Stepping through the single ("One") lamp test with the start button, two lamps flash for one lamp number.

Two lamps flash together in the single lamp test

Cause: The lamp selected has a shorted or reversed diode on its socket.

Checks

• Diode-test the diode on the socket of the lamp the display names.

Fix

• Replace the diode, or turn it round so the band matches the other sockets.

Systems

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

Related tips

• Test each lamp column driver with a test lamp
• Test a lamp socket diode in place
• Lamp lights twice or two lamps act as one
• Testing a lamp diode
• Testing lamp-row drivers with a test lamp
• Testing lamp-column drivers with a test lamp
• Why diodes on 555 lamp boards crack
• Lamp diodes sit on small boards at each socket

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