[volt-nuts] Update on 720A

[Charles Steinmetz]

Chuck wrote:

> The part that bothers me is the lowering of resistance.
>
> I don't believe that the contacts to the wire improved with
> overload, so that leaves shorts between turns, and changes
> in the bulk resistance of the resistance wire... maybe the
> difference between hard and annealed wire?

I'm not sure of the mechanism, but have observed many times that 
overloads can either raise or lower the resistance of precision WW 
resistors.  I'm quite sure it's due to changes in bulk resistance, not 
interwinding issues.

Note that we aren't talking about anywhere near 20%, or even 1% changes 
in value -- we aren't talking about "cooked" resistors in the normal, 
macro sense.  It doesn't take anywhere near the "rated dissipation" to 
knock a precision WW out of spec.  And note that they do NOT adopt a new 
resistance value at the same voltage coefficient of resistance ("VCR") 
and tempco that the resistor originally had -- both characteristics 
typically (in my experience) degrade by a factor of 10 to 10k more than 
the resistance change.  Consequently, you cannot rely on the damaged 
instrument to meet its VCR,  tempco, or linearity specs in future use, 
even if operated strictly within the its ratings.

> There appears to be a switch bank position that will put all
> of the power going into the 1.0 input out the LOW output
> through the two damaged resistors, R302 and R311, their
> trimmers, and R1008, and R1044, and nothing else.  I think
> It is 9.001.  It would, I think, require the calibrator to
> have its low terminal grounded, and the KVD's low terminal
> also grounded.

Interesting.  Good sleuthing, which suggests a very plausible theory as 
to how those two particular resistors were damaged.  However, based on 
the known condition of the instrument (including disconnected wires, 
etc.) I'd be very surprised if that were the only damage.  Anyone who 
did that probably did it (or other things equally bad) more than once. 
Connecting all of the low and/or ground terminals together is a very 
common cause of damage to precision potentiometers.

Best regards,

Charles