Electronics > Repair
Keithley 2002 repair help
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Kleinstein:
The logical ground point for the scope is the negative (com) input. This could be either the plug or maybe a connected point on the board (it should not make a big difference at lower frequencies).

Unless very dirty a leakage in the 0.5 µA is quite a lot for just dirt. So it would need way more than just a missing guard. It would more like a dead short towards a guard that could cause that much leakage.
Even this would not match the measured voltages.

So I would either check the amps circuit first of look for AC (with the scope) on the supply and the critical points (e.g. outputs of U228 and U238) of the Ohms circuit. The control lines (e.g. U223 Pin 9 ) might be interesting too.

The strong drift in the amps part looks odd and could be easier to find. The circuit for the amps part should be relatively simple compared to the ohms part.
nikonoid:
After the last post, I tried quite a few things. Let me list them somewhat chronologically:

I did detailed review under microscope of guard traces and also checked continuity of them and possible shorts between them and microchips - nothing suspicious.

Then I did measure a Drain on Q214 for AC component with oscilloscope. I placed scope ground on Common test post that sticks up next to voltage reference under ADC. I see some low frequency and high frequency components, but I am not sure if this is significant, or I am just picking up some unrelated spikes. See attached.

I let meter to warm up for 3.5 hours and remeasured voltages at att transistors involved. See the Transistors1.jpg attached. Interestingly the meter drifted closer to be in spec, yet some voltages are almost double of previous measurements of good 2002 meter. Is it possible that from 1994 to 1997 they changed not only some microchips to their equivalents but also changed operating modes?

Today, being out better options, I replaced U234 (DG411) and two diodes next two it. When I removed U234 I noticed that R277 was pushed to a side and down (not by me) and basically laying on some traces and vias under it. I measured about 10 to 20MOhms between either lead of the resistor and its metal body. I did not like that for a 750k resistor. I used a small screwdriver to push the resistor higher. Afterwards the resistance between leads and the body became greater than 100M. I thoroughly cleaned the area around and reinstalled fresh U234 and diodes.

After U234 change and this last round of cleaning around U234, performance of the meter improved quite a bit in both Ohms and Amps. Another positive is that it now passes all self tests, even when cold! :-+

If comparing 1 hour warm up readings, it used to read 98.6k and now is reading 99.95. The phantom current on 2A range after 1hr used to be -950uA. After initial cleaning it went down to -337uA and now I get -23uA.

After 2 hours the meter is very close to be fully functioning. I get 99.985k that is no longer improving, +6uA reading on 2A range.

I am still not sure if I am dealing with bad component/components or dirty board. When I got the meter the board was dirty to the point that I was initially removing mice excrement and dead insects from it.

The temperature dependence of performance is quite puzzling. I am considering putting a gloved finger on microchips one by one, trying to find one that reacts to temperature the most. Also I can use cold canned air to cool different areas of the board.

What else can I try? How can I distinguish components trouble from board trouble?
Kleinstein:
The drain of Q214 is not a good test point: it is an essentially floating point in normal operation. So the readings are essentially noise picked up by capacitive coupling and DC is determined from non critical leakage currents. I don't really understand why they have Q214 at all - it should have very little influence. I would not be surprised if the meter would even pass the self test if it is missing or broken (shorted).

A suitable point to check for a superimposed AC signal would be more like drain (or source - should be essentially the same) of Q213. Another interesting test-point would be the emitter of Q251. Checking the supplies (e.g. +-15 V for the DG411 / max326) for spikes would be a good idea too.

For the current part it might be interesting to see if there is some AC residual at the terminals when in the lowest amps range. Also the measured DC voltage in this range might give an indication. There might still be some cleaning needed in the path (e.g. upper end of highest value shunt) of reading the voltage at the shunts.

I still can't see a path from leakage around R277 towards the R272, were the odd leakage toward ground was initially measured. So far I don't see a path coupling the the ohms and amps part for something like an automatic adjustment or just an extended self test. So it is odd to see an error in both parts that is so close coupled. For me this somewhat point's to an AC problem, of some noise / oscillation that couples to both areas.

Much of the different voltage readings for the good and bad meter around Q213 can be due to a different type of diode for CR217 (higher forward voltage for the bad meter). Such a change would not have a significant effect on the meter operation. Also slightly different levels of the reference voltage can change the readings (e.g. around Q251).

In TiN's circuit drawing, I just noticed the guard connected through R370 to U238. This would be rather odd. The more logical guard potential for the resistor part would be the output of U228. I would more guess this is an error in the plan: the output of U238 would be good for the guard around Q213, CR217 and connected, but not for the range resistors. There might be a second guard for the resistor part too.
nikonoid:
Kleinstein, I did a number of AC measurements today, as you suggested. Please see attached.

I do see spikes and sometimes even combination of several frequencies but I am not sure what is important and what is not. I noticed one strange thing, though: the power on U220 is +20 (pin 13) / -15 (pin 4), while U223 has +15/-15, as expected. Not sure if this is important. I checked and working 2002 has same power on these +20/-15 on U220 and +15/-15 on U223.

I also used K2000 to measure voltage between Amps and Lo inputs of broken K2002. I did this as both AC and DC voltage. They are in the last attachment here. Please note that K2000 has a 40uV AC offset with inputs shorted, so supposedly 40uV AC should be subtracted from AC measurements in attached table.

I do see at least two guards around ohms circuitry. I will look into tracing them in a bit.

Thanks a lot for taking a look.
nikonoid:
Kleinstein, you were spot on about the guards. There are at least 4 different guards in the ohms circuitry. I see one guard going from output of U238 through 10k resistor, I also found another guard from U238 though 100k resistor. That one covers Q213, Q210 and other nearby elements.

I also found that output of U228, as you predicted, through R371 (10k) resistor drives guard covering majority of OHMS switching including U220, U223, U232, U233 and precision resistors. and some legs of Q211, Q212. I am making some changes to my drawings and I am planning to pass them to TiN.

I also found a bit of circuitry that is conected to Q226, including precision divider R335, R339 and also involving compactor U239 and register U203. I am not sure though if they could be considered part of OHMS circuitry.   
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