Originally Posted by unitron
A Metal-Oxide Varistor (variable resistor) is supposed to have very high resistance, so high that it reads as if it were "open".
That only changes when there is a "spike", that is, a very high difference in electrical potential, a very high voltage, between the two places where it's connected, which is usually across the AC line.
120V AC actually varies (60 times per second) from 0 Volts up to about 170 Volts and then back to 0 and "down" to about 170 Volts but with polarity reversed from the first half of the cycle.
Only if something, like lightning somewhere vaguely nearby, for example, puts a spike up above say 300 Volts or so on the line does the MOV suddenly become extremely conductive, essentially shorting that higher voltage to ground at which point the MOV becomes very "resistant" again until the next spike comes along. The spikes are much shorter in length than either half of a regular AC cycle.
Generally a spike isn't strong enough to push much current, but the much higher voltage will burn out semiconductor junctions and that's how it does damage.
If you get a strike near enough that you wind up with scorch marks, that's not what I'm talking about, and that's something that the MOV wouldn't have been able to prevent.
I have seen a MOV short out and since it's across the AC in I think it would be a good idea to remove it just for testing. I think he is going to have a hard time finding the short on this on. I had one that was popping fuses it was the Xformer shorted out and I have no idea where to find one. It would be nice to have schematic on this unit. By the way how have you been?