In sound test one of the sounds (sound 2, used for 100 points) never plays while the others are fine.

One sound missing in sound test (the 100 point sound): failed 7408 on the sound board

Cause: A failed 7408 logic chip on the sound board was blocking that sound select line.

Checks

• Run sound test and note which numbered sound is missing.
• Follow that sound's select line through the board with a logic probe.

Fix

• Replace the 7408. Members have also fixed the same symptom by replacing an LM324 or a CD4013.

Systems

• Stern MPU-100

Games

• Nugent

Related tips

• Sound module -32 and -50 test points
• Stern SB-100 sound board screeches until a game sound plays
• New sound board gives only a faint crackle: set MPU DIP switch 23 on

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