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
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