Once again I need help from the pros. I'm working on a Williams System 4 MPU. Not sure of its origin, I'm just working through a stack of bad boards. No obvious corrosion damage on this board. On this one I cleaned all of the header pins, de-soldered / re-soldered all of the headers, replaced the chip sockets at IC20, 17, 14, & 19, burned and installed new flipper & game (Gorgar) EPROMs, installed an NVRAM, removed the battery holder, and fired it up on the bench with a good driver board. The LEDs flashed so I stuck in in a good Gorgar. Here's what it does in Gorgar: Powers up normally and goes into attract mode (kind of, attract mode is missing a lot of the lights and I'm not sure that the lights that are working are flashing correctly). You can add credits and start a game, you get the "Gorgar speaks" callout, then the game goes back into attract mode. It gets stranger: If there's a ball in the ball trough it won't go into attract mode. A couple of the playfield lights strobe rapidly, only the player 1 and player 3 displays are lit, and they are both kind of scrolling zeros for lack of a better description. The switch for the center middle rollover causes the same symptoms. Here's a link to a video of what it's doing:
One more symptom: Flexing the MPU board on the right side will cause the game to lock up or reset. I know, that one should be easy to track down, but so far I haven't been able to. It seems to be an issue with the upper right quadrant of the board but I haven't been able to narrow it down more. No obvious cracked or cold solder joints, at least none that I've found. Also all diagnostics run correctly. All of the controlled lamps work, all switches (including the two that cause the weird symptoms) register correctly. BTW those two switches are not in the same row or column. One last thing, the fuse for the logic voltage has blown twice while I've been working on this, once when I was flexing the MPU and once right after powering the game up after installing the MPU.
Here's what I've tried so far: Swapped in known good EPROMs, NVRAM, PIA, and processor. New sockets for PIA and processor. New socket for IC13 (had been previously replaced, badly). IC5, 16, &26 are also socketed but I haven't replaced those sockets (yet). None of the sockets I replaced were RM, or seemed bad. The good news is that I haven't caused any new problems , bad news is it's doing exactly the same thing. Anyone have any ideas where to look next?