The large resistors in the lower right of the driver board are scorched, cracked or have come unsoldered; feature lamps may be out.
Burnt 27 ohm lamp matrix resistors R149-R156 on the driver board
Cause: The eight 27 ohm R149-R156 resistors run very hot when the software leaves feature lamps unstrobed in attract mode or when the CPU locks up. System 3 used 2 watt parts and later boards 3 watt; both are undersized.
Checks
- Look for discoloured board, cracked resistor bodies and cracked joints around R149-R156.
- Measure each one; it should read about 27 ohms.
Fix
- Fit eight new 27 ohm resistors rated 5 watts (ceramic or sand-cased wirewound).
- Mount them about 1/4 inch above the board for airflow.
- Use #47 bulbs for the feature lamps to cut lamp matrix current by roughly 40 percent.
- Or fit MOSFETs in place of the TIP42 column transistors (see the separate entry).
Systems
Related tips
- Replace burnt 3 watt lamp resistors with 5 watt
- Fixing the burnt lamp matrix resistors R149-R156
- Physically blown 7408 on the driver board
- Solenoid fuse blows at power-on and solenoids fire: driver board logic chips
- Outhole and saucer won't kick out: cracked joints on the driver board pins
- Test a 2N5060 lamp matrix SCR with a meter
- Test a 2N6427 (MPSA14) lamp matrix transistor with a meter
- Kicker coil locks on again after a new TIP122: dead 7408 gate on the driver board
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)