| Electronics > Repair |
| Keithley 2002 repair help |
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| macboy:
Just thinking... Monitor the control register outputs (which drive the various relays, analog switches, and FETs) while manually switching between 20k, 200k, 2M, 20M ranges. You will be able to determine which switches or FETs are used differently for the different ranges, and hopefully narrow in to specific FETs or switch input-output combinations only used for 200k and 2M ranges. Then look for leakage in or around those components. |
| nikonoid:
Today I measured 200k and 2M ranges for cold meter. For 100k resistor on 200K range I got stable 7.11670V across R277 (750MOhm) or 9.49uA - within 1.1% off the target - good. I got 295uV across R322 (33Ohm) or 8.9uA - 6.2% off target not so good. Resistor read at 96.8k For 100k resistor on 2M range I got stable 1.42343V across R277 (750MOhm) or 1.9uA - within 1.1% off the target - good. I got 35uV across R322 (33Ohm) or 1.06uA - almost 45% off target horrible. Resistor read at 55k instead of 100k. So far it looks like we have a proper current on R277 and loose it by R322. There are only few transistors inbetween. I also liked the idea with similarity of 200K and 2M ranges. Today I retested the meter cold instead of hot, here is what I got: 2K range off by 160ppm 20K range off by 2200ppm 200K range off by 3% 2M range is off by 40% 20M range is off by 30ppm So really the problem is not on the 2M range, but on every range below it too. It just gets smaller on lower ranges. It is just small enough on 200k that self test error is not catching it. Now this also reminds me of the current problem I have. When measuring 0 current I get: 200uA range: -000.0461 uA 2mA range: -0.013751 mA 20mA range: -00.03802 mA 200mA range: -001.7962 mA 2A range: -0.018315 A So far I can see one similarity all failing modes including current should have U221 pin 4 set as 1, and 20M, 200M and 1G have it set as 0. I previously could not trace what that pin controls, so there is more work to be done. As always suggestions are very helpful and welcome. |
| saturnin:
--- Quote from: nikonoid on September 01, 2017, 04:41:33 am ---Today I measured 200k and 2M ranges for cold meter. For 100k resistor on 200K range I got stable 7.11670V across R277 (750MOhm) or 9.49uA - within 1.1% off the target - good. I got 295uV across R322 (33Ohm) or 8.9uA - 6.2% off target not so good. Resistor read at 96.8k For 100k resistor on 2M range I got stable 1.42343V across R277 (750MOhm) or 1.9uA - within 1.1% off the target - good. I got 35uV across R322 (33Ohm) or 1.06uA - almost 45% off target horrible. Resistor read at 55k instead of 100k. So far it looks like we have a proper current on R277 and loose it by R322. There are only few transistors inbetween. --- End quote --- It seems at least the core of the constant current source is working properly. --- Quote from: nikonoid on September 01, 2017, 04:41:33 am ---So far I can see one similarity all failing modes including current should have U221 pin 4 set as 1, and 20M, 200M and 1G have it set as 0. I previously could not trace what that pin controls, so there is more work to be done. --- End quote --- If U221 pin 4 is not set according the manual, I would definitely check what input data go to U221. U221 might be faulty. Btw. Don't you mean rather pin7 (Q4 signal)? |
| TiN:
Is there any reason why you not willing to reverse engineer related circuitry schematics? I'd see that much more beneficial, than guessing correct/incorrect voltages/currents around parts. There are not lot of parts involved, so I'd expect that to be just few evenings effort. You don't need to trace all back to 4094's, just up to LM339's would do just fine. |
| nikonoid:
--- Quote from: TiN on September 01, 2017, 04:51:29 pm ---Is there any reason why you not willing to reverse engineer related circuitry schematics? I'd see that much more beneficial, than guessing correct/incorrect voltages/currents around parts. There are not lot of parts involved, so I'd expect that to be just few evenings effort. You don't need to trace all back to 4094's, just up to LM339's would do just fine. --- End quote --- TiN, the reason is simple.... I simply do not know how to draw circuits. I have never done that. It is on my TODO list, but so is many other things. I know how to use Photoshop, so I used that to reverse engeneer connections to the best of my abilities. If someone skilled at drawing circuits can work together with me on that that would be great. Anyone interested? Sent from my iPhone using Tapatalk |
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