What a bad coil diode does
Every CPU-driven coil needs a diode to absorb the voltage spike, up to about twice the supply, when the coil switches off. A shorted diode blows the coil fuse. An open or missing one causes erratic play and resets, and can destroy the driver transistor or other parts.
Systems
Related tips
- Testing the diode on a coil
- Coil diode polarity
- Where the coil diodes are on System 11
- Coil-mounted diodes: replace rather than test
- Testing coil diodes on the auxiliary power driver board
- Checking for power at a coil
- Testing flipper coil diodes
- System 11 coil outputs
Source: Repairing Williams System 11 Pinball 1986 to 1990, Deel 1, Clay Harrell (pinrepair.com)