Without seeing the circuit its all guesswork... have you had the top off and checked for internal fuses ?
If the mains switch (the on/off switch) is connected to the incoming supply (rather than the amplifier having a standby feature and perhaps a relay to perform on/off switching) then you should be able to read continuity across the live and neutral terminals of the wall plug when the amp is switched on (and not connected to the mains of course). The resistance should be low, perhaps as low as a few ohms. If there is no reading, or if its very high, then there could be a thermal fuse that has failed within the transformer.
Other checks would be to confirm that internal supply voltages are present across the main reservoir capacitors.
Totally dead though suggests a basic problem such as an open thermal fuse (or simply an internal fuse)
If you look at page 6 of the service manual, there is one power transformer feeding two power supplies. I would start there, looking for an AC output from the transformer. My guess is the 1A6 fuse, mains fuse, has blown.