Are we talking the little board where the AC power cord plugs in? Those things often crack their solder, and then they arc, causing burn. QUite possible the varistor itself is OK. Or Thermistor. There are severa all in series. If one is actually damaged, just eliminate it and replace it with a jumper. The amp has plenty of inrush protection even if it is missing one of those. Yes, do grind away any carbonized board.