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| Keithley 179A resistance measurement is slowly degrading, is there a way to fix? |
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| LinuxHata:
Hello. Got this bench some time ago, working fine (just 7905 was busted but quickly replaced and everything is fine). But recently I started to note the following - resistance measurement is giving increasingly wrong (lower) results. Say if for 20K resistor on 20K range it says 19.15K (when resistor is exactly 20K, 0.01% tolerance), for same resistor on 200K range it will say 13.35K, and around 10K at 2M range. And these are going more and more inaccurate, as time goes by. Also, if I touch any of contacts during resistance measurements, results will become wildly different - like for that 20K resistor, it will say 68K and so on. So what might be causing this? I haven't done calibration according to manual yet, but I just want to get prepared, to know with what to start. Volts and amperes measurement are ok, no issues so far. Any ideas? |
| AVGresponding:
First thing to rule out is the test leads, the flexing over time can cause the wires to gradually harden and break, with physical movement making the reading unstable. If you can, try the leads in a different meter, and try different leads with the meter. |
| LinuxHata:
Yes, leads already replaced/tested. No change. |
| Kleinstein:
An unstable reading is nothing that a calibration can fix. One should not recalibrate a broken meter. Beeing out so much is a sign of a defect and not drifting out of calibration. To do a proper calibration one needs quite some instruments, even for only a 4.5 digit meter. It is somewhat normal that touching the DUT can effect the resistance reading, though usually down. A higher reading is unusual, but possible, e.g. with poor contacts or too much capacitance to ground that may make the resistance converter unstable so that it oscillates. Added hum could also be an issue. The switches in these old style Keithley meters tend to get kontact problems. So the question is if repeated switching of the ranges / function changes (often it makes things better if the switches or relays are the culprits) the reading. Another easy test is checking for excessive input bias current. So it the 200 mV range reading the same for an open input and a short ? Another point would than a visual inspection and check of the supply, inclucing a check for ripple. |
| lowimpedance:
--- Quote from: Kleinstein on May 03, 2023, 06:11:13 am ---The switches in these old style Keithley meters tend to get kontact problems. So the question is if repeated switching of the ranges / function changes (often it makes things better if the switches or relays are the culprits) the reading. Another point would than a visual inspection and check of the supply, inclucing a check for ripple. --- End quote --- My bet would be the switches too, I have had two K 177 DMM's with unstable readings that a switch clean and careful lube fixed after treatment. |
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