Bally-35 MPU: remove the on-board rechargeable battery before it leaks
The original rechargeable battery on the MPU will eventually leak and gas, and the damage spreads to the board and connectors. Remove it now: the game runs without it and only loses saved settings and high scores. To keep memory, the simplest option is an NVRAM module in place of the 5101 RAM, which plugs into the existing socket with no soldering (fit it the right way round). A remote battery pack also works, but then R12 is removed and replaced by a small signal blocking diode (1N914 or 1N4148) so the game does not try to charge the alkaline cells; check the schematic for the diode's direction. A memory capacitor or coin cell are other options.
Battery leakage is corrosive: handle a leaking battery with gloves and clean and neutralize any residue on the board.
Systems
Related tips
- Converting a -133 MPU to a -35
- Bench-testing a -133 MPU: swap the R113 diode for a resistor
- MPU boots only some of the time
- Convert a -35 MPU to a -133 for Baby Pac-Man and similar games
- MPU LED locked on with no flicker: likely causes
- Testing the MPU reset circuit at U9 pin 40
- Keep spare 6800, 6821 and 5101 chips
- Single reset parts that hold the LED on
Source: Pinball Repair help group!! (Facebook group)