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 > > > _______________________________________________ > 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. > _______________________________________________ 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.
