I totally get your point that ideally you should use any KVD in null-balance 
mode.  I do note however that you can load it up to 11mA without damage though 
I've not once gone anywhere near that (worst case insult has been about 0.1mA). 
  However it does also say that to avoid loading errors you should load with an 
impedance >= 1TeraOhm (10**12).

Clearly a 3458A on the 10V and lower ranges is merely > 10GOhm and will show 
loading effects.

So, yes, I do recognise I was "doing it wrong"!

I don't believe I've inflicted any damage, and have managed to restore it to a 
useable state from a fairly sorry condition:   When I got it, two oil bath 
resistors were *way* off value (202 and 100 ohms low respectively), and the S2 
shunt resistors were about 1.3 ohms high which was far enough off to prevent S2 
calibration.   There were were also sundry other problems like two open 
circuits in the final decade, a badly worn trim pot, and sundry wires broken at 
solder joints.

Ideally I'd like to replace the KVD "Bridge Balance" pot as that's a bit noisy 
but shudder to think what that might cost if I could even find one.

-----Original Message-----
From: volt-nuts [] On Behalf Of Chuck Harris
Sent: 08 August 2017 13:34
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Update on 720A

Hi Dave,

Fundamentally, any input impedance from your 3458 is going to throw your 720A 
out of wack.  the 720A is not supposed to have even picoamps of loading.... 
nothing, nada, zip.

I would suggest that before you damage your 720A further, you read the manual 
for the 720A and follow the instructions to the letter.  An HP3458A is not a 
substitute for a null detector and a voltage calibrator.

The calibration of a 720A requires that no current, none, zip, be drawn from 
the 720A on any port.  It can only be done by using a voltage calibrator, and a 
null detector, as a differential voltmeter.

Remember, and heed my words, *no* current may be drawn from the 720A.

-Chuck Harris

David C. Partridge wrote:
> 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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