that need to be replaced). I think my Wavetek 166 at about 5ns risetime will do for the time being. I can also make a few circuits with fixed frequencies that have about <2ns risetime to really finish off the vertical checks/adjustments. First I'm going to buy a $50 specific voltage standard box to get a general check on the cal gear and build up a quick board for my STP2145A OCXO so I have a second freq standard to check against the Bulova in the TMG.
But I'm not quite ready for the cal work just yet anyway. I still need to finish troubleshooting and fixing this 468 and then I will cal my Wavetek 166, check the cal gear against other gear at the college, and then get on to calibrating this 468 and my 465.
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Changing out all of those caps made a HUGE difference in the PSU loading. As expected, the waveforms are still off from something which I think may be in the trigger or vertical section involving the NORM trigger source, but other than that it was worth it to replace them all. The transfo is getting just mildly warm as it should now. Not so hot that I could fry an egg in 30 seconds!! ha
In NORM trigger mode, it still gets a trigger when I have NORM source selected. It also has what looks to be a 60 or possibly 120hz "wobble" to it in that source setting. This only happens with 1ms or longer selected in the timebase. With any other trigger source setting selected I get a perfectly stable horizontal line when it should be there, and it's not present when it should not be - in any timebase selection. I think this might be my final issue to fix - all others seem to be fixed. :-+
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You have to remember this equipment might be 30+ years old now and if it's spent a lot of it's time turned on, most components will be well past their MTBF.
Parts that were readily attainable back then are not so easy to source now, at least with Tek there is some NOS available, probably more so than any other manufacturer.
So one should be prepared to substitute after careful cross referencing.
For your zener a post or 2 back, these were the early days of zeners, noise or/and reliability might have been reasons for change.
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Fortunately there are a few databases specifically for semiconductor parts cross referencing, so that isn't a problem for the most part. Of course there are a few IC's and custom transistors that can't be substituted.
I'm attaching a pic of the area affected by the zener mod so you get a better idea of what it says to do.
It is saying to replace VR187 with a 270ohm resistor and install an 8.2V zener parallel to C186 with cathode to Q285. As far as I understand, that will change the voltage level reaching Q285, thus adding some power to the input signal going back into the other circuits. I don't think it's a reliability thing, but I could be wrong of course. Let me know if you think this is unnecessary or worth doing. I guess I could always try it later on and see what affect it has..?
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Being early in the signal path, it may change vertical amplitude, however a resistor/zener configuration is the common method used, additional stability should result. Note the DC +8.2V on pin 6. ;)
As your Cal is way off, definately do the mod and your new cal will null any changes the mod makes.
Make a note/list of mods you do.
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gotcha. sounds like a plan. Now to order some 8.2V 1/2w zeners (can't find 400mw unless I order from digi or mouser... NXP china junk - blehhhhh. MOTOROLA for me!)
I believe I know where that "60-120hz like" wobble is coming from.. I looked on the timebase diagram for any setting that is 1ms and slower, and...well... it's the 1.5uf 10V 150D that I didn't replace. DOH!!! :palm: Of course the one and only PITA-to-access tantalum that I forgot to order!!
The triggering issue.... the waveforms and voltages at the vertical outputs to trigger are quite a bit off and have some noise, so I think that is where the trouble may lie. Of course CH1 is way worse than CH2 due to usage and all, so it's time to finally track that down now that I can get a clean signal into the circuit. YAY!
One other thing I noticed is that in the parts list - for part 151-0341-00 (2N3565) it specifies "TO-106" and in my scope there are all crappy ol PN3565 TO-92 package transistors. I have 100 or so TO-106 Signetics 2N3565's..... I feel inclined to replace them! They may not change a whole lot, but at least they will be correct...
I used this for the cross referencing.
http://www.reprise.com/host/tektronix/reference/transistor.asp (http://www.reprise.com/host/tektronix/reference/transistor.asp)
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One other thing I noticed is that in the parts list - for part 151-0341-00 (2N3565) it specifies "TO-106" and in my scope there are all crappy ol PN3565 TO-92 package transistors. I have 100 or so TO-106 Signetics 2N3565's..... I feel inclined to replace them! They may not change a whole lot, but at least they will be correct...
If they are used as pairs be sure to attempt to match Hfe, any old DMM with a transistor tester will give you indicitive gain readings for a "close enough" match.
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it doesn't say anything about matching them and I'm checking their Hfe as I take them out to be sure. thus far their Hfe's are wildly different so I don't think it matters. for each channel there is a pair around the "position" op-amp and one used w/ voltage divider to put 15V into the signal. (I'm sure there's a term for this setup but I don't know what it is .. yet.)
I'm going to match the pairs and match them all from channel to channel anyway. Might as well since I can.
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I'm going to match the pairs and match them all from channel to channel anyway. Might as well since I can.
:-+
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one of the delay line leads decided it was ready to fall off so I had to spend the last hour redoing that whole deal. Now my delay line is 5mm shorter! AHHH. hope that doesn't matter.... cuz I'm not replacing it!!
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Anyone happen to know if this model/type of resistor is worthy of signal path? I'm trying to stay thru hole but go more reliable than carbon comp.
http://www.ebay.com/itm/231374168010?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT (http://www.ebay.com/itm/231374168010?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT)
Datasheet:
http://www.vishay.com/docs/31027/cmfmil.pdf (http://www.vishay.com/docs/31027/cmfmil.pdf)
Doesn't say anything about being non inductive or low inductance, but considering that they are MIL spec Vishay/Dale one would think that they are going to be straight cut high end instead of the spiral cut type. I know I can get 1000 for 100 instead of paying $1 each then sell some for my money back. If I can find out if these are worthy for scope/gear repair, I may start ordering different values here and there as I need.
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That they do not tout inductance says they are spiral cut, so are inductive. If they were slot cut they would proudly advertise capacitance and inductance value maxima and typical values for them.
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ah ok... so it would seem that I am more or less limited to carbon comp or SMD resistors unless I want to go into the 1W+ area to get the metal film type I would want.
Heh... interestingly enough changing out those 2N3565 transistors has almost completely eliminated that "60-120hz - like wobble" that I had. Glad I did it!!! I wonder what other transistors I could benefit from changing out..... I will be keeping a close eye for transistors that have been substituted in place of correct replacements so I can continue bringing this scope back into spec/original condition. :-+ Another well-worth-the-effort swap!
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ah ok... so it would seem that I am more or less limited to carbon comp or SMD resistors unless I want to go into the 1W+ area to get the metal film type I would want.
