Trace the 12 V switch supply with the playfield disconnected

Steps

• Leave only J101 (J129 on WPC-95) connected on the driver board; unplug J114, J116 to J118 and the ribbon cable (on WPC-95 unplug J101 and J138 to J141).
• Measure TP8 (TP102 on WPC-95). Voltage there means BR1 and C6/C7 (D11-D14 and C11/C12) are fine; none means F114 (F106) or an open bridge.
• Check D1 and D2 (1N4004): about 20 V on the unbanded side of D1 and about 16 V on the banded side of D2. Nothing means a diode has opened.
• Check regulator Q2 (7812): about 16 V in on pin 1 and 12 V out on pin 2. Input but no output means replace Q2.
• Check C2 (100 uF 25 V; C40 on WPC-95) for leaks and the trace near it for cracks.
• Finally confirm 12 V after F115 at TP3 (F101 and TP100 on WPC-95).

Systems

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

Related tips

• Driver board LED1 off (+12 V regulated missing)
• WPC-95 LED100 off (+12 V regulated)
• Driver board supply test points and normal readings
• Replacing a driver board bridge rectifier without tearing traces
• Pre-WPC-95 bridges, filter caps and fuses by supply
• WPC-95 rectifier diodes, filter caps and fuses by supply
• WPC-95: a dark +20 V LED at TP104 means the flasher supply is not leaving the board
• Row 1 short error and F114 blows at once: fault in the 18 V lamp supply

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