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409, 41x, 42x, 44x, 48x -
14x, 18x, 1801 Nanovoltmeters
15x, 18x Null Detectors/Microvoltmeters
I recently acquired the Keithley 610C analog electrometer. It seems generally in very good shape. I went through the calibration sequence as specified in the user manual, and everything checks out except the main meter cal. The meter calibration pot R179 (nominally 2KOhm) is at the high end of it's range and the meter still reads about 4% low on the 1V full scale voltage test. It is as if I had just 10% more resistance available in R179 , I could nail the calibration. There is another resistor R180 (500 Ohm) in series with R179. It is tempting to replace R180 with a little larger value to give a little higher total resistance in that branch. Do you see any problem with doing that?
You might be interested in the following data measured on a Meter ME-81 used in the model 602
(the meter circuit is the same as in the 610C):
100 Ohms, 1 mA at 100 mV full scale
With this data you will see that there is not much headroom left for increasing the resistors in parallel to the meter
(full scale current is approx. 1.1 mA).
Indeed if I calculated right just leaving out the parallel resistors would give an increase in reading of approx. 4 % - just
the amount you need. So please give it a try.
(Just a remark: my guess to why your meter reads low is that the magnet used in the meter suffered from aging.
Other faults might be possible - please first check the instrument measuring 10 V and see if it is 4% low again.)
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