Diagnostics for hum


My Celeste HT-3 amp has recently developed a hum in both the left and right channels. What are some basic (non repair shop) diagnostics I can do to help find the problem. I have removed the preamp, and the hum is still there with just the speakers.
bones45
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Sounds crazy to put a $.99 adapter on a $200.00 power cord, but I'll give it a shot. Do I need to screw the ground on the Cheater plug to the receptacle?
Well, I am rather relieved to say that the cheater plug didn't work. Any other options before the trip to the shop? If I can even find one in OKC.
Sounds like possibly a bad cap in common power supply. Not something playing around with lifting ground, etc. is going to solve.