LED1 (test point TP3) on the power driver board is out; the switch matrix 12 V is missing.

Driver board LED1 off: switch matrix 12 V missing (WPC-S and earlier)

Cause: Fuse F115 has blown, often because chip U20 on the CPU board has failed. The supply comes in on J101 pins 4,5 and 6,7, through F114, bridge BR1, caps C6 and C7, regulator Q2 and F115, and leaves on J114 pins 1,2.

Checks

• Measure TP3 against TP5 (ground).
• Check F115 and F114.

Fix

• Replace the fuse; if it blows again, check CPU board U20 (see the switch matrix section).

Systems

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

Related tips

• Driver board LED1 off (+12 V regulated missing)
• Driver board LED6 off: lamp matrix 18 V missing (WPC-S and earlier)
• Driver board LED5 off: flasher 20 V missing (WPC-S and earlier)
• WPC-89/WPC-S driver board LEDs and test points
• Voltages made by the WPC power driver board
• Bridge rectifiers to use on WPC-S and earlier driver boards
• Supplies made on the WPC power driver board
• Bridge positions on WPC-S and earlier driver boards

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