A switch shows up together with a second switch in switch test, and the fault stays after new diodes, new matrix chips and even a different CPU board.
WPC: phantom switch closures traced to a 4N25 on the -1 driver board
Cause: A damaged 4N25 optocoupler on the -1 version of the power driver board (used to sense flipper firing on pre-Fliptronics games) loads the switch matrix through the ribbon cable.
Checks
- Simulate the switches at the CPU board connectors with a wire and diode, playfield unplugged; a phantom that remains is not under the playfield.
- Unplug the ribbon cable to the power driver board; if the phantom goes away, test the 4N25s.
Fix
- Replace the bad 4N25. On Fliptronics games, where they are unused, the damaged one can be clipped off.
Systems
Related tips
- Phantom switch closure that survives a new CPU board: bad 4N25 on the driver board
- Row or column "ground short" errors
- Flipper buttons not seen by the game (pre-Fliptronics)
- Upper right flipper only buzzes while the magnet holds the ball: failed opto-isolator
- Lane change not working on Taxi and later System 11 games
- Pre-Fliptronics: flipper lane change does not work
- Funhouse: flipper buttons won't step through high score initials
- Switch matrix chips on the CPU board
Source: Pinball Repair help group!! (Facebook group)