Electronics > Repair
Keithley 2002 repair help
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Kleinstein:
Have you checked input leakage / bias in voltage mode ?
A jumpy test current could also be due to leakage at the input. The ohms current source is reasonably well protected and thus not that likely to show a defect. Input protection and switching is much more exposed to things like ESD.
Le_Bassiste:

--- Quote from: nikonoid on June 07, 2017, 06:34:03 pm ---
I checked the noise and so far it seems around +/- .5uV and the offset is around -4.2mV even after warmup.

--- End quote ---
looking good. mine is around 3.4µV offset at 23°C, hovering about 40 nV/°C


--- Quote from: nikonoid on June 07, 2017, 06:34:03 pm ---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.

--- End quote ---

 it's a siliconix J2611 JFET.
note colour dots on top. these are factory-selected and marked, so it might be better to not try and change/remove them, until you are absolutely sure that they've gone south.


--- Quote from: nikonoid on June 07, 2017, 06:34:03 pm ---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)

--- End quote ---

on the ADC, the CPLD can get quite warm, no reason to worry. same holds true for the three electrolytics next to the smal silver box. the temperature of that box is actually very high, but its nickel plating is fooling your TIC into false temp readings (because emissivity of metallic platings is much lower than that of the other things around the box).

did you try to slightly(!) knock with a popsickle stick on the board? can that provoke a jumpy ohms reading?
are the current jumps still noticeable when measuring a resistor in non-auto-ranging mode?
(i must admit, a little bit poking in the dark here  :-\

EDIT: did you read this?
https://www.eevblog.com/forum/repair/keithley-2002-repair-help/25/#msg1227932
nikonoid:

--- Quote from: Kleinstein on June 07, 2017, 07:08:39 pm ---Have you checked input leakage / bias in voltage mode ?
A jumpy test current could also be due to leakage at the input. The ohms current source is reasonably well protected and thus not that likely to show a defect. Input protection and switching is much more exposed to things like ESD.

--- End quote ---

How do you check for input leakage?

Also, I am dealing with two separate issues, one is 4mV offset with shorted input and this might be normal.

Another is non working 2MOhms range (selected manually) and 306.2 error. The current source sometimes is at 1.98uA and is stable as it should be and sometimes is as high as 7.9uA and floating up and down by a lot.




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nikonoid:

--- Quote from: Le_Bassiste on June 07, 2017, 08:26:47 pm ---did you try to slightly(!) knock with a popsickle stick on the board? can that provoke a jumpy ohms reading?
are the current jumps still noticeable when measuring a resistor in non-auto-ranging mode?

--- End quote ---

I manually set 2MOhms range. While in this range sometimes current jumps and someones is it stable. I have seen it change both ways without any mechanical interaction on my part. I will watch it a bit more. It almost seems thermal rather than mechanical. However I will try the knocking test tonight.


--- Quote from: Le_Bassiste on June 07, 2017, 08:26:47 pm ---
 it's a siliconix J2611 JFET.
note colour dots on top. these are factory-selected and marked, so it might be better to not try and change/remove them, until you are absolutely sure that they've gone south.
--- End quote ---

Thank you, yes I do see the yellow mark. Actually on Q242 and Q244. Now that I know they are JFETs they are less of a suspect.

Here is my test setup: I put the K2002  into 2Wire Ohms on 2MOhm range. I use Fluke 187-II in micro current mode (100 Ohm impedance) to measure current produced by K2002 between Input High and Input Low. K2002 shows 0.000100MOhm when working well or higher number when not. While monitoring the current, I use 34401a to measure voltages relative to Input Low of K2002.

One noticeable difference is on CR217. When working well one leg is at 304.3mV and another at 0.64mV and fairly stable. When not working properly one leg is still at around 300mV but fluctuates a bit, while another is at 1.5mV and fluctuates a lot.

Also, when working well very stable 12.7017 is seen on legs on Q211 and Q212 and in contrast when misbehaving I can see around 9v on these legs and it fluctuates a lot.

It would be much easier if I had schematics. Any help is appreciated. Thank you.



Kleinstein:
The changing voltages around Q211, Q212 might be a clue to the issue. However without a schematics, it is not that easy. There is a reverse engineered schematics for the ADC part around. Chances are the input stage is somewhat similar to the K2001.

Measuring leakage / input current is relatively easy. There are two options:
1) use a 10 M or similar resistor across the inputs in the volts (e.g. 2 V) range.
2) have a low leakage capacitor (e.g. 1 nF) across the inputs an watch is charging / discharging.
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