I'm a newbie to receiver maintainance. I get a hum through any speakers I use only with this HK receiver ever since I bought it used. After 2 seconds on, the hum loudness drops to half strength and stays there; but its still loud. I've tried alot of configurations. Its there no matter what knobs I turn; and with nothing connected to it. FYI - there are 3 fuse knobs on the back, and a GND screw. Any simple things I can try? Thanks.