Battery leakage near the MPU J4 connector.

Corrosion under MPU J4 can destroy every chip

Cause: J4 brings 43 V onto the MPU. A trace of corrosion under the J4 header can leak it onto the 5 V bus and kill every chip on the board.

Checks

• Lift the J4 header's plastic base with a small screwdriver and look for any corrosion under it.

Fix

• Remove and replace corroded J4 header pins and clean the area before powering the board.

Battery leakage is caustic; wear gloves and eye protection while cleaning it off.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• Repairing battery corrosion on the MPU
• Repair battery corrosion before chasing MPU flash codes
• Switch faults from corrosion under MPU J2 or J3
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Reset section parts typically ruined by battery corrosion
• Removing corroded chip sockets
• Fitting a remote AA battery pack
• U8 (5101) connections for ringing out corrosion damage

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part One, Clay Harrell (pinrepair.com)