I had a look at some of videos, much later model, 1986, than mine, which has 1979 date codes internally. Thing is that the psu assembly can be first level verified with no other cards in the box. Verify the voltages are correct and even dummy load the rails to make sure. In particular, if the power amp is faulty, that can kill the psu board, so taking it an assembly at a time can save a lot of time and effort. Take all the mainbox boards out and check for shorted tants (yes, a lot of them :-). Get the psu right first, then put boards back one at a time, far left reference board first and verify the voltage looks ok. Then the rest, power cycle between each. Leave the power amp until last and set the slide switch to debug, as it can be checked for stability in that state. Also, the other parts of the system can be checked out by removing the power amp and fitting a jumper. Buying a bad one can be a really bad idea, if you don't have at least one spares donor for card swaps and robbing for spare parts. Far more complex beast than it looks and there are loads of dependencies and gotchas for the unwary. Lot of volts as well...