Tektronix 2432A Repair

[TheDefpom]

Hi Guys and Gals,
I thought I would just post a recent issue I had with my old Tek scope, it suddenly failed recently, and so naturally I had to pull it apart to fix it, I put a video up on YouTube showing the issue I found, which was due to good old Tantalum capacitors failing.

[george.b]

I had the exact same capacitor blow up on my 2430A. Some other capacitors were shorted, so it wouldn't turn on. I removed them, and it came back to life. I haven't replaced them yet, so CH1 is all wonky - hopefully that will be fixed after I replace them.

Edit: Here's a picture I just took. What the heck, it's been a long day, might as well replace them caps and end the day on a lighter note. Will report back.
I cut the other capacitor's leg so I could test the scope without risking it shorting somewhere else.

Update: I replaced the capacitors with two 22µF tantalums instead of the original 27µF ones - I guess she'll be alright? The wonkiness is mostly gone, but the CH1 trace wobbles vertically a little. Do you get that on yours? Maybe one of the transistors got unhappy with all the shorted caps?

[TheDefpom]

I didn't have any other faults, just that one shorted cap, in my case the trace wouldn't show up for ages, and when it did it was very dim and was in a circular pattern near the Centre, it effected the sweep, once I replaced the caps shown in the video it was fine.

[David Hess]

Update: I replaced the capacitors with two 22µF tantalums instead of the original 27µF ones - I guess she'll be alright? The wonkiness is mostly gone, but the CH1 trace wobbles vertically a little. Do you get that on yours? Maybe one of the transistors got unhappy with all the shorted caps?

The difference in capacitance should not matter.

A wobbly trace might indicate a problem with the low voltage power supply like worn out aluminum electrolytic capacitors.  Switch to line triggering and about 1ms/div and see if the wobble is power line related.

When operating this oscilloscope with the cover removed, use a fan to blow air across the bottom to prevent damage to the hybrids which will otherwise overheat.

[george.b]

The difference in capacitance should not matter.

A wobbly trace might indicate a problem with the low voltage power supply like worn out aluminum electrolytic capacitors.  Switch to line triggering and about 1ms/div and see if the wobble is power line related.

When operating this oscilloscope with the cover removed, use a fan to blow air across the bottom to prevent damage to the hybrids which will otherwise overheat.

I thought so, but CH2 is okay. What happens on CH1 seems to be random, not related to the line.
I was looking at the calibrator signal; I decided to couple both channels to ground and take a look. CH1 seems to be glitchy, in addition to being wobbly. I took a few pictures. Any ideas? 

Edit: Should I start a new thread for this, maybe?

[David Hess]

It does not look like a power supply problem.  I see spiky noise and what looks like massive drift.

Was it doing that before?

Sometimes the connections between the hybrids and printed circuit board go bad.  The hybrids can be removed and the contacts cleaned by rubbing with a swab and alcohol.

I have a 2440 that I refurbished by replacing the NVSRAMs and recalibrating.

[george.b]

It does not look like a power supply problem.  I see spiky noise and what looks like massive drift.

Was it doing that before?

Sometimes the connections between the hybrids and printed circuit board go bad.  The hybrids can be removed and the contacts cleaned by rubbing with a swab and alcohol.

I have a 2440 that I refurbished by replacing the NVSRAMs and recalibrating.

And the noise looks digital in nature... I mean, it seems to jump between two discrete levels, in addition to the wobble/drift (it drifts within a couple of seconds, not sure what to call it ).

I wouldn't actually know; this is a dumpster rescue I was diagnosing the power lines when suddenly that capacitor blew up, and some of the lines went back to normal. I then suspected there were other faulty capacitors, which I confirmed and, upon removing them, the voltages went back to their nominal values. It would usually fail some combination of CCD, PA and TRIGS, until I replaced the caps. Now it passes self-test, but the trace does that weird stuff.

I will try cleaning up the hybrids, then. Are they in any kind of weird fragile package? Can't really see under those heatsinks

Are the NVSRAMs particularly prone to failure? Sometimes it comes up with weird waveforms instead of those previously stored, I found that somewhat suspicious. Other settings seem to be stored normally otherwise.

[David Hess]

And the noise looks digital in nature... I mean, it seems to jump between two discrete levels, in addition to the wobble/drift (it drifts within a couple of seconds, not sure what to call it ).

Because of the complex CCD based sampling system this oscilloscope uses, problems can manifest as noise spikes as shown without it being a digital logic problem.  While from a DSO using a design closer to a modern one, the photographs below are from a completely *analog* problem.

