There's a burnt out resistor.  I was listening to this, when I heard a small "pop" and it wouldn't transmit sound.  This does turn on, but does not function.  I took a photo of the burnt out resistor.  I looked up the cost, and it's $1.10 to replace.  I thought about fixing this myself, but I'd have to unscrew every board and then reassemble, which is beyond my skill set.

The knobs are about $15-$20 each, and the power supply is about $100, so it's well worth the parts.