Do you hava a red light on when you switch on ? If not then check this.
There are 2 fuse on the power board. if either onne is blown then it could be the output transistors short Q14-Q17 . 2PNP/2NPN . Replace all four even if one is short cause they are connected in parallel.
Check the FET's Q22-Q24 in the possitive rail and Q19-Q21 in the negative rail. Even if one is short replace all 6 FET's . Check for 13v supply on the power board. Check for a short diode near the capacitor area.
Check the comapator IC BA-10393 2 nos if Pin no 8 and 6 measures less than 100 ohms either side. if so replace the IC ,the 13 supply will become stable.
Good luck ! let me know if you need any more detailed help.