The PL400C board is a bit different. Let's take the Left Channel (left side of the board) first. The potentiometer at the top is R14, it is the Bias Adjustment. With the amp off connect a pair or alligator clip equipped wires across R24 (the 10 Ohm 1/2W resistor in each output bank) and then to your digital voltmeter. Turn on the amp and let it warm up for 15 minutes. With no inputs or outputs connected, read the voltage and adjust R14 to get 350mV on the meter.
You will then conduct the same test but connect the alligator clips across R41 (10 Ohm 1/2W) and verify the voltage reading is within 20% (350mV +/-70mV), so minimum 280mV and maximum 420mV.
The potentiometer on the left side of the board is R8, it is the DC Offset Adjustment. With the digital voltmeter connected to the left speaker outputs, adjust R8 for 0V or as close as you can get with the amp at operating temperature.
Repeat these tests for the Right Channel. The corresponding adjustment pots are just moved over to the right side of the board.
I hope this helps.
