SOLVED Tek 475A DM44 Excessive Horizontal Delay Time Jitter

[Hornethandler]

My Tek 475A with DM44 has excessive horizontal delay time jitter.  The spec. calls for 1 division or less jitter, but I’m getting around 2 to 3 divisions.  There is no adjustment in the cal. procedure, i.e., it’s a “Check” step only.

All power supplies are within voltage and ripple limits and there are no interfering light sources.

The jitter is less and within spec. at slower sweep rates, but at the “Check” setup (set DLY’D SWP to 0.2us, set  A TIME/DIV to 1ms, and set time-mark generator for 1ms time marks) it’s much worse.

I’m not really sure where to start and would appreciate any help/insight you can offer.

Thanks,
Mack

[David Hess]

If you need it, the service documentation for the DM44 is available here:

https://elektrotanya.com/tektronix_dm44_for_465_475_szkop_dmm_puttony_1976_sm.pdf/download.html

It looks like the horizontal jitter tests are identical.

I would temporarily disable the DM44 delay time control circuits which requires just disconnecting one cable and adding a jumper and then use the built in oscilloscope circuits although this requires temporarily adding 1 microfarad C923 back on schematic 8 of the 475A.  This will resolve if the problem is with the 475A or DM44.

The people over on the TekScopes@groups.io or TekScopes2@yahoogroups.com will have additional ideas.

[Hornethandler]

Thanks for the advice, David.  I looked over the schematics and scope and found the cap location, connector and jumper location.  Hopefully I'll get at it today and let you know how it went.

Thanks again,
Mack

[Hornethandler]

Hi again David,

I found instructions to install a DM44 on a 475 and 475A on the Tekwiki site (see link below).
http://hakanh.com/dl/docs/kitinstructions/040-0817-01.pdf

Many modifications (parts added and deleted) to the scope were carried out to accomplish installation of the DM44.

Do you recommend doing a complete reversion back, or will adding C923 and a jumper on P1120 pin 2 and 3 be sufficient to isolate the DM44 and operate the Delay function solely from the scope circuitry?

Thanks David,
Mack

[David Hess]

I found instructions to install a DM44 on a 475 and 475A on the Tekwiki site (see link below).
http://hakanh.com/dl/docs/kitinstructions/040-0817-01.pdf

Many modifications (parts added and deleted) to the scope were carried out to accomplish installation of the DM44.

Do you recommend doing a complete reversion back, or will adding C923 and a jumper on P1120 pin 2 and 3 be sufficient to isolate the DM44 and operate the Delay function solely from the scope circuitry?

For the test, I think you can get away with just disconnecting P1120, adding the jumper, and adding C923.  The idea is to restore just the original delayed sweep configuration.

[bd139]

Just a heads up on the original question. I had a 475 (not A) with DM44 that suffered from this as well. Got tired of it before I fixed it but most of the rest of the triggering problems were related to dead or dying tantalum capacitors.

[Hornethandler]

Novice question so please be kind...

The “Jitter Check” is preformed with the DMM in the Volts mode.  Therefore, the Time and 1/Time functions on the DMM are disabled.  Does this effectively take the DMM out of the loop for the purposes of isolating the snag, or do I still need to isolate the DMM as described above, i.e.,: “disconnecting one cable and adding a jumper and then use the built in oscilloscope circuits although this requires temporarily adding 1 microfarad C923 back on schematic 8 of the 475A.” ?

Thanks,
Mack 

[David Hess]

The “Jitter Check” is preformed with the DMM in the Volts mode.  Therefore, the Time and 1/Time functions on the DMM are disabled.  Does this effectively take the DMM out of the loop for the purposes of isolating the snag, or do I still need to isolate the DMM as described above, i.e.,: “disconnecting one cable and adding a jumper and then use the built in oscilloscope circuits although this requires temporarily adding 1 microfarad C923 back on schematic 8 of the 475A.” ?

When the DM44 is not performing its delta delayed sweep function, the delayed sweep control signal from the 475A still goes through all of the DM44 delayed sweep circuits.  Essentially with the DM44 installed, the 10 turn delayed sweep potentiometer on the 475A always feeds directly into the DM44.

I am actually surprised that the jitter specification is the same with the DM44 installed although I can see some improvements which could be made in the DM44.

[Hornethandler]

Roger that, David

Next few days are booked, but hope to get at it soon and will advise results.

Thanks again,
Mack

[David Hess]

There are lots of parts which I could suggest are causing the problem but the test I outlined should be done first because it is so easy.

bd139's suggestion about the decoupling capacitors should be taken seriously.  However testing it rather than just shotgun replacing them could be difficult (and non-obvious failure of solid tantalum capacitors is not common) which brings up this question:

All power supplies are within voltage and ripple limits and there are no interfering light sources.

How did you measure ripple?  Ideally both a ripple measurement using an AC voltmeter and a peak-to-peak noise measurement using a 20 MHz oscilloscope is made.

Do you have another oscilloscope for testing?

This is actually one of those cases where the oscilloscope can measure the ripple on its own power supply outputs using AC coupling but make sure the probe's ground lead does not flap around and short on something; I often just remove the ground lead for safety in this case.  DSOs are actually better for making this measurement because they more readily show peak to peak noise.

[Hornethandler]

Dave,
Thanks for asking the ripple question; it made me go back and look again.