Heh... interestingly enough changing out those 2N3565 transistors has almost completely eliminated that "60-120hz - like wobble" that I had. Glad I did it!!! I wonder what other transistors I could benefit from changing out..... I will be keeping a close eye for transistors that have been substituted in place of correct replacements so I can continue bringing this scope back into spec/original condition. :-+ Another well-worth-the-effort swap!
Do you think that the problem was fixed by the new transistors or by the fact that you matched the gain of the new transistors.
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I was actually misled by my setup. I had them both in the GND input coupling setting, and it doesn't appear there at all. It's only in AC or DC input coupling, and I'm fairly certain it's caused by the 1.5uf 10V 150D capacitor on the timebase board that I didn't replace yet. As I mentioned before, it only happens with 1ms or slower on the timebase, and that cap is involved with all settings 1ms and lower. So once I replace that it should go away. I have to order them and play the wait for USPS game and then I will know for sure.
Now that I've checked a bit more, though, it appears changing those transistors out and matching them up has improved the waveforms and voltages throughout the signal path. I assume that any new transistors in this old scope is going to make improvements anyway though. Unless I have to, or they are not the proper replacements, I won't be doing all that! haha that would be a bit.... expensive.
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Other than the hot rectifiers it seems like everything is almost wrapped up. I have that one 1.5uf cap to deal with and some 110 unreg loading from the CRT and then I think everything else is just in the adjustments.
I just did the CRT grid bias adjustment and that cleared up the wide, fuzzy trace issue I was having. That made me think.. once I change that cap and fix the 110 unreg loading, maybe it would be best to go thru the calibration procedures to tighten everything up. Then I think it would be easier to find any remaining problems it might have since all of the waveforms and voltages in the schematics are based on a fully calibrated scope. That should help avoid any more unnecessary part replacements.
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I think that the booger in the CRT circuit is Q109. I've checked all the components around it and even changed the all the transistors around it to mil spec. That Q109 "buzzing" and CRT flickering I had when I touched Q530 is gone since I changed out both Q530 and Q533. However the voltages surrounding Q109 are still way off and unstable.
The voltages surrounding Q109 (2N3053) are supposed to be: -0.6V on Collector, -3.3V on Base, and -4.5V on Emitter.
My measurements: -0.19V Unstable on Collector, -3.8V on Base, and -4.4 on Emitter.
I've changed CR104 and it helped a tiny bit. The only other thing I can do is change CR110, and I don't think that will make any difference but I may do it anyway.
When I put my finger on the can of Q109, the voltage on the Collector climbs closer to normal, but it does not go past -0.35V. It also helps to stabilize it a little bit.
I checked it with the diode tester and Hfe tester, and it appears to be OK, but that still doesn't tell me that it's definitely good obviously.
The high voltage side of the transformer is within spec and stable. It reads -2414V.
Q1108 (2n3055) also may be on it's way out. It's collector reads 3.4V low with a small amount of instability, and it's base is tied into TP112. The Emitter to ground is reading 0.9mV while everything else in the vicinity that hits chassis ground is reading less. C108 seems to be good according to it's ESR and value, but I plan to replace it anyway. I have some ST 2N3055's on hand if I decide to replace Q1108. It's kind of a PITA to get out without taking the boards off again, though.
What are your thoughts on all this? Q109 & Q1108 to be replaced?
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Hmm.
I'd be suspicious of the high value resistors although they are not under the same stresses as those on the secondary side, they probably have drifted some from new values.
Q1108 is under the most stress in this part of the circuit, second will be the E caps associated. ;)
Zeners possibly. :-//
Have you setup as specified to check those DC voltages, they won't match unless you do.
BTW snippets of the schematic as you do make help easy. :-+
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I"ve been checking all the resistors as I replace parts and also in circuit where I can, and it seems that most of them are close enough. The 22Meg that I thought was way off is actually 22.5Meg, which is def in spec (31.1Meg max I believe).
I've replaced all of the HV caps except for the .0047 6kv and the .0012 4kv to ground at the focus pot. It focuses nicely and, since it's to ground and the voltages there are correct, I didn't feel the need to replace it. I do have another one to do so if I decide I want to though. I had previously stuck in one of the big fat ceramic 0047 6kv caps temporarily to check and see if the original was bad, and it's not.
I want to replace Q1108 anyway even if it isn't bad, but all I have on hand are the ST 2N3055's from China which I don't feel too confident about using. I don't know if they are genuine (don't say "MALAYSIA" under "ST" logo) but I know they worked fine in some other circuits. Can't decide if I want to risk it temporarily or not...... I'm going to order some MOTO or RCA ones.
The 75V and 160V zeners are on the other side of the transfo near the CRT bias, and it adjusted perfectly. the voltages around there are good, except the waveform at TP86) is "doubled". The waveform at TP86 with CRT blank is always displayed in addition to the Unblank waveform (which is the correct waveform). (I will add pic of this scopeshot shortly)
--The scopeshot below is different and is what it looked like before I replaced C318, and the larger amplitude WF shown with the correct WF is the waveform on the other side of C210 (which I ordered a replacement for). I figured I would mention this possible issue as well in case you thought it might be related to the issues on the primary side of the HV transformer. I don't think it is, but as always I could be wrong. Also could be wrong about the cause as well, and maybe it's CR323 by C318. I hope not though, because those diodes are very hard to find and expensive!
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Oh, and the waveform and voltages at TP84 are correct. Actually, all of the waveforms except for TP86 are perfect. I am using the waveforms and voltages from the 465 manual and 468 manual.
As one might guess, there are no waveforms specified in either manual for the primary side of the transformer, which frustrates me. Either way I believe that problem may be narrowed down to either (or both) of those two transistors ( Q109 or Q1108) unless there is something I am missing entirely here.
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I need to start getting my cal gear into working order.. the calibration procedure jumps directly into the time mark gen with the CRT y axis and geometry adjustments. Just a quick off topic question...
Does anyone have a Tektronix 067-0502-01 or -00 model amplitude calibrator? Mine's Ne2 Neon on the underside is nearly black, and I would like to know if this is to be expected over the life of these units, or if there is likely something in there in need of repair. My unit works, but I'm just not sure if that means there is some funkyness going on in the power supply.
I also need to find the value of the dead cap in my 184, but I can manage that from the manual.
But anyway, back to the 485 - I ordered the 2n3055 and 2n3053. Both RCA. :-+ Even if one of them is still good, I want new in there since they are constantly under a good bit of strain. After replacing those and the 47uf cap, the entire primary side of the HV transformer will have new semiconductors.
