A second or two after power-up the speaker starts to buzz (not a mains hum, unaffected by lamps), and the sound is thin with little bass. All electrolytic caps are new.

Bally -51 sound board buzzes and sounds thin: wrong small caps in the audio path

Cause: A previous repairer had replaced small ceramic capacitors in the amplifier and audio signal path with polyester ones.

Checks

• Compare the small caps in the analogue section with the schematic and a known good board.

Fix

• Put ceramic capacitors back in the audio signal path.

Systems

• Bally MPU AS-2518-35

Related tips

• No sound at all: start with the amplifier hiss

Source: Pinball Repair help group!! (Facebook group)