[volt-nuts] Update on 720A

[Chuck Harris]

Interesting.  The R302 and R311 that have lowered in value,
are each comprised of 4x 2.474K wire wound resistors,
and are each in the middle of a series string of the same
sized resistors.

I suppose one could over dissipate them, but being in oil,
that would be pretty hard.

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?

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.

It's kind of twisty and turny in the schematic, and I can't
spend a lot of time on it, but after a quick look, it looks
possible.

So, maybe it is simply abuse.

-Chuck Harris

Charles Steinmetz wrote:
> Chuck wrote:
> 
>> I wouldn't expect physical trauma to be the issue, more something
>> like a metal flash due to an arc, or tin whiskers... Something
>> like that.
> 
> The damage to the resistors in the unit was almost certainly caused by an electrical
> overload (the "trauma" to which I referred).  Or, more likely, to a number of
> overloads perpetrated by one or more clueless previous owners on more than one occasion.
> 
> The other damage (disconnected wires, etc.) was caused by someone hamhandedly
> attempting to diagnose or repair the original overload damage, or contemplating
> harvesting parts from a unit they knew was damaged beyond any practical repair.
> 
> The only cure is to replace all damaged resistors with new resistors of equal or
> better quality.  If they are in the oil bath, the complexity of any repair is raised
> exponentially.  Note that there are almost certainly other damaged resistors in
> addition to the ones that the OP has identified.
> 
> As a practical matter, an instrument with damaged (overloaded) resistors will never
> work properly again.  The only 720As I have ever seen that worked properly after
> damage of this sort were repaired by Fluke at astronomical cost.
> 
> Best regards,
> 
> Charles
> 
> 
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