Electronics > Repair
Keithley 2002 repair help
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Kleinstein:
Usually the test for zero current would be with open inputs. For a classic solution with shunts this should not make a significant difference though - it can be a big factor for a configuration with TIA (for low currents).

The data look like quite some offset - it might be just a question of adjustment / calibration, but could also be a problem with amplifier offset or similar.  Before doing any adjustment / recalibration, the rest should be working of cause.

For the Ohms source, one could externally measure the test current.
TiN:
Any update? :) Can't want for next episode of 2002 repair.  :-DD
nikonoid:
TiN, thanks for checking on me. Yes. it had been a long saga.

I had been stumbling for a week, trying to figure out a problem with 2M Ohm range (Test 306.2). This is also the same range that I had a problem repairing first 2002.   :-DD

What I know so far is that on 2M Ohm range I get between 0.5uA and 1.8uA of current between Input High and Low, instead of 1.92uA. In many cases it starts at 0.5uA on warm up and within minutes would get to 1.5uA, then to 1.7uA and fluctuate from there.

When I check voltage drop across R271, I get 1.42V instead of 1.44V in manual - close enough and R271 being 750k it corresponds to correct current.
When I do the same thing across R322 - 33 Ohms I calculate the current that corresponds to what I have on the "Input High".

Somewhere in between we seem to lose some current. From reading the manual in between these resistors we have Q211, Q212 and Q213. My best guess is that either one of them is at fault or a circuit that controls one of them. I am having hard time tracing the circuit in that area.

Any suggestions are welcome. Thanks.
TiN:
I'd suggest to reverse ohm schematics, for our all enjoyment. Then it will be obvious what currents are expected.
nikonoid:
The process is slow. Attached images are as much traces as I was able to redraw so far.

Meanwhile I have some additional information. 306.2 error seems to be highly temperature dependent.
I plugged 100K Ohm standard resistor and used 2M Ohm range. When unit is cold it would start reading at 20k, then in minutes gradualy rise to 65k, in an hour the reading went up as much as 95k. At that point when I reran self test every OHM test passed successfully, including 306.2.
If I let the meter cool off a little bit is goes back to reading 80k, if it cools a lot is goes back to 20k.

So the issue appears to be stable, repeatable and temperature related.

I compared voltages to working Keithley 2002. The performance is identical for all pins on:
U220  DG411
U221 MC14094B
U223  MAX326
U228  AD711
U231  AD706
U232  MAX326
U233  AD705

Where I get the difference is:
U238 pins 2,3,5 and 6, respectively
0.787  0.787  0.788  0.859  on the bad meter
0.476  0.476  0.476  0.539  on the good meter

Q210
10.159  0.785  0.785  on the bad meter
10.005  0.475  0.474  on the good meter


Q211
12.792  10.198  0.787  on the bad meter
12.694  10.174  0.475  on the good meter

Q212
12.809  12.792  0.785  on the bad meter 
12.707  12.698  0.475  on the good meter

Q213
10.155 0.786  0.784  on the bad meter
10.011 0.475  0.469  on the good meter

Q214
0.000010  between (63mV and 125mV) 0.004  on the bad meter
0.000009   0.001  0.004 on the good meter   

The major difference is the Q214 transistor and 0.786 voltage instead of 0.475 voltage.

It is almost like performance on one of these transistors (Q210-Q214) is changing with temperature. I do not yet completely understand the role these transistors are playing.

Can someone please suggest a next thing to try?

Thanks a lot.
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