A single matrix lamp stays dark though a known good bulb is fitted.

One controlled lamp never lights, bulb is good

Cause: The socket diode has gone open or come loose from its lug.

Checks

• Look for a diode lead broken off the socket.
• Diode-test the socket diode.

Fix

• Resolder or replace the 1N4004 diode.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Lamps in a row or column light twice per sweep
• Test each lamp column driver with a test lamp
• Lamp lights twice or two lamps act as one
• No score displays light at all
• Optos test good but still don't work: the opto board
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Testing a lamp diode
• Testing lamp-row drivers with a test lamp

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Three, Clay Harrell (pinrepair.com)