Hello:
Most common failures:
1. Normally closed switching jacks on the back panel (any that say "in").
2. Inter-connect cables between circuit boards.
3. Broken solder joints.
4. Bad potentiometers.
Intermitant problems are rarely caused by component failure. A good cleaning / service, inspection for bad connections and broken solder joints fix 99% of bad units.
The power board is either a 20-0135 or 20-0166. You will find schematics for these in the manuals for other products (20-0135 = BETA & 20-0166 = SA20-21). I do not have a schematic for the mix board.