Tektronix 468 repair/refurb

[Addicted2AnalogTek]

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

[PaulAm]

Page 496 of the service manual on K04bb:

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.

[Addicted2AnalogTek]

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.

[MarkL]

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.

[Addicted2AnalogTek]

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!

[poot36]

You can also search the BAMMA website for the schematic for this scope.

[Addicted2AnalogTek]

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?

[tautech]

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.

[MarkL]

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?

[macboy]

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.
...

^^^ 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.

[Addicted2AnalogTek]

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.

[macboy]

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:

[PaulAm]

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

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
 

[MarkL]

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.

[Addicted2AnalogTek]

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.

[Addicted2AnalogTek]

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? 

[rqsall]

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.

[Addicted2AnalogTek]

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!

[rqsall]

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.     

[MarkL]

...

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.

[Addicted2AnalogTek]

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!

[MarkL]

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).

[Addicted2AnalogTek]

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

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...

[PaulAm]

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.

[Addicted2AnalogTek]

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.