[Elecraft] Humming KPA500

[Dauer, Edward]

KPA500 started a new routine today after several years of perfect operation.  It emits an audible hum at a frequency of 60 Hz with a rise and fall time of 3 to 4 seconds.  It’s just loud enough to be annoying.  It continues whether the front panel power switch is on or off.  It ceases only when the rear panel power switch is off.  Neither the temperature of the amp nor the duty cycle have any no bearing on it, as the noise persists even when the amp comes on from dead cold.  Anyone heard this in a KPA500, or have any ideas about it?  Is something expensive about to happen?

Ted, KN1CBR