In single lamp test, lamps from two different columns light at the same time. Connectors look fine and no burn marks are visible.

Lamps in two columns light together in single lamp test: failed lamp column transistor

Cause: A failed lamp column driver transistor (here Q56, a TIP42) on the CPU board.

Checks

• Check for continuity between the two column wires and for lamp sockets touching each other under the playfield, to rule out a wiring short.
• Run the lamp matrix test at the board to tell a playfield fault from a board fault.
• Test the column driver transistors for that column.

Fix

• Replace the failed column transistor. Take care removing it: a lifted or burnt trace has to be repaired before the board goes back in.

Systems

• Williams System 11C

Games

• Diner

Related tips

• CPU and driver board transistors by position
• A column of controlled lamps out: failed lamp column transistor Q60
• Test a TIP42 lamp column transistor with a meter
• Diode-test the lamp column and row transistors
• Test each lamp column driver with a test lamp
• Lamp and switch matrix transistor positions on the CPU board
• Lamp matrix column test at 2J5 with Leon's code
• How the CPU-controlled lamp matrix is wired

Source: Pinball Repair help group!! (Facebook group)