I wouldn't actually know; this is a dumpster rescue I was diagnosing the power lines when suddenly that capacitor blew up, and some of the lines went back to normal. I then suspected there were other faulty capacitors, which I confirmed and, upon removing them, the voltages went back to their nominal values. It would usually fail some combination of CCD, PA and TRIGS, until I replaced the caps. Now it passes self-test, but the trace does that weird stuff.

Which other capacitors did you replace?

The aluminum electrolytic output and maybe input capacitors of the switching power supply are old enough to be suspects when odd problems start to crop up.  They are a lot more work to change though compared to cleaning the hybrids.

I will try cleaning up the hybrids, then. Are they in any kind of weird fragile package? Can't really see under those heatsinks

I have not had them off myself but from descriptions, I think they are just pressed onto the printed circuit board and held on by the visible fasteners at their cornerss.

Are the NVSRAMs particularly prone to failure? Sometimes it comes up with weird waveforms instead of those previously stored, I found that somewhat suspicious. Other settings seem to be stored normally otherwise.

The NVSRAMs have a rated operating life of 10 years.  Mine lasted almost 20 years.  Not all of these oscilloscopes used NVSRAMs however.  Older ones used a lithium backup battery and low power SRAMs.

The two NVSRAMs (or SRAMs) are located on the bottom of the processor board which is the top board which tilts from one side for easy access.  Inspection will reveal if a battery and SRAM or NVSRAMs were used and the date codes on the NVSRAMs if present will be visible.

If the calibration constants stored in memory are lost, it is not a huge deal.  External calibration can be done with a modicum of lab equipment; it just requires some accurate voltage levels and a fast rise or fall pulse; the sync output on a function generator is good enough.  For non-critical applications the external calibration can be ignored.

Also, do not operate this oscilloscope with the cover removed *without* air blowing across the hybrids on the bottom circuit board.  Otherwise they will overheat!

[george.b]

Which other capacitors did you replace?

C282 and C284, which seem identical in function to C181 and C182 - the latter are in the vertical vector generator section, and the former are their horizontal vector generator counterparts.

The NVSRAMs have a rated operating life of 10 years.  Mine lasted almost 20 years.  Not all of these oscilloscopes used NVSRAMs however.  Older ones used a lithium backup battery and low power SRAMs.

Aha, mine seems to be an older one, then. There's a lithium battery, which measures at 3.7V, so I guess that's not a worry for some more time.

I have not had them off myself but from descriptions, I think they are just pressed onto the printed circuit board and held on by the visible fasteners at their cornerss.

Yeah, indeed they are. I cleaned them up with IPA and cotton swabs, to no avail. I started swapping them around - lo and behold, CH1 went normal and CH2 got weird. Seems I got a faulty U440 (peak detector). Now to hunt for a replacement...

Thanks for the help! I will replace the power supply output capacitors at some point, as a matter of preventative maintenance.

[David Hess]

Which other capacitors did you replace?

C282 and C284, which seem identical in function to C181 and C182 - the latter are in the vertical vector generator section, and the former are their horizontal vector generator counterparts.

So you did not mess with any of the power supply output capacitors.  The capacitors you changed should not affect anything except the display output circuits and they will probably run well even without the capacitors.

The NVSRAMs have a rated operating life of 10 years.  Mine lasted almost 20 years.  Not all of these oscilloscopes used NVSRAMs however.  Older ones used a lithium backup battery and low power SRAMs.

Aha, mine seems to be an older one, then. There's a lithium battery, which measures at 3.7V, so I guess that's not a worry for some more time.

The oscilloscope has a low battery warning function.  A measurement of the battery voltage may reveal if it is approaching the end of its life.

Yeah, indeed they are. I cleaned them up with IPA and cotton swabs, to no avail. I started swapping them around - lo and behold, CH1 went normal and CH2 got weird. Seems I got a faulty U440 (peak detector). Now to hunt for a replacement...

It is not unknown for these to apparently fail although the same symptoms can present if the CCD clock driver signals get out of calibration.

Thanks for the help! I will replace the power supply output capacitors at some point, as a matter of preventative maintenance.

Changing the power supply capacitors without need runs the risk of damaging something so I do not recommend it unless you are comfortable doing this kind of work.

[TheDefpom]

This discussion should really be under a thread for the 2430, not in the 2432A thread, in case it confuses people about references made to parts.

[David Hess]

This discussion should really be under a thread for the 2430, not in the 2432A thread, in case it confuses people about references made to parts.

These oscilloscopes all use the same basic design and especially so with what we discussed except for the non-volatile memory.