Hi Dave,

The 11ma loading is a threshold where over dissipation will
occur, letting the magic smoke out of the box.  The 720A won't
be physically damaged, but will be far from usefully usable
long before that point.  No load is the only acceptable loading
of a 720A.

The damage I am referring to is the damage done by you in your
attempt to diagnose and fix the 720A through faulty techniques.
A snip here, a solder joint there, and you can quickly, though
unintentionally, turn the KVD into a worthlessly unstable piece
of junk.

I know that the 720A is yours, and you can treat it anyway you
please, but if your intention is to bring it back to its design
specifications, you are going to have to follow the rules.  No
shortcuts.  The 720A is a bridge device.  It works in that
magical region where no currents are sourced or sinked by the
720A.

Typically, trim pots don't wear out.  They are accessed so
infrequently that they never get a chance to wear before something
else renders them unuseable... usually corrosion.  It can probably
be saved by a little cleaning.  The bridge balance pot may not
be so lucky, though.  I don't think it needs to be anything very
special.

-Chuck Harris

David C. Partridge wrote:
> Chuck,
> 
> 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.
> 
> Cheers Dave -----Original Message----- From: volt-nuts
> [mailto:volt-nuts-boun...@febo.com] 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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