Electronics > Repair

Solartron 7050 Multimeter

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Tenderfoot:
Hi folks.
I'm trying to calibrate this meter, which is working but not accurate.
However I'm confused by the first step (of many). If I'm understanding the manual it tells me to take a voltage reading with a 1k resister in series with a voltmeter, this makes no sense to me. Does it mean to put the resistor in parallel with the meter maybe? I'll attach a couple of screenshots. Many thanks for any ideas.

Kleinstein:
The adjustment steps should be there for the offset and than the bias current at the input.
The 1 K resistor may be useful to avoid capacitive loading to the amplifiers output. So the capacitor should be an the amplifier side. Than the 1 K make absolutely sense.

The 2nd step (RV4) is for the input current - not sure why they use such a small resistor ( 27 K) and not more.

Tenderfoot:
Thanks for your help. To be honest, what RV4 does is above my pay grade. Not that I need to know to adjust it.

Kleinstein:
RV3 is to trim the amplifier offset. RV4 is there to trim the LM310 input current compensation. RV4 adjusts a small voltage, slightly more positive than the output / input of the LM310 and the 22 M resistor than provides a few nA of current to compensate the input current.
Depending on how good the 2nd meter is that one has for the adjustment, one may want to use more than 27 K for the RV4 adjustment. I see no problem using something like 1 M and this way getting more gain for the trim step (e.g. trim to +-200 µV instead of +-20 µV and still get a more accurat compensation).

Tenderfoot:
Thank you for that clear explanation, I will try your 1M suggestion later tonight.

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