although i have not fixed it, i have learnt a few things which may help.
have on/auto/off switch in on position.
measure voltages on con4, specifically the +- 7v supplied by the bias supply, if the +7v rail is low (about 6V)then you have a problem on the amp pcb, in my case it was the micro ic pulling down the 3.3v rail causing the 3,3v regulator ic to current limit and the increased load drops the 7v rail.
i was too quick to blame the amplifier ic ( a circuit would have saved me the trouble).
hope you find it helpful. i cannot obtain a replacement amplifier in Australia
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