A coil was locked on or keeps killing its driver transistor.
Coil overheated: check its resistance
Cause: Heat burned the enamel on the windings and they shorted together, lowering the resistance.
Checks
- Measure across the coil lugs on the lowest ohms range: normal coils read 3 to 150 ohms; below about 2.5 ohms there is an internal short.
- Try to slide out the nylon coil sleeve; if it is stuck, the coil has been hot enough to swell.
Fix
- Replace the coil. Thick power wire(s) go on the lug where the diode band sits; the thinner driver wire goes on the other lug.
Systems
Related tips
- Check the wire from the coil to its driver transistor
- A cracked coil diode keeps killing its driver transistor
- Check the coil diode after replacing a driver
- Check every coil resistance before first power-up
- Which way a coil diode goes
- Pop bumper or slingshot locked on
- Which way a coil diode goes
- A low resistance coil points to a bad driver transistor
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)