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TP86 WF would worry me, it's not jitter, there are 2 distinctly different waveforms. :-//
Waveform on the primary around Q1108 are generally a sinewave @ 20-40 KHz and amplitude close to the supply voltage of 15 V.
Have you seen Dave's vid of cal :-/O he did on a Hameg:
http://www.eevblog.com/2013/08/03/eevblog-502-19-hameg-analog-oscilloscope/ (http://www.eevblog.com/2013/08/03/eevblog-502-19-hameg-analog-oscilloscope/)
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Yep I saw Dave's Hameg cal video and also W2AEW's Heath cal vid. Dave's is more of a quick adjustment. W2AEW's is an actual cal but on a much, much less complex scope. Both are/will be quite helpful, and it's probably a good idea to just go thru and do some quick adjustments to this scope once this CRT issue is solved and get it a little bit tighter to spec to aid in the rest of the checking/troubleshooting. I still don't have a ceramic screwdriver yet.... :palm:
yeah, everything in the HV area is and should be running at 50khz. First, before replacing C318, that second waveform was as pictured.. take a look at what it is now...
Sorry bout the somewhat fuzzyness. the camera didn't want to focus.
The 2nd waveform is different now. It's the same as the correct waveform, but with the unblank amplitude. The correct waveform changes with the intensity setting as it should, but the second one stays the same no matter what I adjust. Before, in the other pictures, that second waveform acted the same way, but it looked exactly like the waveform where "300V p-p" is on the schematic. That is what lead me to believe it was the 47pf cap, but I have no way of knowing without just replacing it since I don't have a 100x scope probe to check around in the HV side. All I can do is poke around with the 80K-6, and I'm not about to hook that into my scope! ha.
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Ok so there's a little more going on than I thought. I'm fairly certain that Q1108 is loading the 15 unreg and pulling down the emitter of Q109, and Q109 is pulling down the 110 unreg. Q1108 probably aided in killing Q109.
I decided to start checking supply foldover again, and all of the supplies are folded. Disconnecting the vertical preamp board eliminates ALL of the over-current issues. The rectifiers run a bit warm still, but nowhere near as hot. So now I also have that to track down and fix. From what I remember about the waveforms when I first started, it's going to be in the switching logic.
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I've definitely isolated the majority of the loading (besides the 110/55 from the CRT) to the regulated supplies to the vertical preamp. I pulled up the jumpers for the 5V and -8V to the switching logic, and nothing changed. Isolated the Vertical Output Amp, no change... Vertical Mode Board... no change... I also isolated the 5, 15, and -8V on the trigger board as well (in case it was related to the signal interconnection) and nothing changed. Nothing got any worse when I changed all the 2N3565's. I had overlooked some negative offset in several waveforms across channel 1 before, so I am going thru the schematics before I start testing again.
My initial observations regarding what to look for:
1) The one power supply rail that is capable of loading down all of the others is the 5V rail, and it's pass transistor's Vbe is the highest at 1.34V. Something directly off of the 5V rail is likely what I should look for.
2)The Vbe of all of the other pass transistors is almost the same at about 0.62V to 0.64V, except for the 110reg which is close to folding. It's Vbe is 0.54V. From what I understand, these transistors should all have a Vbe in the low mV - such as 3.6mV, which is the value when the Vert Preamp board is isolated.
3) Previously (before parts swaps) the worst part of the channel seemed to be closest to the output signal to trigger. I think I should work my way back from there.
Now I have something else to fix while I wait for the CRT replacement parts. YAY! ::)
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now that I've had some time to think, maybe this problem is why the modifications were added.... the majority of them are to the vertical preamp board.. I need to do the mods anyway. I found and bought the exact specified zeners, so I'm going to go order the resistors now. the other mods don't sound as important, so I will deal with those a little later. There's also a full page of potentiometers that get changed as well........ I really hope that those have already been dealt with!!!!!
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did old school zeners like the original ones for these tek scopes have a diode drop around .6-.7 like a regular diode??
Im testing out the trigger board, and there is a +8V rail that gets split off the +15 via a 6.2V zener. Thus giving me 8.6V due to the .2V drop of a modern zener. this +8V is used to feed the Fet pairs and the trigger level pots, and I am wondering if I should find a diode that will get me much closer to an actual 8V or not. (I'm having some issues with the trigger circuitry, obviously.)
The original zener was only clipping off 5.9V so I installed an actual 6.2 on point zener. It seemed to make a very little difference in the waveforms and voltages.
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well, only 1 out of 4 of the trigger FETs are good. I've been playing with them a bit, and I am able to turn them off and on with my finger, and they also turn themselves on sometimes. When they are turned on, the voltage drop across Source and Drain starts at almost 2V and steadily drops to about 0.2V and stays. Sometimes the V drop will climb a few mV and then drop a few - back and forth. Only the 1 JFET on the B trigger seems to function "properly".
I'm pretty sure that these aren't supposed to function this way if I am reading the datasheet correctly.. Their part number is 151-1042-00 and generic part number is 2N5454. I found some for pretty cheap so I bought 4 pairs. If I happen to be wrong and these are OK, at least I'll have some of these anyway since quite a few scopes use them.
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I found the bastard that was dumping SMPS frequencies into the 5V rail on the vertical board. Good Ol' 10 cent 2N3906 being used as an amplifier. It's emitter was leaking the chop freq back into the 5V. Of course the DMM reads it as good still, even with the resistance measurements. I really need a real transistor tester.....
That output goes directly into the CRT circuit and is at the same frequency as the "double waveform" I was seeing at TP86, so I have a feeling I am about to solve that issue. I remember saying " the second waveform is the CRT blanking waveform, and it stays there when the CRT unblanks. " I just didn't put it together that CRT blanking is CHOPPED blanking! (sad, because it's right there on the schematic!!!!!)
Now that I know what I am looking at and looking for, I should be able to identify more leaky junction transistors that have decided it's time to break down. :box:
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I think I may have found a way to tell when a diode is leaky using only a dmm. While I was messing around testing some different NPN and PNP ones with hfe, diode, and ohms settings, I noticed that when I have it in Ohms connected across a junction, the resistance increases when the transistor is cooled and decreases when it's warmed up. I noticed that the resistance did not drop when my leads were connected across the base/emitter. It stayed almost completely stable while I held it between my fingers and when I blew on it. I guess I could have gone a step further and did some tests with freeze spray, but that would be a waste.
I have no idea how valid the results of this test would be on other leaky junction transistors, but I thought it might be worth sharing my observations in case it might help someone else out who doesn't have any other way to test transistors either.
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I found the bastard that was dumping SMPS frequencies into the 5V rail on the vertical board. Good Ol' 10 cent 2N3906 being used as an amplifier. It's emitter was leaking the chop freq back into the 5V.
