I have been working on this unit that came in with a fault where the speaker muting relay would click off after it had been running for a while. I first thought that it was a fault on the power amps that was triggering a speaker protection circuit. I was wrong. I eventually found out that it was a poor contact on the headphone mini-jack switch. When a headphone jack is plugged in, the main speaker outputs will be disabled. This is normal behaviour. The remedy for this fault is to first try squirting some switch contact cleaner into the jack and push a jack in and out a few times. If this does not work, the jack will need to be replaced.

Good luck!