LED locked on; U9 pin 5 is steady.

U9 pin 5 (VMA) is not pulsing

Cause: The CPU is not running: a bad 6800, or a fault on the address or data lines that crashes it. A shorted CPU output pin can do it too.

Checks

• Logic probe on U9 pin 5 (or about 2.8 V on a meter).
• Watch pin 5 across an off/on cycle: a brief burst of pulses lasting a second or two is expected before it settles.

Fix

• Try a known good 6800, then look for shorted address/data lines.

Systems

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

Related tips

• No clock at the CPU: U15, U16, C14/C15
• Checking U6 and the jumpers with the U9 VMA line
• Making the test LED for the test EPROM
• Flicker but no first flash
• Tell a -35/-133 MPU from a -17 by counting J5 pins
• MPU LED never lights at all
• Power and ground pins of the main MPU chips
• Dark MPU LED: follow +12 V through R29 and Q2

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