Re-biasing a PA amp

Question

Have an MCM PA amp model MPA-120 used one hour/week at low power (about 20W) for 30 months.

Now it overheats in 35 minutes.

I have no expertise in audio, but am guessing the crossover bias is off. Found a pair of trimppots (VR3 and VR4) on the amp board.

Can I tune the bias by 'scoping the output and going for a linear crossover, or is a THD meter required. Or should I check the voltage across the gate resistors (R57, 57, 128,129).

Would appreciate advice, as I do not want to damage the unit.

Jan Cumps · Answer

PA amps aren’t designed for low THD. If you have fiddled with the bias settings, try to get a measurement of the current going through the output stage and turn the bias back as much as possible. Just above the no current point with no input signal.

Then listen. If too much distortion, you can increase the bias a little as long as you stay under the overheating point.

Those are the best measures I can give without service manual to indicate set points.