when my sons uses his amp the sound goes?

I would check out the power supply, it looks like it can't supply enough power when used. I would try replacing the large filter capacitors in the power supply.

David Hall
June 2008

It seems obvious that the fault is in the power supply stages of the amp. The fault allows enough power to light up the power light but little else. As soon as a note is struck, the amp demands more power from the power supply and forces it to its knees. That's why the power light flickers. The DC voltage rails in the amp will immediately collapse and that will cause the distortion. I would check out the power supply's rectifier first. If just one diode in this was dead, it would produce the symptoms you describe.

bwriter
October 2007

Use an oscilloscope to trace the signal path from the preamp to the power amp and note at which point the signal turns bad.
Your fault is the component between the good and bad signals. If that fails, check the decoupling.

Colin
September 2007