Sound works but a strong hum comes from the speakers.

Loud hum from the speakers

Cause: A poor ground, or a failed main filter capacitor (typically 1000 µF 16 V, across +5 V and ground) on the sound board.

Checks

• Tighten the screws holding every board to the backbox.
• Find the large capacitor joining +5 V to ground on the sound board.

Fix

• Replace that filter capacitor if the hum remains.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Check the sound board's regulated -5 V and +5 V
• Sound board versions are not interchangeable
• The 520-5050-00 sound board variant
• Data East and Sega sound boards by game
• Sound board power input connector CN2
• No sound with all voltages present: the PALs
• No sound, only amplifier hiss with the volume up
• No sound though sound board voltages are fine: unreadable sound ROM

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Three, Clay Harrell (pinrepair.com)