Thats when you wonder if the 10M resistor has been doing it's job. :-//
What is the voltage past the 1.2K ? In excess of 2N3906's capabilities?
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I'll do some more testing and measurements in the switching logic circuit in the morning and get back to you on that. it's already 2AM... time flies when you're havin fun!
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Finally just got back to testing this thing, and right off the bat I believe I may have found another rotten apple. Switching Logic as well, but it seems to be originating from the 74LS08N - U508.
It is on the 5V input at pin 14. It's also on all of the 5V inputs, but the magnitude of the oscillations are much, much higher on U508 and U409. (Their V inputs are tied together: U508 Pin1 & U409 Pin 1&16) The amplitude is a few mV higher at U508 pin 14. The main frequency is 125Khz @ 60mV p-p. As shown, there is also some 3mV p-p 250Khz.
Also, the 5V rail is lowest at U508 Pin 1. I believe this to be the lowest point on the entire vertical board.
Picture 1: 20mV/DIV @ 1uS/DIV.
Picture 2: 10mV/DIV @ 1uS/DIV & B timebase @ 100nS/DIV.
468 setup:
CH1, CH2, CHOP
20 mV/DIV
A HORIZONTAL
.5 mS/DIV
AUTO, AC, NORM Trigger
No Input signal. CH1 & CH2 separated by 2 DIV.
Also CH2 has some small oscillations riding it, making it a bit bigger (was not able to adjust out w/ HF & LF reject pots/caps). Not too worried about that right now though.
Since these IC's are <$1 each from Digikey, I may as well replace all 3.
It's starting to make some sense now why I had to disconnect the entire vertical section to unload the PSU rails...
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Just check the logic levels are as expected in case some erroneous levels are causing the additional current draw. :-//
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there's definitely some variance in the High voltages and low voltages, and some of the waveforms are off (acc to the manual's specified waveforms). What worries me is that U408 gets fairly hot when the scope is on long enough. I'm really hoping that has something to do with U409/U508/U509 and there's not something wrong with it. I found one just in case, but it's $40. I'm really wishing that I would have bought that other $100 468 so I could make 1 out of 2.....
I could have bought a fully working 468 for what I've spent, but then I wouldn't have a pile of extra new parts, the experience & gained knowledge... and of course the fun of figuring this all out and fixing it.
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I could have bought a fully working 468 for what I've spent, but then I wouldn't have a pile of extra new parts, the experience & gained knowledge... and of course the fun of figuring this all out and fixing it.
Or more importantly, you won't find one in as good condition and as reliable as when you have it finished.
BER is the term commonly used, but who gives a shit when the knowledge gained and gratefully shared is so valuable. :-+
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Or more importantly, you won't find one in as good condition and as reliable as when you have it finished.
That's definitely true. Is BER an acronym for something? Can't think what it would be..
I don't know if/how the 3rd unit of the 74LS175 (U409) is terminated, but the voltages on those pins are allllll over the place!!
U408 seems to be good still - possibly more resilient since it's a Cerdip and made by TEK, not Signetics like the others. I'm pretty sure U409 is bad though. the 3 digital channels between U408 & U409 have erratically varying voltages acc to my Fluke 87III, and D2 channel has noise on it. What makes me think that it's not U408 is that the voltages are much more stable on it's digital pins, and a bit higher as well. Also less noise.
There's quite a bit of noise/variance on U508D, which I have no way of telling if it's that unit itself or the outputs of U409.
There's also quite a bit of noise/varying voltages on various pins throughout the chopped clock, and at the input to the first NAND gates of the chopped clock - the ALT/CHOP Select from the vertical mode switch. This makes me think that not only are U508 & U509 bad, but one or more ICs in the mode switch circuit are likely bad. Fortunately I left the shield off of it so I could poke around in there while it's running.
Another thing is that the external triggering and trigger view circuits don't work. I'm pretty sure I've already got that sorted for the most part though - just waiting on parts. I'll add to that info once I have them. For now I'm going to try and confirm my suspicions about the vert mode board and chopped clock and see where I land.
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Or more importantly, you won't find one in as good condition and as reliable as when you have it finished.
That's definitely true. Is BER an acronym for something? Can't think what it would be..
Beyond Economic Repair.
There's also quite a bit of noise/varying voltages on various pins throughout the chopped clock, and at the input to the first NAND gates of the chopped clock - the ALT/CHOP Select from the vertical mode switch. This makes me think that not only are U508 & U509 bad, but one or more ICs in the mode switch circuit are likely bad. Fortunately I left the shield off of it so I could poke around in there while it's running.
Decoupling caps?
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no, can't be decoupling caps. I've replaced almost every single one. there's 2 51pf and a 47pf 100V left in the entire vertical section that I haven't replaced, and they are glass encapsulated. I checked them, but my meter cant accurately measure anything under 100pf. All of the ones in the logic circuit and mode switch are new.
BUTTTTTTT..... I was just reading a little into the new 87III thread and saw your post about the cracked traces/components and decided to look over it all again. I found one cracked resistor in CH1 so far. It was really hard to spot with 2.5X mag. It's a really, really small crack and it's diagonal off of one corner. It wasn't bad enough for me to find with the pencil-eraser-pushing test, but probably bad enough to make some noise.... so I'm going to keep looking. I took a new xacto blade and gently ran the back edge of the tip over the crack to confirm it was really a crack and not just paint. it definitely catches on it. I'm sure I'll be finding more cracks around. hope I can find the values I'm going to need!!!
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I was just reading a little into the new 87III thread and saw your post about the cracked traces/components and decided to look over it all again.
Wise IMHO.
I'll spend some real time inspecting a PCB, get a comfy chair and some magnification and you'll be surprised what you can find, partly dry and cracked joints, solder balls, cracked or improperly soldered component mountings, cracked joints on headers(not uncommon) etc.
Often identified deficiencies don't matter much in the scheme of things, it's just good to tidy things up and know they're right.
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I'm definitely going to go over it again. 2.5X mag will have to do. I wish I had 5X.
I don't think that resistor had and effect on CH1's performance. The crack seems to be more of a superficial thing. I did turn it into a chip, though.... it's going to get replaced anyway, but I'm reinstalling it for now.
HOWEVER............. I did find a real cracked resistor right there, SMACK next to the position op-amp that I replaced!!!! bad part about it is that I had already pulled up one of it's leads to work around it and never even noticed it! I bet this is the oscillating chump that is causing all the fuzziness of my CH2 trace! of course it's an 8.06K 1%!! haha at least it's film.... I'll just buy a bunch of some value close to it and measure them until I find a match... unless I can get a few for cheap off Digi or maybe ebay....
