LED will not flash, or the board works only some of the time.

Intermittent or dead MPU from old closed-frame sockets

Cause: Original closed-frame brown or black IC sockets, or sockets marked "SCANBE" or "RS", make poor contact. U11 also sits in the battery corrosion zone.

Checks

• Test continuity from each chip leg down to its pad on the solder side.

Fix

• Replace all such sockets with machine-pin sockets.
• Until they are replaced, a machine-pin socket stacked into the old one, with the chip on top, makes a workable stopgap.
• Avoid gold-plated sockets; the mismatched metal against tin chip legs causes intermittent contact.

Systems

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

Related tips

• Intermittent MPU faults from old closed-frame chip sockets
• MPU stops before the 7th LED flash (no 43 V)
• Flicker but no first flash
• MPU LED never lights at all
• M-200 flashes seven times but the game misbehaves or repeats one number in audits
• Dark MPU LED: follow +12 V through R29 and Q2
• Seven flashes, then it starts over endlessly
• Six flashes only: 43 V, U10 or U14

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