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34410A Ohmmeter problem

Question asked by Tardz on Mar 11, 2017
Latest reply on Mar 15, 2017 by tomc

Hi,

I have a problem with my 34410A: everything is OK except Ohmmeter.I tested with 0.01% resistor, 100, 1k, 10k, 100k, 1Meg and 10Meg and used my 34970A for DC current measurement (equivalent to 34401A).
If I execute TEST ALL, I have error 612 (500nA current source FAIL), 613 (DC high voltage divider FAIL) and 621 (pre-charge amp zero FAIL).

I verified current source in the ohmmeter circuit with result below

• 100 Ohms range seem OK: current source was 0.997mA (nominal 1mA)
• 1K range seem OK: current source was 0.997mA (nominal 1mA)
• 10k range near OK: current source was 100.7µA (nominal 100µA)
• 100k range not OK: current source was 11µA (nominal 10µA)
• 1Meg range not OK: current source was 6µA (nominal 5µA)
• 10Meg range not OK: current source was 1.56µA (nominal 500nA)

In fact, I have 1µA too, maybe on all range, but effect appears only on higher range resistance.
I supposed this leakage came from opamp U300 AD706JR, but after replacement, problem was still there.
I don’t know the part number of JFET include in current source (Q313), maybe a 2N5461 (obsolete part), but I'm not sure. 

Somebody may help me about this problem... Ohmmeter circuit on page #4 of PDF

 

Thanks.

Regards

Yves

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