The flasher fuse (F107) blows as soon as the game powers up and the flashers die. Unplugging J111 lets the fuse hold.

Flasher fuse F107 keeps blowing and a flasher transistor shorts: stray washer in a socket

Cause: A short on one flasher output on the playfield side. In this case a loose metal washer had fallen into a bayonet flasher socket, which shorted the circuit, blew the fuse and took out the flasher's drive transistor (Q25, right-side lower flashers on J111-7).

Checks

• Unplug flasher connectors one at a time to find which one makes the fuse hold.
• With the game off, check each pin of that connector to ground with a meter; a pin that rings to ground marks the bad circuit.
• Test the matching drive transistor on the driver board; a shorted transistor will lock the flasher on or blow the fuse.
• Pull the wire for the suspect output from the connector and confirm the fuse now holds, then follow that wire to its sockets.

Fix

• Find and remove whatever is shorting the socket or wire (here a washer inside a bayonet socket).
• Replace the shorted drive transistor and the fuse, then refit the wire.

Systems

• WPC-95

Games

• Medieval Madness

Related tips

• WPC-95 power driver board LEDs, test points and fuses
• WPC-95: regulated 12 V fuse F101 blows
• Won't boot and AV board fuses F601/F602 blow: check solenoid fuses out of circuit
• Flasher glows full bright until the bulb blows: shorted driver transistor
• A coil stays energized

Source: Pinball Repair help group!! (Facebook group)