Reducing hum on a type 1 sound board

Steps

• First make sure all board screws and brackets are tight and clean, and replace old electrolytics (C10, C29, C30).
• If the hum stays, change R18 (TDA2002 feedback) from 220 ohms to 100 ohms.
• This lowers the maximum volume, so turn the volume up to suit; it is usually still loud enough for home use.

Systems

• Williams System 3
• Williams System 4

Related tips

• Hum from a type 1 sound board
• Type 1 and type 2 sound boards and their switches
• Sound EPROMs work in either sound board type
• Which sound board each game uses
• Using the sound board test switch
• Powering a sound board for testing
• R21 value on early power supplies
• Add two test LEDs to a System 7 CPU board

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)