Before you begin your journey to explore possibilities of repair, open your top cover and bottom cover. With clean paint brush sweep off debris and DeOxit the unit and let it rest for half-day. Turn back on and see if problem is solved(you may get lucky!)
You can short high-current output of relay(find specifications of this or similar relay from major electronics catalogs such as mouser, newark or digikey) and see if your poweramp section turns on. If not, it's not discrete unit(only output stages are) and has bunch of logic IC boards that are far less durable than discrete devices. The way to test these boards or schematics or even simple block diagram are under the heavy secrecy of the manufacturer unobtainable by neither independent techies or casual buyers.
In my opinion such unit is never worth repairing besides the fact that it's sold for in working condition near $300.
If you go to HiFiengine.com, you will find most of great discrete vintage units schematics and service manuals. These units ARE serviceable and can be brought to PERFECT operating condition.