Lamps misbehave, or part or all of the matrix is dead, yet all the TIP107s and TIP102s test good. Lamps flashing on at power-up are a clue.

Lamp matrix faults with all lamp transistors good

Cause: The ULN2803 at U19 (U11 on WPC-95), less often the 74LS374 at U18 (U10 on WPC-95), then the LM339s at U15/U16 (U16/U17 on WPC-95). If the entire matrix is out, a lost ground on resistors R150-R153 and R172-R173 (on WPC-95: R225, R228, R231, R234, R237 and R240) or capacitors C13-C20 (C32-C39 on WPC-95).

Checks

• Watch the lamps at power-up.
• Check the ground side of the listed resistors and capacitors.

Fix

• Replace the ULN2803, then the 74LS374, then the LM339s.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• BR1 (D11-D14 on WPC-95) and the board around it scorched
• GI strings won't dim in the GI test
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Row or column "ground short" errors
• Switch matrix dead after a screwdriver slip under the playfield
• Switch matrix chips on the CPU board
• "Slam tilt stuck closed" at power-on
• Why CPU U20 fails and how to replace it

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Two, Clay Harrell (pinrepair.com)