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 _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
