the receiver amplifier Sony K740P, has a very loud hum for all modes except Sterio, with the Trebel and base at 0. If I move off the 0 settings the hum comes in. If I try any of the quad modes bad hum. The controller board looks like the 5v and 3.3v supplies are fine. Could it be digital signal processor. If so where to buy a replacement controller? I'm an old Electronic Engineer, thanks

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