Tektronix 468 repair/refurb

[Addicted2AnalogTek]

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:


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)

[Addicted2AnalogTek]

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.      I don't think that there is any reason to do that, either.

[MarkL]

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?

[Addicted2AnalogTek]

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

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.

[Addicted2AnalogTek]

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?

[Addicted2AnalogTek]

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.

[tautech]

found something very strange.....

Does it change in amplitude with different input attenuator settings?

It's a square wave, but at what frequency?

[Addicted2AnalogTek]

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.

[Addicted2AnalogTek]

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. ***

[Addicted2AnalogTek]

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. 

[tautech]

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. 

[Addicted2AnalogTek]

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?

[Addicted2AnalogTek]

well, the trigger board or the vertical preamp board?   I shall start probing power rails to see if I can confirm and trace!

[Addicted2AnalogTek]

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.

[Addicted2AnalogTek]

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.

[tautech]

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. 

[Addicted2AnalogTek]

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..... 

[MarkL]

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.

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...

[Addicted2AnalogTek]

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.

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.

[Addicted2AnalogTek]

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.

[tautech]

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. 

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.

[MarkL]

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).

[MarkL]

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.

[Addicted2AnalogTek]

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. 

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. 

[Addicted2AnalogTek]

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.