The CPU board LED neither flashes nor comes on and the game does not boot.

CPU board LED never lights

Cause: The game EPROM at U2 is bad or missing.

Checks

• Make sure U2 is fitted and seated.
• Verify it with an EPROM programmer.

Fix

• Replace or reprogram U2.

Systems

• Gottlieb System 3

Related tips

• Booting a CPU board on the bench
• CPU board 26285 versus 26285-1
• Main chips on the CPU board
• CPU board connector uses
• Missing P6 or P7 header on the CPU board is normal
• CPU board EPROM and RAM size jumpers
• Logic chips worth stocking
• U11 must be a 74HC123AN

Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)