Luxman L-80

[PA-Pete]

I bought this unit at a repair shop and the it was on consignment. The problem is the unit keeps shutting off, not powering off but I hear a click then nothing. I took it back 3 times and the old retired Air force veteran says it plays fine. I get it home and it plays fine for 3 secs til I want to adjust the sound and then CLICK...nothing. I dont have a schematic and I am not taking it back because he complains about it was on consignment. So I will service it myself. I build tube amps and that is where alot of my recent education consists so I am willing to try and do it myself with help from my friends...you!
Like I said the unit will turn on and play for a few seconds then shuts off. Is there some sort of protection circuitry? Suggestion?