Electronics > Repair
Keithley 2002 repair help
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saturnin:
I am sorry, but "self test passed" is necessary but not sufficient condition to say that K20xx units work perfectly. As I wrote performance/stability verification needs to be done. This is especially true about nikonoid's unit which was affected by severe leakage due to surface contamination (0.5 uA!). Ideally, F5700 or similar calibrator should be used, but even hobbyist can reveal a lot issues by using a decent set of DYI standards.

If I were at nikonoid's position I would check long-term stability of all DC and ohm ranges (because they have tightest specs) before declaring the unit to be close to perfect state...

I would like to add it is not so rare a DMM has an issue although it passes its self test. Let me mention two examples I solved some time ago:

1/ K2000 - very clean unit, never repaired before, it passed all built-in tests, at first glance everything was ok. During stability tests, I found out there were glitches in readings (see K2000_noisy_test3.png). Reason? A zener in an auxiliary power supply was dying...

2/ K2010 - very clean unit, never repaired before, it passed all built-in tests, very stable and quiet on DCV ranges. After some time, I realized there was a drift on all <10 MOhm ranges. (see K2010_ohms_drift_issue.JPG). Reason? Slowly drifting TF-245 resistor network...

That's why I don't exaggerate relevance of built-in tests...
Zucca:
Thanks saturnin, very interesting, yes you are right.

Now it looks like you have more experience than me, quick questions: how many devices you saw passing self test and have no problem then? More/less then 90%?

Thanks!
nikonoid:
I might have overstated the "perfect" state.


Saturnin, K2002 self test conditions are very wide. I think some of them are as wide as 3% accuracy, so I am completely aware that passing self test is not guaranteeing anything. My assessment of the condition was based on measurements outside of the self test. I got it to provide believable, relatively stable readings, considering absence of shield and case.

Questions of calibration, time stability and especially temperature coefficients still need to be addressed. My other K2002 is @CalMachine, for low level calibration, so it will be fun to compare them against each other when it is back.

TiN, I wish I had 5700....     However I can check stability and consistency even before going for low level calibration. I am also quietly experimenting with a DIY temperature controlled chamber.

Zucca, I did use 50/50 mix. My previous attempts with 99% IPA were not as successful. By the way, my original K2002 was sometimes failing self test. In the end it was MAX326 chip that sometimes worked properly and sometimes not. So I actually share the saturnin's concern. Self test, especially if it ran just once, does not guarantee anything.


saturnin:

--- Quote from: nikonoid on October 17, 2017, 03:33:20 am ---Saturnin, K2002 self test conditions are very wide. I think some of them are as wide as 3% accuracy, so I am completely aware that passing self test is not guaranteeing anything. My assessment of the condition was based on measurements outside of the self test. I got it to provide believable, relatively stable readings, considering absence of shield and case.

--- End quote ---

That's my point exactly. Sorry, it seemed to me you relied on results of built-in tests only. Thanks for posting updates on the actual status of your repair project! Hopefully, you will get fully functional and reliable unit in the end.
nikonoid:
After fifth, yes fifth, round of cleaning and baking I did measurements for two modes that gave me problems before:
1) AMPS with open inputs and
2) 100k Ohms in 2M and 200k ranges.

Both measurements were done from cold meter (off for 24 hours). I did measurements without top shield but I borrowed the metal casing from a donor K2001.

Amps took 45 minutes to an hour to completely settle. After that it would fluctuate from time to time by about 3ppm of the range.

Ohms took a bit longer - 60 to 120 minutes to settle.

Can someone try something similar with fully functioning K2002 from cold, so we can compare? Thank you very much.


Today I am getting a parts 2001 unit that will donate the case and top and bottom shields. I got it for $150 - cheaper than replacement parts.

PS. The score so far on this K2002: Two blown capacitors, one shorted zenner diode, 3 traces killed by a hole burned in the board, two traces eaten away to electrolyte, two bad vias, and LOTS AND LOTS OF CONTAMINATION!
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