Electronics > Repair
Keithley 2002 repair help
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nikonoid:
Thank you, Le_Bassiste.

After reading your post I decided to put the instrument back together and test. I did not bother to put push rods back in, since I will be disassembling it again.

The good news is that it runs ok. On 10 volt range it is about 15ppm away from my 34401a. I measured a 10 Ohm standard and got 10.000065 - not too bad either.

The bad news is that after being assembled it repeatedly failed the self test with 306.2 error: 1.92?A ohms source. This corresponds to 2 MOhm range. This range was not working properly even before recapping.

I used 34401a to check the current on the 2 MOhm range and it was fluctuating wildly between 2.8?A and 4.0?A. Then all of the sudden it snapped to 1.90?A and became stable. I checked the resistance measurement in 2 MOhm range and it worked fine. Ran self tests and they all passed.

I power cycled the meter and again everything passed.

This situation with an intermittent current source problem has me completely at the loss. I would appreciate any troubleshooting ideas. Thanks.


nikonoid:
Today I checked the base level noise and got 4uV. This did not improve after an hour of a warm up.
I used a copper strand with Q-tip to short the inputs. This seems too high.

My 34401a starts at 4uV but fairly quickly settles to 0.3uV.


Le_Bassiste, I think you reported something similar with your 2002. How did you get it resolved?
Is there something I can do to troubleshoot this? Thanks.
Muxr:

--- Quote from: TiN on May 08, 2017, 04:55:11 am ---Welcome to forums. K2002 is my second favourite meter  :).

--- End quote ---
What would be your most favourite meter? :)
Le_Bassiste:

--- Quote from: nikonoid on June 05, 2017, 02:58:51 am ---Today I checked the base level noise and got 4uV. This did not improve after an hour of a warm up.
I used a copper strand with Q-tip to short the inputs. This seems too high.

--- End quote ---

not sure as to whether you are talking about "noise" or "offset" here, so i assume that you mean "offset" :)
after cold start, (instrument was off for 24 hrs) my specimen needs about 2.5 hrs to fully settle from approx. 6µV to some constant offset value (set to DC, 200mV, 10NPLCs, no FILT, input shorted).
i'm not concerned about the constant offset, as that will be fixed, once the instrumet has got a calibrate/adjust run in a cal lab. also, that constant offset value is "rock stable", i.e., even after a continuous run for a month, that offset didn't change at all (well, it did change with ambient temperature, but always came back to same value for same temperature).
but i am wondering whether the amount of initial drift of several µV has to be considered too high, and might point to some hidden fault. otoh, the instrument passes all self tests.  :-//
so, would be great if you could post some of your offset values wrt time here  :popcorn:

as for "noise", have look here, where i posted some values that may be helpful for further performance verification of your 2002. note that i've used quite a small sample window of just 50 samples, so take them with a grain of salt. should be fine for a q&d performance check, though.
https://www.eevblog.com/forum/repair/keithley-2002-repair/msg1188312/#msg1188312


 


nikonoid:
Thanks.

I checked the noise and so far it seems around +/- .5uV and the offset is around -4.2mV even after warmup.


I am still trying to troubleshoot 1.98uA source and currently suspecting problems with either Q244, Q242 or CR217. Do you know what parts they are? Here is a photo of one of them.



Also here is a thermal shot of the meter after warmup. Surprisingly ADC is very hot as well as area around 3 electrolytics by display board (they themselves are cold - cold horizontal line in the lower right corner)




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