Electronics > Repair
Keithley 2002 repair help
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TiN:
I'd remove DG411 near diodes, remove two diodes (big ones at right), remove 1Meg resistor in metal case, clean everything thorougly with IPA.
Once you done cleaning, dry it. Clean again. Then carefully put new DG411 in, clean 1Meg resistor, clean diodes, put them in.
Test again :) If same, buy JFETs and transistors to replace Q210-Q214, and iterate again.
saturnin:
Before starting replacing parts using "try and error" approach (which you can use as a last resort anytime), I would suggest to analyze how ohms circuitry works actually...

In K2002 Repair manual, you can find quite detailed ohms circuitry description (see info on tests 304.1 and 306.2). You have already tracked multiple connections on the PCB, so put these information together and try to draw a partial schematic of ohm circuitry. Then, it should be much easier to understand how it works and find error...
Kleinstein:
Even if the voltage reading seems to work, it might be worth checking for input bias, as too much input bias could be an issue that can also effect the high ohms readings.

Having it working better when hot, somewhat points to a problem with dirt / electrolyte / flux residue. This would be one of the very few cases where current leakage would decrease when hot (as humidity will go down).

The suggestion from TIN is thus not so much about replacing parts, but cleaning in a area that might be influenced be leaking caps. A new DG411 is just more convenient / easier than a really good cleaning of a part that might already be damages from unsoldering.

A little more understanding of the Ohms circuit would really help. Having an idea from the manual could help a lot here. One would at least know if the control of the FETs should be OK.

As the 2 M range is off so much, a leakage current of that size should be visible in the 200 K range too. Not that obvious, but still well higher than uncertainty specs. One could at least check if the missing current flows through the resistor for the 200 K range.
nikonoid:
Guys, thank you for the feedback.

I think D411 next to metal 750K resistor is actually part of current measurement, not resistance. I will try to verify it. The actual board is much cleaner now than what you see in the picture. I even have a replacement part for it. However it was so close to precision network, I was not sure if I can remove it without accidentaly touching it with soldering iron. I may have to try if everything else fails.


I just ran few more tests. My resistance standards are not the greatest, some are +/-100ppm, some are +/-20ppm.
2k range off by 30ppm
20k range off by 150ppm
200k range off by .25%
2M range off by 2.5% this is after 1 hour warm up. It is much worse before warm up.
20M range off by 30ppm
200M range off by 30ppm
1G range off by 40ppm

Seeing that performance improves at 20M, I doubt it is leakage that is responsible for the problem.

I also checked bias voltage with 10M resistor and got -0.3mV

I see the pattern with two parallel diodes pointing in opposite direction is used in at least 3 places:
CR210, CR211   (by that DG411)
CR212, CR213   (by large rad COTO relay)
CR214 - actually 2 diodes in one case (by large rad COTO relay)

I think it is used for some sort of voltage clamping. Would someone know what exact use is? Thanks.
saturnin:
Both 200k an 2M ranges use the same setup of the ohm circuit. The only difference is 7.2V appears across R277 (=750 kOhm) for 200k range, which gives 9.6 uA and 1.44V appears across R277 for 2M range, which gives 1.92 uA (see description of 306.1 and 306.2 tests). According specs these currents have absolute tolerance of 5%. R277 has 1% tolerance, so voltages across should be 4% close to nominal values (this is a rough estimate).

I think the cause both 200k and 2M ranges are way off is the same. Do 200k measurements drift upon warm up as 2M measurements do? Since high ohm ranges (20M and above) are basically spot on, I would not suspect surface contamination by electrolyte (it would have to be very localized to affect only these two ranges).

At first, I would check stability of voltage across R277 for 2M range  (has to be very stable). Is it stable or does it change? If it is changing, does the change correlate with change in 2M readings? From R277 I would follow path of the constant current and draw a partial schematic to get ideal where the current could be leaking...
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