Last resort on a locked CPU: replace IC11 and IC12
If everything else checks good, the only remaining support logic that stops Leon's code from running is IC11 (74LS10) and IC12 (7408) in the chip-select circuit. Replace those two before changing other gates at random.
Systems
Related tips
- Damage after the black and white plugs were swapped
- Boot stops after the ninth LED flash: IRQ fault
- Background sound never stops at game over: sound-line driver chip
- Kicker coil locks on again after a new TIP122: dead 7408 gate on the driver board
- Outhole kicker locks on whenever the service menu is entered: bad 7408 at U20
- How a coil or flasher is switched: PIA to TTL to pre-driver to driver
- A coil or flasher comes on and stays on at power-up
- Which logic chips drive which solenoid transistor pairs
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)