I'd like to try and put together as much of a comprehensive list as possible by tomorrow night so I can get the majority of my parts early next week. Of course there will be a few parts here and there left to get. I've already been searching for and buying piles of the heavily used transistors in these scopes just so I will have them.
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This is what the display looks like on the 468 with 1Mhz 20mV positive going square wave input. The output of the Wavetek 145 is clean acc to my 465. Adjustments don't do much, and it's there no matter what vertical setting I use or whether I use A or B triggering. I just noticed it and thought I'd post a couple pics before I start looking for the source.
I should have checked this thing out more thoroughly than I did, but hey.... always learning. :-+
Anyone have the 'light bulb' going off? have a good idea of what's going on hither?
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raising the input frequency made the issues in the vertical section much, much more visible and easy to pick out. I'm sure there's a few, but now I am 100% certain that the oscillations I saw on the Vcc pin of U508 are the cause of a large number of problems on the vertical board. The oscillations are the exact same frequency as the oscillations measured directly before the amplifier stage (?) before the delay line. I was able to match up every detail using points on the screen of my 465, and if I had a spectrum analyzer I'm certain that they would match up on that as well.
Since this noise has flooded the 5V rail on this board, I am going to replace U509 & U409 and pray that U408 hasn't been damaged. If it has, I'll be searching for a quick parts scope! Oh, and I'll certainly be flush-mount socketing any IC's that I have to replace. I'm a bit peeved that they took out almost all of the sockets on the 468 model to 'improve reliability'. I can understand the reasoning, but it's a PITA for the troubleshooter!
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It helps a lot that I am actually starting to understand how transistors/diodes function and how their characteristics are supposed to look. I remember when I would look at a datasheet and be completely clueless as to what any of it means.... It's been a big weekend. hah
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Wondering out loud.......snubbers used to be added around zeners to reduce noise, just a thought....
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not sure I understand what you're getting at.... I have no idea what a snubber would be. I'll look it up... but there are no measurable oscillations coming from any of the zeners on the vert board. That's not saying anything about the vert amp, which I already know is at least half of the problem with the trace I took pics of. I'm about to poke around there right now, actually.
Also, there is cross-talk between channels. Presumably coming from the 5V rail and the logic section, but I'm not certain. It's also measurable on the 15V and -8V rails as well, but it's worst on the 5V near the logic stuff.
I also know that the thermistor for CH1 (100kohm 4mW) is bad. figured this out by making use of the CH2 signal bleeding into the rails. I was able to measure the little bit of 1mhz signal coming thru it and noticed that the trace on the 465 floats up and down (ac coupled). I already replaced the one on CH2 (should have ordered 2 but they were $5 each for the good ones I picked..) I tried to get the closest dissipation that I could, (5.4mW) and that's what the price ended up being. and I could only find it at Mouser.... horray for more shipping charges..
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ohhhhh boy... this pup has some ills in the logic, that's for sure. nothing on the mode switchboard so far is a clean looking signal. there's logic signal's piled on top of each other and offset on several pins of several chips. also trigger view enable doesn't work at all - and it's not in the discrete components. this should be quite an interesting continuation of the troubleshooting considering I haven't even begun to teach myself how logic circuits work. all I know thus far is what a clean signal looks like (and obviously it's inverse), and what high voltages and low voltages are. I don't know what frequency to look for, how NAND/AND gates work, etc etc etc.
anyone care to point me towards some tutorials that are fairly simplistic, yet thorough enough to understand? I'd like to try and weed out the videos/pages that are useless for a beginner in logic like myself. Just for reference, I have an easy time understanding this guys vids:
https://www.youtube.com/channel/UCdhGKS28LbzxebigAlA2rDg (https://www.youtube.com/channel/UCdhGKS28LbzxebigAlA2rDg)
but he doesn't have any vids on logic circuitry.
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My apologies that I forgot to mention this, but this whole time I have been running the scope without the storage mode isolated - but for a very good reason. The scope will not operate unless everything is connected and not in test mode!! The pictures below are what happens when I put it into test mode:
I am not able to do anything or get a semi proper trace displayed on the scope when I have it in test mode. This is likely due to the faulty logic IC's on the vertical board that I have been testing. The manual says that the scope is supposed to operate in CHOP only when it is put into TEST MODE, since what I am basically doing when I put it into test mode is breaking the connection between the vertical mode switchboard and the vertical switching logic. It powers on and everything when in test mode, but does not respond to any CH1 controls or signal, and CH2 will only show two fuzzy horizontal lines depicting the amplitude of the input signal to channel 2. The only thing that changing the timebase does is dim the trace and shift it to the right as I turn it to shorter time settings. I am not sure what is going on, so I am going to leave it in test mode for right now and poke around to see if I can figure out what is happening. After I figure out some more about what is happening I will likely make a video to show it.
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Alrighty......
In the test mode, U408 and U509 are not getting the correct voltage sent to them in order to activate CH1, CH2, and CHOP. The only settings getting any voltage at all are CH1, CH2, and XY mode, but only 0.73V. depressing the CH1, CH2, and CHOP switches only increases the voltage to these by 2mV. There is 50Khz riding those and several other inputs/outputs of the other 74 series ICs, and also some 5.6Khz (very low amplitude) as well.
The 5V rail still has the same noise (5.6Khz) and still seems to be originating from U508. I'm replacing them regardless, but I would still like to know what exactly has gone wrong. I really feel like replacing all of the 74 series ICs and any others I can get quality for cheap, but even if I do so I still want to find out what the actual problems are. It's not a learning experience if I just keep pulling and replacing everything that I can!!
I have arranged some jumper links and 90 degree header connectors so that I can get better access to the mode switchboard. I just wish I had extensions for the coax interconnection cables... I will be on the lookout for a deal on those females and cables for future repairs - and also a longer ribbon connector cable.
any tips on what I should be looking for based on the info I've provided? or is there anything specific you want me to check/measure so you can get a better idea of what the deal is?
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You'd wonder if all the 74* IC's have been damaged by PSU problems at some stage. :-//
If this IS the case, then what else is hurt too? :palm:
Getting to the bottom of all the lingering niggly faults can be very frustrating and I feel your pain.
I'll be away for a few days and mightn't be much help, although it seems you're on the right track IMO.
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well well, another MAJOR failing on the part of Tektronix in the 468 design and manual!!!!
