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. >

Chuck wrote: > 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? I'm not sure of the mechanism, but have observed many times that overloads can either raise or lower the resistance of precision WW resistors. I'm quite sure it's due to changes in bulk resistance, not interwinding issues. Note that we aren't talking about anywhere near 20%, or even 1% changes in value -- we aren't talking about "cooked" resistors in the normal, macro sense. It doesn't take anywhere near the "rated dissipation" to knock a precision WW out of spec. And note that they do NOT adopt a new resistance value at the same voltage coefficient of resistance ("VCR") and tempco that the resistor originally had -- both characteristics typically (in my experience) degrade by a factor of 10 to 10k more than the resistance change. Consequently, you cannot rely on the damaged instrument to meet its VCR, tempco, or linearity specs in future use, even if operated strictly within the its ratings. > 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. Interesting. Good sleuthing, which suggests a very plausible theory as to how those two particular resistors were damaged. However, based on the known condition of the instrument (including disconnected wires, etc.) I'd be very surprised if that were the only damage. Anyone who did that probably did it (or other things equally bad) more than once. Connecting all of the low and/or ground terminals together is a very common cause of damage to precision potentiometers. Best regards, Charles