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| Keithley 617 Electrometer faulty behavior |
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| matthuszagh:
--- Quote from: alm on July 29, 2022, 08:42:32 am ---Looking at the resistor, it was the fuse :D. Though the parts list does not list it as a fusible resistor. F102 is supposed to be located on the inside of the rear panel according to section 7.3.2 of my manual. You could try spraying some contact cleaner (careful to keep it away from all the high impedance bits), and/or exercising the switch. Clearly whatever blew the resistor damaged the switch. I would be worried that whatever blew the resistor and damaged the switch might also have damaged something else. Though so far everything else seems to work fine. Do you see any blown traces or other damaged components, particularly between S103 and the inner shield on the triax connector? My guess is that someone connected an external voltage source between COM and the inner shield with guard disabled. --- End quote --- I don't see anything else in that region (or anywhere else) with visible damage. Using contact cleaner on the switch brought the resistance down to roughly the same as the other switch position. I also replaced the 100ohm resistor (apparently all this needed was a 5% 1/4W carbon composition resistor) and cleaned up the gunk. I couldn't get all of it, but I got the large majority of it. I reassembled the unit and have performed some basic current (using the DMM method you proposed earlier), resistance, and voltage (using the built-in source) measurements. Everything seems fine with guard on and off. I did run into a little difficulty at first that I think is attributed to the switch. The open resistance measurement was showing 100ohms, but switching it a few times brought it back to OL and it hasn't reappeared since. Any other tests you think would be worth running to ensure correct operation? The leakage current is presently showing about 500fA (pretty stable), which seems high, but I haven't given it the full warm-up time yet. I'll probably leave this on for a day or so and see how it settles. |
| matthuszagh:
After about half a day, the current leakage is sitting stable at about -550fA, which is about an order of magnitude greater than it should be. I'd originally thought it was only the electrometer board that I needed to be very careful not to touch, spread flux, etc. But, the manual says to be careful with the ranging amplifier section of the motherboard too, which is precisely where I was doing rework. I guess the next step is to perform the cleaning procedure described in the manual in that location and see if that improves the leakage current. |
| Kleinstein:
A relatively easy point to test is for a DC offset. A relatively large DC offset (e.g. 10 mV range) could also cause input leakage. |
| matthuszagh:
--- Quote from: Kleinstein on July 30, 2022, 03:13:31 pm ---A relatively easy point to test is for a DC offset. A relatively large DC offset (e.g. 10 mV range) could also cause input leakage. --- End quote --- If I place the electrometer in voltage mode and short the inputs I see a reading of about 0.1 mV on the 617 display. If I place the electrometer in current mode and measure the voltage offset at the input with an external DMM I see roughly the same. Did I correctly interpret the test you described? |
| Kleinstein:
--- Quote from: matthuszagh on July 30, 2022, 03:36:19 pm --- --- Quote from: Kleinstein on July 30, 2022, 03:13:31 pm ---A relatively easy point to test is for a DC offset. A relatively large DC offset (e.g. 10 mV range) could also cause input leakage. --- End quote --- If I place the electrometer in voltage mode and short the inputs I see a reading of about 0.1 mV on the 617 display. If I place the electrometer in current mode and measure the voltage offset at the input with an external DMM I see roughly the same. Did I correctly interpret the test you described? --- End quote --- That are 2 ways of measuring the amplifiers offset. 0.1 mV should be OK in at least not causing much extra input current. One may be able to adjust the offset even more, but I would not expect this to be needed. The offset is well smaller as the the about +-5 mV that can be applied to the resistor for the bias compensation. |
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