Add a ground jumper on every -17/-35 MPU

Logic ground reaches the MPU only through J4 pins 18 and 19, joined on the component side under the header, where corrosion or cracked joints break it. On the solder side, run a short insulated wire from J4 pins 18-19 to the ground plane at the board edge.

Systems

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

Related tips

• Why the MPU has ROM jumpers
• Get the MPU working before changing jumpers
• Get the MPU booting before you change any ROM jumpers
• Power and ground pins of the main MPU chips
• Jumpers for a -35/-133 MPU running two 2732 EPROMs (U2 and U6)
• Bally -35 MPU jumper settings by ROM type
• Set a -35 MPU up once for 2732 EPROMs instead of re-jumpering per game
• Testing an MPU in the game with only J4 connected

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