One CPU controlled coil never fires.
A CPU controlled coil does not fire
Cause: Open coil, blown fuse or broken power daisy chain, bad driver board connector or trace, or a failed driver transistor or chip.
Checks
- First measure coil resistance; replace any under 2.5 ohms before powering up.
- With a game running, measure DC at both coil lugs. Voltage on one lug only means the coil is open; on neither means a fuse or the supply chain.
- In the backbox, briefly ground the metal tab of its driver transistor with a clip lead (on the Q29/Q45 pair ground Q45, not Q29). If the coil fires, the wiring is good.
Fix
- Coil fires when grounded: test the transistor, buffer chip and CPU in turn.
- Coil does not fire: repair the driver board bottom-edge connector or a broken trace.
This check is done with the game on; keep the clip lead away from other parts and touch only the named point.
Systems
Related tips
- Firing coils from CPU buffers Z6 and Z7
- Why System 1 coils lock on: the ground shift
- Lamp test can hold coils on for five seconds
- A shorted coil means replacing its whole driver path
- Coils or driver transistors run very hot
- How many coils the driver board can fire
- Which driver transistor fires which coil
- How CPU-controlled lamps are driven
Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)