Random ball search, 'pinball missing' with a full trough, or extra balls kicked out. In switch test, as soon as more than about four switches in one matrix row are closed (trough balls plus a ball in the shooter lane, for example), every switch in that row reads open.
Whole switch row drops out when a fifth switch closes: damaged row drive on the CPU board
Cause: The drive circuit for that switch row on the CPU board was damaged: a surface-mount 1K resistor in the row driver was visibly burnt. On a scope the bad row rose slowly and sat at a higher base voltage instead of switching cleanly to 5 V.
Checks
- In switch test, close switches in the suspect row one at a time and watch whether they all drop out after the fourth or fifth.
- Compare the row drive signal with a good row on a scope or meter.
- Swap in a known-good CPU board to confirm the board is at fault.
- Look over the row driver area of the board (resistors and the small driver transistor) for burnt parts.
Fix
- Repair the switch row drive circuit on the CPU board (replace the burnt resistor and test the matching transistor), or replace the board.
Systems
Games
Source: Pinball Repair help group!! (Facebook group)