No Sound: Signal Trace Leads to 74LS139 at 7M

Repair Case Summary

This page documents a firsthand No Sound: Signal Trace Leads to 74LS139 at 7M repair case. It is intended for technicians and collectors troubleshooting original Pac-Man or Ms. Pac-Man arcade PCB hardware.

• Date: 2019-08-09
• Board: Original Pac-Man PCB
• Category: Sound / logic tracing
• Symptoms:
  • Board booted and played perfectly but had no sound at all.
• Diagnostic observations / tests:
  • Speaker/connections were ruled out first.
  • With volume high, rubbing the solder-side pins of amp IC 11A produced speaker crackle, indicating the downstream amplifier path responded.
  • Sound-circuit resistors checked normally.
  • 8K pin 5 was high as expected.
  • At 2M, pins 3, 4, 7, 8 and outputs 6, 9, 12, 16, 19 showed bad signals.
  • 2M inputs 3, 4, 7, 8 receive signals from 2L.
  • 2L pin 3 was missing a pulse; that input comes from 7M pin 11.
  • At 7M (74LS139), output pins 5, 11 and 12 had irregular signals while the remaining pins checked normally.
  • Piggybacking a new 74LS139 at 7M restored all sounds.
• Components / locations investigated: 11A, 8K pin 5, 2M, 2L pin 3, 7M, 74LS139, 7M pins 5, 11, 12
• Root cause: Bad 74LS139 at 7M.
• Repair: Replaced the 74LS139 at 7M.
• Result: All sounds returned and played correctly.
• Search terms: Pac-Man no sound; 7M; 74LS139; 11A crackle test; 8K pin 5; 2M; 2L; logic probe; sound tracing
• Evidence: Physically verified repair / firsthand repair log

Related Pac-Man / Ms. Pac-Man Repair Cases

• Missing and Out-of-Tempo Sounds From Broken Trace at 1M
• Slightly Distorted Sound Effects and Intermission Music
• Crunchy and Fuzzy Sound Effects
• Missing Background Siren and Hum With Aftermarket Ms. Pac-Man Daughter Card

How to Use This Repair Record

This is a firsthand physical repair case. The same symptom on another Pac-Man or Ms. Pac-Man PCB can have a different electrical cause. Use the documented diagnostic observations and tests to guide troubleshooting rather than replacing a component solely because it repaired this board.

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