An LED on the trough opto board flashes erratically and its fuse (F116 here) blows, and the unregulated supplies measure high: about 18 V on the 12 V supply and about 83 V on the 50 V coil supply.

12 V and 50 V supplies read far too high: transformer line-voltage jumpers set wrong

Cause: The jumper plug on the transformer primary was wired for a lower line voltage than the house supply, so every secondary voltage was boosted. That stresses boards, coils and bulbs. Some aftermarket trough opto boards also have components that can touch the metal trough frame and short.

Checks

• Measure the unregulated 12 V and 50 V supplies; on these games roughly 14 V and 70-something volts is normal.
• Compare the transformer jumper plug wiring with the chart for your line voltage in the manual or a service guide.
• On an aftermarket trough opto board, check whether any component leads on the back can touch the trough frame.

Fix

• Rewire or replace the transformer jumper plug for your actual line voltage and re-measure (here the readings dropped to about 14 V and 73 V).
• Trim any long leads on an aftermarket trough board and insulate it from the metal frame, or replace the damaged board.

The transformer jumper plug carries line voltage. Unplug the game before touching it, and measure supply voltages only with the meter leads well insulated.

Systems

• WPC DCS

Games

• Judge Dredd

Related tips

• Using LED2 and LED3 to set the transformer tap
• Shorted or open bridge rectifiers
• After playfield work: optos, crane and Deadworld dead, trough dumps balls (J116)
• Pre-WPC-95 bridges, filter caps and fuses by supply
• Driver board LED7 off (+12 V unregulated)
• Driver board LED7 off: unregulated 12 V missing (WPC-S and earlier)
• +20 V filter capacitor C11 (C10 on WPC-95) blows apart
• WPC-89/WPC-S driver board LEDs and test points

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