What could be the causes of intermittent output signal of audio amplifier?

Answer 1

Any number of things can cause intermittent output, but the most common are bad solder joints or bad sockets, bad switches or controls in the signal path, or, much less frequently, a broken capacitor or resistor that is letting moisture in. Examine solder joints and sockets on power components first; the heat cycles that output transistors and power supply components go through will put more mechanical strain on the device. It can be very useful to use a 10x magnifier to examine solder joints. Switches and controls can be cleaned with tuner cleaner or contact cleaner, available in a spray can from nearly any electronic supply store.