+5 V is low or missing although C1 and D1-D4 test good.

+5 V still bad with good C1 and diodes

Cause: Failed 78H05C regulator U1 on the big heat sink, or small caps C3 (2 mfd), C4 or C5 (.1 mfd) gone open.

Checks

• Check the small caps C3, C4 and C5.
• Measure the input and output of the U1 regulator.

Fix

• Replace the open cap or the regulator; nothing else is in the +5 V section.

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

Systems

• Bally 6803

Related tips

• Strange intermittent faults from power supply solder joints
• 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
• Score displays dead or dim: high-voltage section parts
• Missing lamp fuse changes the flash count

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