I saved this from a Grainger post. It will help!
Short the inputs, I use a pair of alligator clip jumpers, one for each channel. Attach your meter to the speaker leads. I put the meter leads through the holes in the 5 way binding posts and tighten the plastic part down on them. This is a hands free method to read the magnitude of the hum voltage.
Set your meter to AC volts, the lowest range if it has ranges. Turn on the amplifier and let it warm up for 15 minutes. If you are just measuring hum, this is what you have. Posting be sure to include the milli-volts (mV) reading.
Start moving the hum balance pot one direction. If the voltage goes up change directions. When you get it to lower find where it begins to climb again while moving in the same direction. You have just passed the null point. Now very carefully make minor adjustments to the pot to get the lowest number, milli-volts, you can on the meter. It is best if you use a 4 digit digital meter. This gives you the greatest accuracy.
The stock pot is wire wound and the limit of resolution is "one knuckle." That means one wrap to the next wrap of wire. They are good pots but don't have the resolution that a 10 turn pot has. The half way point between knuckles shorts two windings together. This is why I went to the 10 turn pots for my Paramours in the hum balance circuit.
BTW, I made RCA shorting plugs from a pair of cheap interconnects. Just cut off the plugs leaving an inch or so of the wire, Strip the wires and twist together to short signal to ground. Add shrink tubing if you want to get fancy.
Cheers,
Geary
