| Electronics > Repair |
| Keithley 2002 repair help |
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| Kleinstein:
The slightly annoying part is not having a schematics. But this is now there for the Ohms part :-+. The only thing that might want a recheck would be the voltages around Q251. The protection part so far looked good. And just for the DC part, it can't be the reason for the current through the 75 K resistor. Worst case Q251 would see some 7 mA (don'T know the extact max current) at 24 V , thus about 150 mW. So it could get a little warm, but no too bad and it won't help that much moving some of the heat to VR216. |
| nikonoid:
--- Quote from: TiN on September 21, 2017, 04:04:59 am --- --- Quote ---This comes to be the most difficult repair I ever attempted. --- End quote --- I bet you learned a lot already as result. :-+ If you have more schematics works, I'd be happy to digitize as well., :-DD +38.6 sounds fine to me, I saw up to +42/-42 few times. It depends on line frequency/voltage a little. --- End quote --- I added few small details to schematic. I am trying to collect more of them before passing them to you. Meanwhile, if you or anyone else has any pictures of 2002 analog board with components off it, please send me these pictures. Even if it is not OhmS section, if would still be useful. I am putting together a drawing for analog PCB, and sometimes what is happening under microchip or even resistor is hard to decipher. If I find time, I may also draw something for amps section, as it has problems on my meter too. PS. The design of Keithley 2002 reminds me a way how some software developers write code. They managed to squeeze performance that is close to 3458 into a space that is less than a half of it, if you take space for multiplexing cards into consideration. It is Smart, highly optimized and agile, yet very difficult to understand and troubleshoot. Sent from my iPhone using Tapatalk |
| nikonoid:
Another small report. I remeasured voltage at all the pertinent transistors. I did it twice, once on a cold meter and second time after a bit of a warm up. It seems consistent with numbers I posted previously: Transistors.JPG is attached. It seemed that behavior of cold unit vs hot is very consistent, so I measured the progression of Ohms measurements as meter warmed up. It was on 2MOhm range measuring 100kOhms. See KOhms_Drift.jpg attached. Horizontal axis is seconds. Then I measured amps on 2A range with open inputs, again starting from completely cold meter. See mA_Drift.jpg. It looks like these two problems could related. After all, the electrolyte spill from a location point of view affected both ohms and amps circuitry. Seeing that performance improved with a warm up, I let it run for 1.5 hours and then I ran self test and it passed completely this time, where before I got some ohms and amps related issues. I do not see much circuitry in common between amps and ohms. Do you think it is leakage on the board itself rather than bad components? I am considering replacing U234 (DG411) to thoroughly clean everything around and under it with IPA. Under similar considerations are diodes and 750k resistor. What do you think? Any other suggestions? |
| Kleinstein:
Leakage due to the spill is still a possibility. However this would be relatively localized around R272 and the U223 (the max326). I doubt that critical node would extend very far. It is more like there should be a guard trace around it - so maybe check that guard. Guard line are sometimes rather thin and might got damaged despite of the gold coating. The other point to look for would be AC noise - this could be a shield missing / disconnected and thus just 60 / 120 Hz hum or maybe an OP oscillating or to much noise on the supply. Reading the 20 mV of AC at Q214 - which is essentially a high impedance node, suggests that there is some AC floating around. The reading of cause depends a lot on the probes - so hard to tell if 20 mV AC is a lot or little. Except for the close proximity to the spill I see not much in common with the amps part. Especially the 2 mA range should not be that easy to disturb by leakage. So it might be worth looking at the amps part first - it might be easier to find the trouble there. An oscillating amplifier in the amps part might cause trouble with the ohms. |
| nikonoid:
Kleinstein, I did a quick check of guard traces. They look Ok. I will spend a bit more time on it tomorrow. By the way, my traces are led covered - no sign of gold flash. Maybe too old of a unit. Today I did a pretty thorough IPA cleaning with ESD safe Q-tip equivalent. I covered both amps and ohms section. Interestingly ohms did not improve at all, while amps improved significantly. See attached before/after charts. As the AC noise is concerned, it went down a bit on Q214 to 11mV AC. I am measuring between input LO and collector. I also measured AC noise on U225 (OpAmp in amps section) where I have a pretty ugly bodge (attached) and found them to be under 0.7mV AC. By the way what would be a proper point to attach ground of oscilloscope to? |
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