After the MPU battery was clipped off, the knocker fires during play at what look like random times (here at the first 1,000 points and again at 145,000), even though the knocker driver behaves in test.

Knocker fires 'randomly' in play: no memory battery, so every game sets replays

Cause: Without a memory battery the high scores and replay levels are lost at power off, so each game easily beats them and the game knocks for the awards. Low replay threshold settings do the same.

Checks

• Note when the knocker fires and compare with the replay and high score levels.
• Check the replay threshold settings.

Fix

• Fit a remote battery holder (or non-volatile memory) so settings and high scores are kept, then set sensible replay levels; the knocker can be turned off in the settings meanwhile.

Systems

• Bally MPU AS-2518-35

Games

• Future Spa

Related tips

• Switch faults from corrosion under MPU J2 or J3
• Repairing battery corrosion on the MPU
• Cut the original MPU battery off right away
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Memory cap or battery does not keep scores
• Repair battery corrosion before chasing MPU flash codes
• Corrosion under MPU J4 can destroy every chip
• Fitting a remote AA battery pack

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