After installing a replacement MPU and a known-good sound board, the game is completely silent, though the speaker tests good.

No sound after fitting a new MPU and sound board: check ribbon direction and sound DIP

Cause: The sound ribbon cable had been plugged in the wrong way round.

Checks

• Check the orientation of the ribbon at both ends against pin 1 on each board.
• Make sure the MPU DIP switch is set for electronic sound, not chimes.

Fix

• Plug the ribbon cable in the right way round.

Systems

• Stern MPU-100

Games

• Magic

Related tips

• New sound board gives only a faint crackle: set MPU DIP switch 23 on

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