[volt-nuts] Update on 720A

[David C. Partridge]

> 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