Good luck on running this down. I'm facing a similar switch matrix issue on my Bally Playboy and I've done much of the replacing / troubleshooting that you've done with no luck. Mine started after the game was moved cross-country so I'm thinking that something moved and shorted or that something on the Alltek board came loose. I'm considering sending my board back to Alltek to have them check it out.

I've thought about doing that but if I have some sort of short that took out the Alltek board I hate to kill a good one.

The upside to my problem is that I got a chance to learn about a new aspect of pinball repair. The electrical engineers back then were really creative to minimize the amount of playfield wiring and utilizing the limited computing power that was available.