A new coil with a diode already soldered on is fitted and the driver transistor fails or the coil locks on.

Driver transistor dies after fitting a coil that has a diode

Cause: The diode is wired backwards, so it shorts the drive and destroys the transistor.

Checks

• Look at the coil: the banded (cathode) end of the diode must go to the lug carrying the +50 V wire.

Fix

• Either cut the diode off the coil, or wire the coil so the +50 V supply wire lands on the diode cathode.
• Replace the driver transistor if it has shorted.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)

Related tips

• Why WPC coil diodes are on the driver board
• Launch coil locks on and blows its transistor and fuse every time it is tested
• A coil stays energized
• Auto-plunger coil energized as soon as the coin door closes: coil shorted to its bracket
• Coil-not-working checklist
• Grounding test for WPC coils 1-8
• No 50 V coils work
• Checking the coil supply at TP6

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer