The LED flickers then stays on.

Flicker but no first flash

Cause: A game ROM U1-U6 failed its checksum: a bad ROM, a bad ROM socket, or jumpers that do not match the ROMs.

Checks

• Check jumpers against the ROM types fitted.
• Swap in known good ROMs; check the ROM sockets.

Fix

• Replace the bad ROM (with a burned EPROM if needed) or socket.

Systems

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

Related tips

• Strip the MPU to three chips to get the first flicker
• One flash only: U7 6810 RAM
• Two flashes only: U8 5101 RAM
• Leave the ROM jumpers alone until the LED flickers
• Three flashes only: U10 PIA
• MPU stops before the 7th LED flash (no 43 V)
• Intermittent or dead MPU from old closed-frame sockets
• Five flashes only: U12 timer or open R113

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