[volt-nuts] Update on 720A

[Chuck Harris]

Hi David,

It is amazing what gets done to equipment in the name
of repairing it...

I have trouble believing that the precision WW resistors
in the oil bath did go low.  I wouldn't presume to argue
that you didn't measure them right, but there has to be
something other than their value drifting low. WW
resistors usually go up and become erratic on their way
to becoming open circuit.

[I had a 720A once that did the same thing.  I got
 rid of it in hopes of getting a better example, but never
 did... And no, I didn't chop it up.  I observed that it
 wouldn't calibrate, diagnosed that it was something in
 the oil tank, and decided that I didn't want to go there
 at that time.]

I am thinking that the resistor leads have to leave the
oil tank some way.  Is it possible that the insulator on
whatever sort of feedthru has been compromised?

Otherwise it seems like it has to be something like
misdocumentation, or misalignment of a switch or something
like that.

-Chuck Harris

David C. Partridge wrote:
>> My concern is you seemed to be going at this repair like you were working on an
>> oscilloscope, rather than a super precision instrument.
> 
> More a case of: Oh my goodness how did this end up in this condition, and is there
> any chance I can bring it back from the edge.   No, I didn't use super precision
> resistors for the two padding resistors I inserted but I did take care to use
> metal film resistors with a low thermal coefficient (ppm/C).   If the values I've
> used don't need changing (which I think I will know soon enough), I'll get in some
> really good ppm/C parts (maybe from Edwin Pettis).
> 
> Yes, I know that the two 9.896k resistors sets that have gone low will probably
> remain suspect until they remain stable at their current value for a long while.
> 
> I wish I could afford a new 720A ... but if wishes were horses, beggars could
> ride.
> 
> Aside: I'm still struggling to understand how two sets of four precision 2474 Ohm
> WW resistors (9896 Ohm) in an oil bath can go low as they have in this instrument,
> while the remainder appear to be almost dead on specification.
> 
> Dave