The two under-playfield diverter coils lock on after their first use in a game, and may burn out along with their driver transistors.
STNG: diverter coils lock on during play
Cause: The small auxiliary driver board (above and right of the main driver board) needs +50 V for its tie-back diodes, fed by a thin violet/green wire looped from the lone drop target coil near the playfield's rear. If that wire or the chain breaks, the aux board TIP102s (usually Q7 and Q15) fail within one or two activations.
Checks
- Check the violet/green tie-back wire at the drop target coil and along the chain.
- Test diodes D7 and D15 and transistors Q7 and Q15 on the aux board.
- With the coil fuse removed, check U1 (74LS576) pins 13 and 8: they should be high when the diverters are off; low means U1 is bad. Grounding a U1 output with a jumper should fire its coil.
- Check the CPU to aux board ribbon (driven from CPU U7) is fully seated and undamaged, especially after the backbox has been folded down.
- Disconnect one wire at each diverter coil and measure: about 12 ohm, no less.
Fix
- Repair the tie-back wire and add a second backup wire from the drop target coil (or a nearby coil) to the board.
- Fit a 1N4004 across each diverter coil, band to the lug with the thick power wire.
- Replace failed TIP102s, diodes or U1, and fit the correct diverter coils (AE-25-1000, but confirm in the manual).
Systems
Games
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Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)