Repairing battery corrosion on the MPU

Steps

• Take the MPU out of the backbox and cut off the battery.
• Cut off every damaged part (green or grey), leaving leg stubs, and remove all parts the reset kit will replace. Chips, sockets and transistors suffer more than resistors and caps.
• Add fresh solder to each pad, heat it and pull the stub out with pliers; desolder all the holes.
• Remove green or grey header pins.
• Where solder has turned grey or black and will not melt, scrub it with a small wire wheel, then sand the area with 100-150 grit until the copper is bright.
• Scrub with 50/50 white vinegar and water, rinse with clean water, then rinse twice with 99% alcohol and let it dry.
• Repair broken traces: desoldering braid for wide ones such as the edge ground rail, 24 ga stranded wire for medium, wire-wrap wire for fine ones.
• Fit new parts raised slightly off the board and solder them on both sides, since the plated holes are probably damaged; use rosin flux on bare copper.
• Use machine-pin sockets so you can solder on top too, and replace corroded header pins and the matching connector pins.

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

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• Repair battery corrosion before chasing MPU flash codes
• Corrosion under MPU J4 can destroy every chip
• Cut the original MPU battery off right away
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Reset section parts typically ruined by battery corrosion
• Switch faults from corrosion under MPU J2 or J3
• 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)