i have a vintage Yamaha guitar g100b 212 amplifier powers up i get sound for 5-6 min then begins to distort and then fads to no sound. Power supply? and were can I get a replacement
i would look at the resistor that feed the power transistor. may have a bias problem or overheating causing it to distort
I wouldn't know how to do that any suggections
you would need to know basic troubleshooting and have a multimeter.if there is any hum it could also need filter capacitors.sorry i could not help more.
20 years repairing electronic musical instruments,amplifiers,speakers.