One row of the switch matrix misbehaves even after every switch and diode on that row has been checked and found good.

Switch matrix row fails with good switches and diodes: battery-damaged CPU board

Cause: Alkaline leakage from the old on-board batteries had corroded the CPU board around the battery holder, damaging the switch matrix circuits.

Checks

• Check every switch and diode on the affected row first.
• Then pull the CPU board and inspect the area around the battery holder, front and back, for green or white corrosion.

Fix

• Replace the CPU board (or have it professionally repaired). When fitting a new board, move over the original ASIC, game ROM and processor.
• Fit NVRAM or a remote battery holder so the new board can't be damaged the same way.

Battery electrolyte is corrosive; wear gloves when handling a damaged board and neutralise residue before storing it.

Systems

• WPC Fliptronics

Games

• Fish Tales

Related tips

• Battery power not reaching the RAM: diode D2
• Cleaning up alkaline battery leakage on a CPU board
• Fitting a remote battery holder to a WPC CPU board
• Changing the CPU batteries without losing settings
• Batteries get hot with the game on
• Batteries die within days or weeks: diode D1
• Settings lost because battery voltage at the RAM is too low
• "Factory Setting" or "Adjustment Failure" at power-on

Source: Pinball Repair help group!! (Facebook group)