The +12 V switch matrix fuse blows immediately every time.

Switch matrix fuse F115 (F101 on WPC-95) blows as soon as it is replaced

Cause: A blown CPU board U20 (ULN2803A), sometimes with U14 (U23 on WPC-S/WPC-95); or a short in the playfield wiring or on an under-playfield opto board, such as solder splashes, a stray nut or bolt, or a lamp, flasher or coil lug touching a switch lug (above or below the playfield).

Checks

• Game off, fit a new fuse and unplug J114 (J101 on WPC-95) from the driver board, then power on (ignore the complaints).
• Fuse blows with J114/J101 unplugged: the CPU board is at fault, usually U20.
• Fuse holds and the +12 V LED is lit: reconnect J114/J101, unplug the opto board power (the board has a red power LED), and power on. If the fuse now holds, the opto board is the culprit.
• Otherwise look under and on top of the playfield for anything shorting a power circuit to the switch wiring.

Fix

• Replace U20 in a socket (and U14/U23 if needed), repair the opto board, or clear the short.

Systems

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

Related tips

• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Which fault blows which driver board fuse (WPC-89/WPC-S)
• "Check Fuses F114/F115" (F106/F101 on WPC-95) message
• Fuse-failure message at boot but the fuses are good
• WPC-89/WPC-S driver board fuse ratings
• WPC-95 driver board fuse ratings
• WPC power driver board fuses and what they feed
• Typical causes of each blown fuse (WPC-89 and WPC-S)

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