The game loses track of balls in the trough, intermittently or completely.

Ball trough optos work intermittently

Cause: Cracked solder joints on the small (about 2 inch square) opto boards, at the surface-mount transistors and the header pins, from balls slamming into the trough.

Checks

• Inspect the surface-mount parts and header pins with a magnifier.

Fix

• Reflow the surface-mount transistor joints with a fine, low-heat iron.
• Reflow the header pin joints.

Systems

• Data East/Sega CPU Version 3b

Related tips

• Specification for replacement Sega opto LEDs
• Opto LED part numbers and replacing them in pairs

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