1/2A slow blow, right?
If we already checked for shorted rectifiers in the power supplies and shorted output transistors. AND we checked that none of the power supplies are shorted to ground, then start at the start. Isolate the problem. In the schematic I have the transformer has a couple red wires plus center tap for the 22v supplies and a blue wire for the high voltage. They connewct to the board at terminals X1, X2, X3. Disconnect them and see if the fuse holds. If the fuse still blows with them disconnected, you either have a bad power transformer or a short in the primary wiring. I suppose a shorted pilot light assembly or a shorted 0.01uf cap across the primary could be bad.
If the fuse no longer blows, then connect the blue wire back up. The red/yel center tap needs to be there too, you didn't need to disconnect it anyway. Now does it blow fuses with just the blue wire? If so, then the HV circuit has some issue. Otherwise, move on to reconnecting the red wires, just one at a time.
In the power amp, are the two fuses there? Are they OK? Pulling them ought to disable the power amp, I;d think.
Your basic supply is a pair of 22v rails, but there is also that IC making 15v out of it. Schematic shows a house number. Is it a 723 chip by any chance? I see its 15v output going to several circuits plus "white wire to preamp" at terminal 19. is that shorted to ground?