The speakers hum noticeably while the game sits in attract mode.
Loud hum from the speakers in attract mode
Cause: Loose boards making poor ground contact, or a failed C8 (47 uF) filtering the supply to the IC1 regulator on the power supply, letting ripple onto 5 V.
Checks
- Check every backbox board is screwed firmly to the metal ground plane.
Fix
- Tighten the board screws.
- Replace C8 with a 47 uF capacitor (up to 100 uF works).
Systems
Related tips
- Pinbot sound suddenly dies but the amp hisses
- Game resets when the flippers are used: recap supply and renew MPU power connector
- Random resets, days or minutes apart: burnt power supply AC input connector
- Mystery wheel won't turn and its opto never registers: reflow the power supply headers
- Right-side playfield GI out: a wire not seated in a new power board connector
- Pop bumper, sling and right-side GI lamps all go out at once: blown GI fuse
- Half the backbox lamps and the lower displays dead: check the backbox fuses
- Whole game flickers off and on, then dies: burnt ground pin at the power supply board
Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell