Starting a game sends two balls to the shooter lane, though the right number of balls is installed.

Two balls served at the start of a game

Cause: A trough switch stuck closed, often the last position. The game kicks a ball, still sees the stuck switch and kicks another; it does not look at the shooter lane switch. On opto troughs, a failed zero-ball or ball jam opto does the same.

Checks

• Check each trough switch in the switch test.
• On opto troughs, check both opto pairs and their boards for cracked joints.

Fix

• Repair or replace the stuck switch, or reflow the opto boards.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• "Ball missing" errors or extra balls served (roller switch trough)

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Three, Clay Harrell (pinrepair.com)