Either flipper alone works all day, but pressing both together drops the game back to attract mode, ending it.

Pressing both flippers resets the game: replace big filter cap C15

Cause: The 5 V logic supply dips under the load of both flippers. Here the large filter capacitor C15 on the power supply board had failed.

Checks

• Measure the 5 V at the power supply and again across the largest capacitor on the CPU board while pressing both flippers; a difference of more than about a tenth of a volt points to connectors.
• Check the flipper coil diodes.

Fix

• Replace C15 on the power supply board (a full power supply rebuild is the usual cure for this symptom).
• Reflow or repin the supply connectors if the voltage drops between boards.

The power supply board carries transformer AC and stored charge in its large capacitor; unplug the game and let the capacitor discharge before working on it.

Systems

• Williams System 6 / 6A

Games

• Gorgar

Related tips

• Game reboots when the flipper buttons are pressed
• Game resets when both flipper buttons are pressed
• Random lockups from the 12 V logic pin
• Overnight test to separate connector faults from power faults
• Judging the logic filter cap by AC ripple
• Leave the big lamp filter capacitor unless it is leaking
• Flippers dead: fuse and coil voltage
• Replace the +5/+12 V logic filter capacitor

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