The MPU LED does not give its seventh flash at power-on.

MPU stops before the 7th LED flash (no 43 V)

Cause: The MPU is not seeing the 43 V zero-cross signal: wiring, R113 open, or corrosion damage.

Checks

• Confirm 43 V reaches the MPU on the connector side of R113.
• Measure the other side of R113.

Fix

• If the far side is dead, replace R113 (2K 1/4 W).
• If still dead, look for corrosion damage in that area.

Systems

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

Related tips

• Converting a -133 MPU to a -35
• MPU LED never lights at all
• MPU stops before the seventh flash: check the 43 V at R113
• Intermittent or dead MPU from old closed-frame sockets
• Flicker but no first flash
• Still no seventh flash with F4 good
• Using a PC power supply as an MPU bench supply: which wires
• Power and ground pins of the main MPU chips

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