The two under-playfield diverter coils stay energized once they are first used, or lock on at power-up; auxiliary board transistors keep failing.

STNG: under-playfield diverter coils lock on during play

Cause: The auxiliary driver board on this game drives real coils and needs +50 V as its tieback (flyback) voltage. That feed comes on a thin violet/yellow wire from the lone drop-target coil near the top of the playfield; a break there or anywhere in that coil-power daisy chain removes it. A shorted TIP102 on the auxiliary board locks the coil on at power-up.

Checks

• Check the violet/yellow wire from the back drop-target coil to the auxiliary driver board, and the rest of that power chain.
• With the game off, measure each diverter coil in circuit: 7-9 ohms is normal; lower means replace the coil.
• Use the manual to find which auxiliary-board transistor drives each diverter. In-circuit transistor tests are not reliable on this board.
• Test the related resistors and 1N4004 diode in circuit, and buzz out the traces, especially the 50 V tieback trace.

Fix

• Repair the tieback wire.
• Fit 1N4004 diodes across both diverter coils, banded end on the power lug.
• Replace the TIP102 and its 2N4403 pre-driver together; replacing only one tends to kill the new part at power-on.

Systems

• WPC DCS

Games

• Star Trek: The Next Generation

Related tips

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• Which transistor drives which WPC solenoid and lamp circuits
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• Fliptronics: flipper will not hold up (flutters)
• Fliptronics II board components
• Lock diverter locks on at power-up and gets hot: shorted driver transistor Q22

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Two, Clay Harrell (pinrepair.com)