The manual explains how to switch the scope into test mode to test the functionality of only the basic scope functions - but they FAILED to provide ANY power to the resistor that then turns on the logic for CHOP, CH1, and CH2!!!!!! it specifically says to UNPLUG the jumper that powers the storage mode power supply - the power supply that powers that damn resistor!!!!!!!! GAHHHHHHH!!!
SO - I am going to rig up a lead to put 5V to that resistor and get things moving along...
one would think that Tektronix would have noticed this dilemma in the... oh..... 30 years that this scope was in regular operation???? or AT LEAST added that info to the manual!!
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S for STORAGE power supply!!
P3006 being the jumper from the transformer that has it's fuses based on A12.
DUM-DA-DUM-DUM-DUMBBBBBBBBBBBB
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You'd wonder if all the 74* IC's have been damaged by PSU problems at some stage. :-//
If this IS the case, then what else is hurt too? :palm:
Getting to the bottom of all the lingering niggly faults can be very frustrating and I feel your pain.
I'll be away for a few days and mightn't be much help, although it seems you're on the right track IMO.
I'm hoping that the problems I am having with the logic circuits boil down to a few 74*s and some out of tolerance resistors causing low voltages. I think I may have located a resistor network that is out and causing oscillations to the 74*s already.
There's just sooo many little piddly issues, and what happens is I'll be tracking one down and find another and get distracted from my initial search, and nothing gets done. I'm really trying hard not to let myself get diverted, but make sure that I am also not ignoring little issues that may be adding up to the bigger ones. Then there's the waiting for parts game, like my trigger JFETs - I bought 4 pairs, but the guy accidentally put all 5 remaining pairs together in one bag. So instead of telling me that early friday to get my response, he waited until right before leaving work to tell me and now I have to wait several more days. At least I'll have 5 pairs for 2.50 a pair, even if I have to rematch them.
Well I hope you have a good time on your trip (or whatever it is you are doing). I'm sure I'll have a pile of interesting issues listed and hopefully sorted by the time you return.
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Fear not, I'll be keeping an eye on you, ;) just not as often. :popcorn:
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it's so much easier to debug the basic circuits of this scope with all of the digital & logic completely disabled... I've already found exactly where the channel to channel crosstalk is coming from. Both Q285 & Q245 are leaking signal into the -8V rail, and oddly enough, channel 2 is worse. It's coming directly off of Pin 3 on both Fets.
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the 100x attenuator for CH2 failed. I have a couple 100x attenuators, but they are model 307-1014-02 and it needs 307-1014-05. anyone see a problem with using the one I have for the time being? they're both for 1M 20pf scopes, and I'm not sure what the differences are internally...
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well this is very strange, or very simple.... It seemed like the 100x attenuator died while the scope's digital section was disabled... I thought maybe if I put it back together it would be fine, and in the meantime I opened up the 100x attenuator and cleaned it (there was actually a little bit of some kind of gunk on top of the precision resistor parts). Now that it's back together, it works fine, but it appears that some of the finger contacts have gummed themselves up again, or the 2X attenuator is going funky on me. I find this very odd, but only in 10mV/div does it do this - it will flatline the signal like the attenuator is dead, but there will be huge spikes with short burst ringing.
I mean, it has to be the contacts, but can they really dirty themselves up again that freakin fast?!?!? It was 2 weeks ago that I spent nearly 3 hours on each channel cleaning ONLY finger contacts!
I guess this is something to be fully aware of when restoring any of these old tek scopes!
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A quick update:
I am checking devices on the Vertical Mode Switch board since this seems to be where a number of my issues are originating. This includes the non-working trigger view, the 25khz 80mv ripple hitting the IC's on the vertical preamp board, and others.
Q2 (Pin 5) of U310 has a connection to pin 1, 2, 10, 11, and 20 when measured in resistance. When measuring for V drop in diode setting, there is a 2.85V drop between pin 5 (+) to all of the other pins. This is not the case with any of the working selectors.
This is the only IC I have fully checked out thus far. I really don't know much about logic circuitry, but from what I do understand thus far I am quite certain that this is a problem - most likely the problem why trigger view does not work, and why the voltage at the vertical amplifier pin for trig view.
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I have completed the trigger view repair and made progress on the 110V unreg loading and bad PSU waveforms.
I have discovered that the PSU waveforms are directly related to the CRT bias adjust and the majority of the CRT circuit. I replaced Q109 and it immediately made a difference in the low voltage side of the HV CRT transformer. :-+ I know that Q1109 is dying, but.... the person out in CA that I ordered the RCA 2N3055's from is too cheap/lazy/stupid to properly package them in anti-static bags for shipping, so they were DOA. I wanted him to send replacements, but he was too lazy to test them quickly with DMM and obtain an antistatic bag for shipping, so he decided to give me a refund instead. (NEVER buying from them again!!!!!) So now I have to wait several more days before I can get that issue completely tidied up.
I have managed to get Trigger View functioning fully again by replacing Q114, which is what was destroying my NORM trigger signal.
There is still plennnnty more to do on this pup, so I'll keep posting updates as I make more progress.
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can anyone tell me what it means when the display is not centered about the horizontal graticule line with the input coupling set to Ground? The manual says absolutely nothing about making an adjustment to it, so I assume that means there is no adjustment and it is a fault somewhere, such as the main input capacitor being bad?
This offset appears to be an issue throughout most of the vertical preamp and is causing many of the waveforms to have offset baselines.
Of course as soon as I switch it to AC or DC it goes to center, but that's because I adjusted it to be there.....
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I just double checked, and now I see that it is only doing it when I have no vertical selected, but have triggered the sweep..... so that must mean there is a voltage difference between the two horizontal deflection plates - thus pulling the sweep towards one of the plates instead of it being centered. The voltages to the plates confirm this, but I'm not 100% that I am correct in thinking this. Anyone feel like confirming or denying ?
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I know that Q1109 is dying, but.... the person out in CA that I ordered the RCA 2N3055's from is too cheap/lazy/stupid to properly package them in anti-static bags for shipping, so they were DOA.
I'm flabbergasted, I've never considered jellybean bipolars, particularly the venerable 2N3055 to be delicate in this way. :-//
Sure if abused, you can bugger most things, but I reckon he was trying to rip you off with duds.
I just double checked, and now I see that it is only doing it when I have no vertical selected, but have triggered the sweep..... so that must mean there is a voltage difference between the two horizontal deflection plates - thus pulling the sweep towards one of the plates instead of it being centered. The voltages to the plates confirm this, but I'm not 100% that I am correct in thinking this. Anyone feel like confirming or denying ?
Sweep waveform spot on to spec?
IIRC other scopes I have fixed had both a balance and gain adjustments, balance maybe :-// set in XY mode and gain sets trace sweep width to 10 div long/wide.
