Strip the MPU to three chips to get the first flicker

Steps

• With power off, remove every socketed chip except U9 (6800 CPU), U11 (6821 PIA) and U6 (program ROM).
• On some Stern games the U2 and/or U5 ROMs are also needed for the flicker; leave those in.
• Power up and look for the first flicker.
• Once the board flickers, refit the other chips (power off each time) and count the flashes.

Systems

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

Related tips

• Flicker but no first flash
• Reseat the MPU chips before anything else
• Power and ground pins of the main MPU chips
• Double-check bench leads: a +12 V to +5 V short kills the MPU chips
• Three flashes only: U10 PIA
• Testing an MPU in the game with only J4 connected
• MPU board test points
• Intermittent MPU faults from old closed-frame chip sockets

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