Quoted from urbanledge:
Try changing the swich matrix ICs.
The socketed ones just below your NV ram .
If you put 12v down the switch matrix then will blow.
Its been an age since I did any fault finding on B/W games (or any games at all!)
You were right! I threw the $3 replacement chip in there and she fired up like a charm! I still had to hunt down a burn fuse to get the flashers and toppers to work again, a broken wire in the Yeti motor, and am troubleshooting gremlins in the ball trough but she's 99% there!
Now that my machine is working again I feel comfortable unloading my spare parts including a few brackets that are out of stock everywhere.
Full video of the parts available below.