An odd fault, no audio out as though in mute mode. The user manual says
if in mute the volume knob LED flashes. What LED, nothing lit, just the
power LED and function LEDs.
The 2 thin wires that loop around the rotating pot motor shaft must have
snagged somewhere and tugged enough to bend up the pcb connector socket
and break a wire somewhere. If no LED load , the amp goes into
protect/mute? the next time the amp is powered up, but caries on as
normal until put in standby.

My ex-boss used to say "Don't look for elephants whern you might just have fleas". Kinda stupid but makes a point. Don't assume it is some wierdass problem that only me and three otyher techs on the face of the planet could figure ut, like the vertical height for a greyscale problem. (it was related to AKB and a replaced CRT)

Just troubleshoot it normally. It is not an old Fisher in which you don't get multiplex if the light blub burns out.

Just do what you would normally do, the LED may have been out for ten years before the actual fault showed up. If it's a reciever see if it recieves, does it say it gets a signal ? If you put in a signal does it show up on a scope at the volume control ? The tape outputs ? The PRE-OUTs ? (if any) If it's in protect mode, is there offset at the emittter resistors or the output of the STKs or whatever ?

Just do the usual. I have been fooled enough times tha tI now just check out pretty much everything. And while I DO want a statd complaint from the customer, I don't heed it. they are not only supid sometimes, some are even dishonest. Like bring in a TV and say the picture is shrink and you find out the ****ing CRT is broken.

Voltages, signal, waveforms. Don't let anything phase you.