A ball drain is registered whenever the right crane switch, the left scoop switch and one of the rollover switches are all closed at the same time (for example during Super Crane).
Game registers a drain when three certain switches close together: bad switch diode
Cause: A failing diode on rollover switch 5 let current back-feed through the switch matrix, making the game see a phantom closure on the trough/outhole switch. The diode tested good with a meter but was faulty in circuit.
Checks
- Use switch test to find which combination of closed switches causes the false switch; phantom closures happen at the corners of a row/column rectangle.
- Unplug the switch matrix connectors and test with jumpers on the board side to rule out the CPU board.
- Test the diodes on the switches involved; also try replacing them, since a weak diode can still pass a meter test.
Fix
- Replace the diode on the offending switch (rollover switch 5 here).
Systems
Games
Related tips
- Ball ends early for no reason: missing switch diodes on the ramp switches
- Fit a replacement switch diode the right way round
- An entire switch row shows closed all the time
- False switch readings only when several switches are closed
- Pressing a switch also shows one in another column
- What a bad switch diode does
- Use a real ball in the switch test
- A single switch does not register
Source: Pinball Repair help group!! (Facebook group)