I made the power supply voltage and ripple measurements using a Fluke 289 and a Tek 2232 respectively.  Ripple came in at or just under the 2mv max allowable, and well under the 1v max for the 110v.

HOWEVER, I just now re-measured to confirm before answering your question and discovered a couple of negative going 8mv spikes on the +5volt rail.  The ripple is 2mv, and the spikes are at 8mv. I can affect the pulse with and rise time of the spikes by putting the DMM in Time or 1/Time and turning the Time pot (the one on the DMM).  The pulse width widens and the rise time slows a bit once the pot reaches about 5 turns from fully counter clockwise.

I guess the first thing to do now is check the 5 volt filter caps.  Do you agree?

Thanks,
Mack

[bd139]

Not a bad idea. BTW worth measuring the ripple with a 1X probe if you can.

I would first of all check the big axial capacitor in the middle of the interface board. That's across the 5v rail if I remember and tends to go a bit wonky. Here's one I pulled out of a 465 - definitely a bit sick. Had "vomited" out of one end a bit as well.



It wouldn't surprise me if this was tant related. I had to shotgun mine which took a while.

My original repair thread (minus videos for some reason): https://www.eevblog.com/forum/repair/tektronix-475-repair-assistance/ ... and a 465B thread which is relevant: https://www.eevblog.com/forum/repair/tektronix-465b-repair-97327/

[David Hess]

Thanks for asking the ripple question; it made me go back and look again.

I made the power supply voltage and ripple measurements using a Fluke 289 and a Tek 2232 respectively.  Ripple came in at or just under the 2mv max allowable, and well under the 1v max for the 110v.

HOWEVER, I just now re-measured to confirm before answering your question and discovered a couple of negative going 8mv spikes on the +5volt rail.  The ripple is 2mv, and the spikes are at 8mv. I can affect the pulse with and rise time of the spikes by putting the DMM in Time or 1/Time and turning the Time pot (the one on the DMM).  The pulse width widens and the rise time slows a bit once the pot reaches about 5 turns from fully counter clockwise.

I guess the first thing to do now is check the 5 volt filter caps.  Do you agree?

Making this measurement without a differential probe is tricky.  Often the ground loop will have more noise than the signal.  The 2232 is perfect for it though.

I do not think the +5 volt supply could cause this problem.  What you might be seeing though is ground noise associated with the switching of the b-sweep.  If the switching is synchronous with the sweep, then it should not be causing jitter.

The capacitor in bd139's photograph looks like a wet tantalum and those do fail open if they leak.  You might want to give all of the wet tantalum capacitors a close visual inspection.

[bd139]

The one pictured above is a sprague 150d electrolytic I think. Wasn’t wet tant. Had a wet tant blow up in a 465b though.

[David Hess]

The Sprague 150D series is a hermetically sealed solid tantalum capacitor.  I have never heard of one failing except due to excessive voltage.  They have nothing to leak out.

If one did fail, there would be nothing to see unless it exploded.

[Hornethandler]

I measured the ripple at the +5 volt rectifier (across capacitor 1452, 5000uf)...it was 400mv.

For comparison, the ripple at the same point (C1442, 5500 uf) on the +15 volt side was 100mv.

Is 400mv on the +5 volt side too high?


Thanks,
Mack

[tggzzz]

I measured the ripple at the +5 volt rectifier (across capacitor 1452, 5000uf)...it was 400mv.

For comparison, the ripple at the same point (C1442, 5500 uf) on the +15 volt side was 100mv.

Is 400mv on the +5 volt side too high?

You can always do a quick-and-dirty test: tack solder a "similar" capacitor in parallel. If the ripple improves significantly, then the original capacitor is poor.

Personally I would have thought that a 10% (400mV RMS) ripple on a regulated power rail was awful. I would be looking for a couple of mV or so.

[Hornethandler]

The 400mv ripple is across the cap (across the rectifier), not on the regulated output rail.

The ripple on the output rail is less than 2mv with transient 8mv negative going spikes.

[Gyro]

The Sprague 150D series is a hermetically sealed solid tantalum capacitor.  I have never heard of one failing except due to excessive voltage.  They have nothing to leak out.

If one did fail, there would be nothing to see unless it exploded.

Yep, I think its the "hermetically sealed solid..." which is critical. I'm certain that moisture ingress is the chief cause of bead Tant failure in older gear. Leave them nice and warmed up and they're fine. Leave them to sit somewhere cold for a few months and watch them pop, one by one! I've never seen a 150D fail, or leak electrically.

[bd139]

This 150D had what I can only describe as "burned spooge" one one end. The rest of the scope was immaculate though. Not sure what the deal was there.

[David Hess]

I measured the ripple at the +5 volt rectifier (across capacitor 1452, 5000uf)...it was 400mv.

For comparison, the ripple at the same point (C1442, 5500 uf) on the +15 volt side was 100mv.

Is 400mv on the +5 volt side too high?

Ripple on the input side is usually 100s of millivolts to volts.  The minimum voltage on the input ripple needs to be high enough to keep the regulator from dropping out.

[Hornethandler]

One of the ribbon cables (8 wire) that connects the DMM (P3306) to the Timing Circuit Board (J1130) was intermittent when lightly touched at P3306.

I first swapped the ends of the cable to verify if the cable was bad.  It was still intermittent at P3306 so I re-flowed the solder on all 8 pins of P3306... Snag gone!!!

Thanks everybody for your help.

Mack

[bd139]

Bingo. Nice find