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.
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?
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 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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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?
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.
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.
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.
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.
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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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.
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.
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.
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!
QuoteHmmm... 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.QuoteWhat 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
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...
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.