No +5 V at all, or FU3 blows.

Bad +5 V bridge diodes D1-D4

Cause: One of the bridge diodes D1-D4 (MR751, 100 V 6 A) open (no +5 V) or shorted (blows FU3).

Checks

• On diode setting each should read 0.4 to 0.6 V one way and nothing the other. No reading either way is open; under 0.4 V is shorted.

Fix

• Replace the bad diode with any 6A1-type part and refit FU3 (6 A slow-blow).

Unplug the game before pulling fuses or working on the power supply; the transformer side carries line voltage.

Systems

• Bally 6803

Related tips

• Power supply test points and their voltages
• No +5 V at TP1 or no 14 V at TP5
• No 43 V for the coils
• Replacing the 78H05 +5 V regulator
• Power supply test point ranges
• Strange intermittent faults from power supply solder joints
• Coil fuse FU1 blows the moment the game is switched on
• High-voltage input fuse FU2 blows

Source: Repairing Bally 6803 Pinball Games from 1985 to 1989, Clay Harrell (pinrepair.com)