I recently got hold of a 400 that's not in working condition (unfortunately it's also missing a bit of the case at the back next to where the RF lead comes out, seems to have taken a hit during shipping).
Basically there's nothing on screen (though it's not straight up noise) and no key-clicks, so I set about debugging the issue using the Field Service Manual. I got as far as the step for the black/grey screen chart that says to check for 12V on Pin 2 of the power board -> mother board connector (J201 from memory).
I do not have 12V on that pin, (the +5v and -5 pins seem good). The flow chart says to check 3 caps and a diode, the diode seems good and two of the caps have 12V across them which would be what I expect. One cap however, (C203?) is a smaller green type, wedgedbetween the big black sheild/heatsink and RF modulator. I managed to hook some wires around the legs of the cap to get my multimeter onto it, and on there I'm seeing a voltage of around 26V. I know it's a long shot but can anyone confirm if it's supposed to be that high or if that's an indication of the fault?