Unit turns on but has no audio. I checked the speakers with another amp and they are fine. I tested the source for output I am using on my other amp and it is fine. I tried all of the inputs including CD, phono, tv/vcr; but still no sound at all. Not even a hiss at full volume. I pressed every button on the cotrols on and off and still nothing. I removed the cover, but can find no visible faults or burns. Any ideas, please?