Checking U6 and the jumpers with the U9 VMA line

Steps

• Remove all socketed chips except U9 (CPU) and U11 (PIA).
• Power up. The LED will stay on; put a logic probe on U9 pin 5 (VMA). It should be pulsing.
• Power off, fit the U6 ROM, and power up again.
• Pin 5 should still pulse. If it has frozen at either level, either U6 is faulty or the jumper setting is wrong for it.

Systems

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

Related tips

• Why the MPU has ROM jumpers
• Get the MPU working before changing jumpers
• Add a ground jumper on every -17/-35 MPU
• Check the U9 address and data lines, and suspect U11
• Get the MPU booting before you change any ROM jumpers
• U9 pin 5 (VMA) is not pulsing
• Jumpers for a -35/-133 MPU running two 2732 EPROMs (U2 and U6)
• Bally -35 MPU jumper settings by ROM type

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