Game resets or locks up now and then, though TP3 still shows 12 to 13.5 V.

Occasional resets with a half-working BR2 bridge

Cause: Half of bridge BR2 has failed, so the logic supply is only half-wave rectified and the +5 V regulator cannot hold a clean output.

Checks

• Measure AC ripple across C23 on the solenoid driver board (see the C23 ripple test).

Fix

• Replace BR2 with a 25 or 35 A wire-lead bridge.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• MPU U9 voltages that show a working reset section
• Game resets or locks up when the flippers are used
• Resets from a burned SDB J3 pin 12 (12 V feed for the +5 V)
• MPU shuts down or resets: broken C23 ground return wire

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part One, Clay Harrell (pinrepair.com)