Noisy Variable 10K Resistor Remove Clean Check Reinstall
Using a wooden poking stick, chop stick, I found a variable resistor making a popping sound. The VR is part of the voltage regulator section of the acoustic 260 amp, it regulates 25 volts DC for the preamp boards and effects board. While it's not part of the signal chain in the power amp the parts are located on the final power amp printed circuit board. The bad joint or dirty metal to carbon track caused enough change to effect the output voltage.
I removed the resistor, pealed the case taps back, cleaned the metal parts, and checked the resistor using a noise generator. After installing I check the board using the same poking and tapping method I used to find the issue. The problem went away. What was causing the issue wasn't directly found.
Cold joint, or dirty pot, either way. It's working again.
Acoustic control corp 260
Resistor R120 10K linear pot. Mallory horizontal.