@jea48 : The difference in ground voltage potential between the two halves of an ungrounded AC cord is usually trivial and not a cause of sudden hum!
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True. Except the OP said the hum sounded like a 60HZ hum and not that of a 120HZ hum. I assume the equipment has full wave rectification.
Also as a rule electrolytic caps don’t fail all of a sudden.
True if we were dealing with an AC balanced power system where both AC lines to ground were 60Vac each. We are not though. Or an isolated power system where both AC lines were floating above ground. But we are not. We have a hot to ground potential that measures 120Vac nominal to ground and a neutral conductor that is at ground potential.
The way the primary winding around the core of the equipment power transformer is wound is one lead of the winding is closer to the core than the other. The proper AC polarity orientation will yield a lower capacitance coupling voltage to the secondary winding than the other.
As for the voltage readings from the chassis to ground I have seen the voltage as high as 90Vac when the polarity is reversed. Current is quite low though.
(In reality you are actually reading from the chassis to the neutral conductor. The branch circuit equipment grounding conductor is connected to the main electrical service neutral conductor in the main service electrical panel. Or service equipment panel.)
I tried to find a wiring schematic diagram for the amps with no luck. Depending on how old the amps are it was not uncommon to install a capacitor between one of the AC power lines to the metal chassis of audio equipment back in the old days. In the 1970s Marantz for one installed a resistor from one of the AC power lines to the chassis. In either case the chassis voltage to ground would be lower if the capacitor or resistor was on the AC mains neutral conductor.
IF,.... a capacitor was used the cap could be going bad and leaking,passing, AC voltage to the chassis. In this case we may not only have an AC voltage but current as well.
It won’t cost the OP a dime to preform the AC polarity test, other that a little bit of his time.
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