The +18 V lamp bridge or diodes run very hot, the board is browned or black around them and F114 (F106 on WPC-95) eventually blows.

BR1 (D11-D14 on WPC-95) and the board around it scorched

Cause: A lamp column is stuck on, so it gets the full 18 V instead of strobed power and overloads the supply. The bridge itself is usually fine. Causes: a shorted TIP107 column driver, then the ULN2803 at U19 (U11 on WPC-95), or the 74LS374 at U18 (U10 on WPC-95).

Checks

• Test all the TIP107 transistors.
• Watch the playfield at power-on: lamps that flash on at switch-on point to the ULN2803.

Fix

• Replace the shorted TIP107, otherwise the ULN2803, otherwise the 74LS374.

Systems

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

Related tips

• Lamp matrix faults with all lamp transistors good
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Why CPU U20 fails and how to replace it
• Switch matrix chips on the CPU board
• Switch matrix dead after a screwdriver slip under the playfield
• How the WPC lamp matrix works
• How the WPC lamp matrix is switched
• WPC lamp matrix and GI driver parts

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