One rectifier test point is below its range with the game in attract mode.

Rectifier test point reads below its range

Cause: That output's bridge is failing. If TP4 (GI, no rectifier) is out, the transformer may be at fault.

Checks

• Measure with the game on and in attract mode, ground at the R1 or R2 lead nearest the fuses.

Fix

• Replace the bridge for that output.
• If only TP4 is out, check the transformer.

The score display supply runs at 170-230 V DC; keep hands off the rectifier and driver boards with power on.

Systems

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

Related tips

• Rectifier test point ranges with fuse and rectifier
• Rectifier board test point voltages
• No +12 V at rectifier board TP3
• No +5V on any board and no MPU LED: bad return pin in the solenoid driver board J3 plug
• Controlled lamps dim and TP1 reads about 2.8 V instead of 5.4 V: half the bridge open
• Only the knocker fires and the lower playfield GI is out: replace the rectifier board
• No voltage at any rectifier board test point: burnt mains wire to the transformer
• What each rectifier on the board feeds

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