The kings drop target bank never resets, in a game or in coil test, though the coil fires if its transistor is grounded by hand. A new transistor and coil diode didn't help.

Kings drop bank never resets: under-playfield transistor wired wrong

Cause: This coil is driven from a lamp driver output (lamp 17) through a heavy power transistor mounted under the playfield. The replacement transistor had its base and emitter wires swapped.

Checks

• Identify the transistor's legs from its data sheet: the case is the collector; the base goes to the wire from the driver board; the emitter goes to the coil side as the schematic shows.
• Check continuity from the driver board through any connectors to the transistor.

Fix

• Rewire the transistor with base and emitter in the right places.

Systems

• Gottlieb System 1

Games

• Joker Poker

Related tips

• Games with under-playfield 2N5875 driver transistors
• Joker Poker: under-playfield coil transistor
• A CPU controlled coil does not fire
• Coils lock on from a playfield ground pin
• Testing an under-playfield 2N5875 and its wiring

Source: Pinball Repair help group!! (Facebook group)