I have been working on a few System 6 MPUs for a couple Firepower machines I'm rehabbing and am having a similar failure across all of them (3 to be exact). So at least the issue is consistent across MPUs.
Each board will boot fine and go into attract mode without issue. I can go into audits and check coils/switches/etc. and everything is good. When I hit the start button, the ball will kick into the shooter lane and that's when the displays start displaying garbage, and shortly there after the board completely crashes. I've been able to duplicate this same behavior without power to the coils, so it is unrelated to the ball being kicked into the shooter lane or a coil firing. I have had this same issues across multiple (modified) MPUs and multiple Solenoid Boards.
When I depress the Diagnostic switch on the MPU it does show a PROM fault (Bottom LED is lit).
Per the Combo Rom instructions I have installed two 1n4148 diodes (band up) at jumpers J3 and J4, installed the jumper from from IC14 pin 21 to IC30 pin 14. Cut the traces on both sides of IC14 Pin 21 and verified that it is disconnected on both sides and installed the 4.7K ohm resistor across pin 20 and pin 24. Note: I have used 1/2 watt resistors as that's what I have on hand but that shouldn't matter.
New EPROMS, Sockets, etc. I thought maybe it was some faulty RAM and have changed out the 6810s and 5101 with the same results.
Maybe I'm missing another jumper someplace? The board pictured did have some previous work done but otherwise it's in good shape.
Usually these issue winds up being some silly mistake on my end which is likely the case with this as well but I'm currently out of ideas so any help is appreciated.