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| 34401A ohms drift |
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| strawberry:
Cleaned front/rear switch but still -20ppm drift. Could that be some relay, current source fault? VDC stable to 1ppm ,using LM399 ref and probemaster testleads. What test leads for <1ppm drift |
| Kleinstein:
The ohms drift can be quite different in different ranges. With very high resistance (e.g. 20 Meg) 20 ppm drift may not be that bad. The reference should have not much effect in the ohms measurement. The requirements on the test leads are different depoending on the range: low ohms (e.g. < 1 K) would like low thermal EMF and care handling them, e.g. wait after handling the cables to get back to stable temperature. With higher resistance like >100 K the isolation qualitiy of the leads can become important. Still I doubt the 34401 would be really good in this range. The accurcy in ohms range can be low than in DC volts range. I would not expect better than 1 ppm stability from the 34401 in the ohms range - more like change to a Keithley 2002 or Fluke 8858 for this. |
| strawberry:
fixed transistor fault, 1..2ppm drift after warmup. |
| floobydust:
Which transistor? Guessing the PNP's Q203-Q210 MMBT6520 or the JFETs? There was an Engineering Change to the Ohms Current Source after S/N 3146A30500. I'm not sure why. R207 from 2k37 to 5k62 1% and CR203 from 5.1V to 4.7V zener. |
| strawberry:
I guess it is some clamp, placed J112, will order original or close to it will calibrate with vishay 0.005 resistor 2010 model |
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