After two wires were torn off the auto-plunger coil and resoldered, the launcher misbehaved (serving two or four balls, then none), and later the game hung on its loading screen. A replacement driver transistor failed again within minutes.
Auto-launcher misfires and transistors keep dying: coil wires soldered on backwards
Cause: The wires had been put back on the wrong coil lugs. With the coil's protection diode reversed it acts as a dead short across the supply when the coil is driven, destroying the drive transistor (and the diode).
Checks
- Check which wire is the power wire: it usually matches the wire colour on another coil fed from the same supply, and is often the thicker wire.
- With one wire lifted and the game on, measure each wire to ground to confirm which is hot.
- The banded end of the coil diode must go to the lug with the power wire.
Fix
- Swap the wires so the power wire is on the diode's banded end, and replace the damaged diode and drive transistor.
Games
Source: Pinball Repair help group!! (Facebook group)