I need some help on a System 7 CPU from a Black Knight. The game booted and played but coils intermittently locked on. Different coils at different times. First things first: replace the 40 pin connectors. I did that and the CPU will no longer boot. 0 and the LEDs are locked on. They don't flash and go out, just lock on solid. Replacing the 40 pin header is the ONLY rework I've done to the board.
Here's where it gets weird. Leon's test ROM runs fine. It also runs fine with a driver board attached (used two different driver boards, no issues). Leon's test ROM runs fine with the EPROMs in IC14, 20, and 26. Only when I remove the test ROM and put the game ROM in IC17 does it lock up. PIAs test good with Leon's ROM. RAM passes Leon's memory test. Yes, I've verified the EPROMs, see below.
Here's a synopsis:
-No corrosion on the board
-IC12, 25, 19 previously replaced. Work looks good and checks out with good continuity.
-I've tested the ICs that were replaced: IC 12 (7408), 25 (4020), and 19 (5101 RAM), all are good.
-I replaced the 40 pin header (This is the only rework I've done to the board). I can find no shorts or issues. I even removed again it to make sure I made no mistakes. No difference.
-I erased and reprogrammed the EPROMS. I verified they are good in another System 7 MPU.
-I've checked all traces between the four EPROM sockets (IC14, 17, 20, 26). I admit I haven't checked the unused sockets (IC21, 22).
-I've applied 5v to pin 40 of the CPU, no change.
-I've checked for shorts to ground on all IC legs
Figured I'd ask here before I tear into anything. I can't find anything amiss using my logic probe and Leon's test ROM. I have an oscilloscope but haven't broken it out as I'm not sure what to look for.