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| Keithley 179A resistance measurement is slowly degrading, is there a way to fix? |
| << < (3/4) > >> |
| Wallace Gasiewicz:
The Hi Lo button switches the FET Q 104 or 103 on and offYou could check if they are OK |
| GigaJoe:
for 179 i had measure my , i really didnt use so current for: 2K 85.360 uA 20K 92.285 uA 200K 9.824 uA 2000K 0.989 uA for Keithley 175 for example # a differ completely: 200 , 2K - 811uA 20K 200K - 8.494uA M - 0.121 uA |
| TaylorD93:
there appears to be a full service manual available from Tektronix for the Keithley 179A dated Feb 1980. That states the output current for the Ohms ranges should be; "Hi" range selected Range - Current 2k - N/A 20K - 100uA 200k - 10uA 2000k - 1uA 20M - 0.1uA "Lo" range selected Range - Current 2k - 100uA 20K - 10uA 200k - 1uA 2000k - 0.1uA 20M - N/A Try replicating this test and see what values you read. |
| NoisyBoy:
I would agree with Kleinstein that you should check and clean the switches as well. It will be a quick check, as in my older Fluke bench DMMs, bad contacts in the switch could cause all type of weird behavior. Good luck. |
| Kleinstein:
The meter uses trimmers for the calibration and thus very likely no extra numerical cal constants. The current should still be a little lower than the nominal values, as there is some loading to the reference. So the readings from GigaJoe look about what is to be expeced. So the 80 µA could be correct. I no longer think K105 could be the culprit - in this case the input signal would not reach the amplifier and no resistor reading at all. The is a small chance that C107 is somehow damages (e.g. from cleaning). Really cleaning the switches is quite some effort. If it is the switches I would expect it to be somewhat changing with multiple operations, but this does not seem to be the case. I think it would be worth doing a few internal measurments. As usual first the supplies. TP5 would be interesting to check with a scope, as there is the possibilty that the amplifier oscillates in the ohm mode. The point where Q103 and Q104 meet could also be interesting when in ohm mode, e.g. with a short between the inputs. This should be a little lower than the 1 V / 100 mV ref. (e.g. around 850 mV / 95 mV) and higher with the higher ohm ranges. |
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