Diode-test the 7408 and 7402 gate chips
Steps
- Game off, meter on diode test.
- Red lead on pin 7 (ground).
- Black lead on pins 1-6 and 8-13 in turn: expect 0.4 to 0.6; under 0.2 means a bad chip.
- In-circuit readings can mislead; if socketed, test with the chip removed. With power on, comparing a logic probe on the suspect chip and a similar working one is better.
- A chip with a visibly bulged case is dead; assume its transistors are too.
Systems
Related tips
- Physically blown 7408 on the driver board
- Kicker coil locks on again after a new TIP122: dead 7408 gate on the driver board
- Diode-test the lamp 7408 and 7406 chips
- Coil locks on and new transistors don't help: a failed 7408 on the driver board
- Solenoid fuse blows at power-on and solenoids fire: driver board logic chips
- Slings and pop bumpers feel sluggish: backbox boards loose with missing mounting screws
- Slings and pops all burned at once: CPU lock-up from the 40-pin interconnect
- System 6 and System 7 driver boards can be swapped for testing
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)