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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.
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.