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The A sweep start and B sweep start waveforms are off, and I am working on solving that. I am fairly certain that the issue is originating from the trigger gen circuit. I also cannot reliably do any kind of voltage measurements or waveforms that rely on any output from the vertical preamp other than directly thru to logic then the vertical amplifier, and I am waiting for the parts to fix it. (151-0333-00) they should be here monday according to USPS, but who knows.
The rotten apples are Q154 & Q155, and are not properly amplifying the input signal. The DC voltages are spot on or very close, but once signal is put into the scope the waveforms up to TP5 & TP6 are good, but those two are 1.5V low and their waveforms are 10% to 20% low as well. If replacing those does not fix it, then it's CR258 & CR258. But the problem is almost the exact same on both channels, so I believe it is the transistors.
Also I had mentioned that the input JFETs are likely on their way out. I am not 100% certain on this. I could only get one new 151-1032-00 (only one I could find--- ANYWHERE!!) from Greece. However, according to the following site:
http://www.reprise.com/host/tektronix/reference/transistor.asp (http://www.reprise.com/host/tektronix/reference/transistor.asp)
that dual JFET is the exact same as the 151-1090-00, so I may order some of those. I'm also looking into buying a 464, 466, or 468 parts scope.
Speaking of JFETS, I received my 151-1042-00's for the trigger circuitry.... but.... I have 8 matched but mixed up pairs to re-match, and one proper pair but TO-106 package. I think that their stripes are painted after they are matched, but I still think I should build a test circuit to confirm, such as this one in Post #9:
https://www.eevblog.com/forum/projects/how-to-match-a-set-of-jfets/ (https://www.eevblog.com/forum/projects/how-to-match-a-set-of-jfets/)
Then once I have the input to the trigger circuit back in order I could work on the sweep and some of the z-axis. unfortunately the horizontal amplifiers voltages are taken in XY mode so I can't do that yet.
At least I'm gaining momentum!!! this definitely is a beast of a repair/refurb for a beginner's learning project, but I'm gettin the hang of it! :-+ :-+
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the one thing that I am confused about with the curve tracer circuit is this...
I understand that IDS goes to Channel 2 (or Y) of my scope, but where does the X signal come from? Do I just tee off the signal from my function generator for VDS or VGS (depending on the measurement I am taking)? The schematic and article aren't clear about this.
I believe that I am correct in regards to teeing off the signal from the FG, but I'd like some confirmation that I am doing this right..
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A few posts back you described problems with trace position when Gnd coupled.
In the schematic portion above there is a 20K pot for Vertical adjustment.
It's likely to be the master adjustment and the others you have adjusted just for the relevant attenuators. RTFM. ;)
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In the post after I said that it was off when the channels were in the GND position I corrected myself and said that they are fine in the ground position because I adjusted them to be, but that it is off when there is no vertical selected, but I have triggered the sweep. When doing that, the horizontal line that is displayed is about half a division above the center horizontal graticule line, and changing the timebase has no effect on this at all. When the DC voltages were measured on the vertical plates and horizontal plates, the voltages were not the same on either side as they should be. Also in the post where I corrected myself, I said that I thought the difference in the horizontal plate voltage would be the cause of this - and then I learned a little more about CRO CRT's. The horizontal plates are horizontally opposing (vertically positioned). The vertical plates are vertically opposing (horizontally positioned). therefore, if anything is causing an offset in the horizontal positioning of the triggered sweep, it would be the difference in voltages at the vertical plates - thus pulling the electron beam more towards one of the plates. In my case it is the top plate. Anyway, this "issue" is likely nothing to worry about and will most likely be resolved once I finish fixing the vertical section.
On another note, I have the JFET matching circuit finished and need to know the pinout of these 151-1042's... In the picture below I labeled what I believe they are, but am unsure of whether I have the Source and Drain backwards or not. I know technically the JFET doesn't care much and it probably doesn't matter much for the test circuit, but I would like to know anyway. In the link below, if I am correct about the pinout, that would mean it is similar to the JFETs of this type that were manufactured by Central. Most of the others have it arranged the other way around with the Source as the middle pin. Anyone care to shed some light on this? I've been researching JFETs a lot and think I have it right but I want to be sure.
http://alltransistors.com/mosfet/transistor.php?transistor=19124 (http://alltransistors.com/mosfet/transistor.php?transistor=19124)
[EDIT] Had it backwards. the pinout on these is D S G.
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Just replacing those trigger JFETs has the rectifiers running quite a bit cooler. A trigger/sweep is doing great even though the waveforms are still just a little bit off. B trigger/sweep is partially back to life but still has some big problems that need to be resolved. The A trigger controls are affecting the B trigger/sweep, and I can only get the sweep to trigger with a certain level setting. The problem doesn't seem to be in the adjustments so it seems I have some investigating to do.
I've got my fingers crossed that at least my order of IC's and transistors makes it here tomorrow so I can make some progress on the vert section and open up some more areas to check/fix. However it will be another week or two before my 1032's arrive, but hopefully these originals have enough kick left before they bite it to get me til then.
Since I built those 2 JFET measuring circuits I was able to measure the old ones and see just how bad they had become.. They were still alive, but very unhealthy - especially when compared to the entire batch of TO-92 type and TO-106 type new ones. I'm sure I'll be finding at least a few more battered and fried transistors in this pup..
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I'm baffled....
I kept finding transistors that we're on their way out, so I went thru and replaced all of the 2n3906, 2n3904, 2n2222A, 2n5551, 2n2907, etc etc. I had plenty high quality Moto & Raytheon ones I got for cheap so I figured what the hell - why not. I also replaced several zeners - a few of which were very unstable, and some that seemed OK but too close to their 5% tolerance rating for comfort.
What has me baffled is this - after the repairs when I initially started it up, the PSU waveforms looked like complete hell. 2x to 3x proper amplitude and way, way off of the expected waveforms. As it warmed up a little and I turned on the trace, the waveforms just magically decided they were ready to go to spec... WTH is going on?!?!?! I know these old analog scopes take a little time to warm up, but I have never heard of the PSU waveforms being crap on startup - especially not like they just were.
The only thing I can think of is that there are still a few semiconductors somewhere that are acting up and don't like to function properly until they are warmed up... The rectifiers are running a whole lot cooler now and the scope seems to be acting up a lot less, but there are still some bugs to be found and solved. ( I have not yet received my new trigger IC's or vertical input FETs and diodes ).
Anyone out there that can shed a little light on this matter for me?
