You have been playing with the rectifier board that supplies ALL power to everything else in the machine.

I think it's wise to get out the schematic and your meter and make sure that ALL voltages are coming from the power supply as intended before going much further.

It won't be much use fitting a replacement MPU f the existing one isn't being fed the correct (or any) voltage from the supply. The new one would also NOT work just as well as the board that's fitted now.

It's pretty simple wiring but not terribly well annotated on the board and the wires loosely soldered to the underside of the board from the transformer don't help matters as far as identifying things.

If you identify and work through each set of wires you should be able to confirm correct connection.