[volt-nuts] Update on 720A

[David C. Partridge]

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