Why System 1 coils lock on: the ground shift
Board ground and cabinet ground meet at a single point on the power supply. If that link gains resistance, driver board current lifts board ground; at about 0.5 V above cabinet ground the coil drivers start conducting and coils lock on and burn, taking their drivers with them. The ground modifications fix this.
Systems
Related tips
- Firing coils from CPU buffers Z6 and Z7
- Lamp test can hold coils on for five seconds
- A CPU controlled coil does not fire
- A shorted coil means replacing its whole driver path
- Many coils and lamps on, or no coils working
- Coils or driver transistors run very hot
- Chime coils used with System 1 must be 12 ohms or more
- What boards a System 1 game has
Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)