Peavey keyboard amp--Hum takes over all operation

Question

I have an ancient Peavey KB100 (a solid-state, 75watt w/1-15" + piezo amp) which I've used for years (typical Peavey). Lately, after playing for a while, the amp starts to emit a loud hum. The instrument signal can be heard faintly in the background. Disconnecting everything and turning all the pots to 0 doesn't alleviate the problem. The hum remains, even with nothing in or on.Any ideas on what the prob is, or a possible fix?Thanks much.