How a coil is switched on
Every coil and flashlamp has power on one lug all the time; the driver board fires it by grounding the other lug. The CPU tells a 6821 PIA, which drives a 7408 (or 7402) gate, which turns on a 2N4401 pre-driver and then a TIP120/TIP102 driver transistor that sinks the 28 V coil current. A short anywhere in that chain locks the coil on; the TIP transistor is the usual failure, but a shorting driver can take out the parts before it too.
Systems
Related tips
- Nearly every coil energizes at power-on
- Coils 1-8: connector, transistors, PIA pin
- A low resistance coil points to a bad driver transistor
- All coils or lamps lock on at power-up
- Coils 9-15 and coin lockout: connector, transistors, PIA pin
- Coil still locked on with good transistors and gate
- Coil locks on and new transistors don't help: a failed 7408 on the driver board
- Coil has power and wiring is good but still dead or locked on
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)