Update: After changing the resistor I added in series with R1008 (A decade
position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is
slowly increasing in value back towards nominal) and re-calibrating again, the
A decade is now within 1ppm of linear (using the 10V range and nominally 10V
input).
I note that when I'd done this, with the A decade set to 0.9 (reading 9.000000V
on the 3458A), the actual input voltage needed was 10.000040 or so. Is that to
be expected?
Two of the positions on the B decade wouldn't quite adjust for a null when
calibrating it. In this situation I think I have two options:
1) Add series resistors to compensate for slightly low value 9.898k
resistors in the relevant two positions of the decade and recalibrate.
2) Adjust R203 to change the bridge balance slightly to the 10K ohm
decades and recalibrate.
Your thoughts on which approach to take is much appreciated.
Thanks
Dave
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