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I'm flabbergasted, I've never considered jellybean bipolars, particularly the venerable 2N3055 to be delicate in this way. :-//
Sure if abused, you can bugger most things, but I reckon he was trying to rip you off with duds.
There is a special place in hell for people like that. I've bought components before where they were either obviously fake or used, pulled from gear before, and I am fine with that, goes with the territory. But knowingly selling bad non working parts and presenting them as good, is low. This seller knew it I am sure.
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in hindsight, yeah they probably knew. It took several messages before they finally backed off the "No one else is complaining" & "well how do you know they are bad" rhetoric and stated they would send two more - but when I demanded they test them first and properly package them in a static shielding bag they resorted to " Dear buyer, we have decided to give you a full refund to make things simpler.."
It's the only seller on ebay with a large number of RCA 2n3055s, and there's a picture of numerous ones packed in their foam packing. If you find that seller and look at their negative feedback, there are several cases where they sent broken & misrepresented items. Then there are several replies to the neg feedback stating " Block this buyer! SCAMMER! " etc etc.
There should be a list of rotten ebay sellers who try to rip people off. Of course that won't happen here, though, because ebay/paypal has the backs of those type of scum. That is why I am giving you all the info I can for you to identify them without actually identifying them. It should be easy enough to find them now....... only a few sellers of RCA 3055's, and only one with over 70 or so sold (last I checked).
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I know these old analog scopes take a little time to warm up, but I have never heard of the PSU waveforms being crap on startup - especially not like they just were.
Anyone out there that can shed a little light on this matter for me?
I wouldn't stress too much about it, unusual as it sounds. Remember there is usually a stated time in the Service manual that the scope must be running for before accurate measurements or calibration procedures be commenced. This applies to all scopes, CRO's and DSO's.
Curiosity might make me monitor power-on mains currents accurately or just with a dim bulb tester.
Are the same rogue waveforms produced at re-power after the scope is warm?
It does sound unusual. :-//
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the incredibly bothersome PSU waveforms are only present when the scope is cold-started. After about one minute they go to normal.
After changing out all of those cheap transistors, the psu waveforms for the 110 & 55 are amazingly better. Now there is just a little "lump" on the peak at the beginning of the discharge parts of the cycle. I think that the waveforms are good enough now. Maybe they will improve to exact spec once I've resolved the few other issues, or maybe it's due to the small amount of ESR I measured in the 55V filter cap. (It's not much at all - maybe one-quarter ohm up from the others, which are all very low.)
Another thing I have noticed that is a direct result of the fresh semi's throughout is that it is much, much easier to pick out and locate problem areas. I'm thinking that the crispness of the switching is making the slow & leaky guys stick out like a sore thumb. I'm going to continue to shop around for deals on bulk of the remaining semi-parts. Soon I'll have this scope running like a champ again with many, many more years of use - and a huge stockpile of parts!
The biggest problem that still remains is the CH1 pickoff outputs to trigger, X, and storage that are still garbage. I have a couple new 151-0434-00's (2N4261) en route from Greece that will hopefully crush that problem. If not, I will need more 0434's (freakin 12 on the vert board in total - at $6 each... MINIMUM!!!) Hopefully that clamps down on the drifting horizontal (in XY mode), which travels >4 divisions as the scope warms. Then that should allow me to discern what is the cause of the bad horizontal CRT plate voltages which are high a few volts, differ by 5 volts at best, and I believe is also the cause of the 110V rail being pulled UP (oddly enough). I've changed the resistors that set the 110V, so I know it's not that...
Annnnd last and possibly least, the only rectifier still getting really hot is the 15V.
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How did you know your jfets in your input amp were flaking out? I just purchased a used 466 scope. I've got a good trace on channel A with a proper amplitude in 5mv from the calibration source, however channel B is only about 1/10 of the amplitude. I've only moved the probe from A to B channel, so nothing different there..
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your best bet is going to be to check the waveforms at the test points on both channels and compare. If the waveforms for the entire channel 2 section are bad, then your problem is near the input area. Your problem could be anywhere from directly at the input to somewhere in the vert switching logic where the signal for both channels gets combined for the vertical amplifier. I would mess with the gain adjustment for CH2 and see if I could get it back to spec first. but if you have to max out the pot to get the correct amplitude, you've definitely got something wrong.
As for me being able to tell that my input FETs are flaky: I took them out and tested them after noticing that the gate of the B unit was leaking oscillations into the -8 rail, and the test point waveform at the gate to A (from channel input) was horrible.
Another thing you should do is check different V/div settings to see if any are good. If they are all off in the same manner, then you probably don't have an attenuator problem.
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I've narrowed it down to a couple of transistor's in ch2. everything before that point is the same as ch1 with voltage and waveform. At this pair of transistor's, the voltage is normal on b and e but the c is 1.7v instead of 2.7volt. This is the Q212/214 pair of NPN transistors. I've attempted to check the components in the filter between them, but there are differences between what I have and what the schematic shows. (I'm missing some components). I don't want to play around with the gain however, as the voltage on the base of this pair is exactly what it's supposed to be along with the correct waveform. Its definitely, in the filter I just need to figure out where. attached to the collector is a 300ohm resistor with a 470 capacitor. I'd have to assume that's a 470pf. I am testing before them, so I believe they are ok.
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I know these old analog scopes take a little time to warm up, but I have never heard of the PSU waveforms being crap on startup - especially not like they just were.
Anyone out there that can shed a little light on this matter for me?
I wouldn't stress too much about it, unusual as it sounds. Remember there is usually a stated time in the Service manual that the scope must be running for before accurate measurements or calibration procedures be commenced. This applies to all scopes, CRO's and DSO's.
Curiosity might make me monitor power-on mains currents accurately or just with a dim bulb tester.
Are the same rogue waveforms produced at re-power after the scope is warm?
It does sound unusual. :-//
Even though this thread is abandoned, I feel the need to post a quick update here.
I had set this specific 468 off to the side and moved on to another 468, 485, and 465 until I found some information that may help me solve the main issue with this 468 - the power supply waveforms and their erratic behavior.
As it turns out, the problem was a faulty 110v rail bridge rectifier. It wasnt bad enough to cause half-wave rectification, but it was doing exactly what the bad rectifier that Dave found in one of his (fairly recent) repairs - it was randomly dropping out and blocking the flow of current. This is what caused the 'jumpy' waveforms all over the PSU. The 'rounded off' falling edge of the PSU waveforms were caused by the 55v rail's filter cap, which began showing a slightly high ESR after several hours of the scope being powered on.
The rectifier is replaced and the new filter cap (not the one borrowed from the other 468) is on it's way from Greece. :-+