xodaraP
Legendary
Sega Mega CD - Model 1
Region: Japan
Fault: Damaged traces
Received a Mega CD with trace damage from a BIOS removalremoval/socket installation. Traces were repaired and console now boots to the normal BIOS screen, boots and runs games, but PCM audio is not working
Fault: PCM audio not working
This took a long time to resolve because the issue was inconsistent. Console would boot and have partial PCM sound, then if console is reset would have clicking, another reset and better sound than the first boot, then another reset would kill all PCM sound
I did the following in order:
- Tested with another main board to confirm
- Recap main board
- Verified all traces on BIOS
- Verified traces on edge connector to Mega Drive console
- Verified connections between CD drive and IO/power board and main board
- Verified connections from CN1/CN3 (ribbon cable connectors) to main board components
- Verified all traces on PCM chip
- Verified all traces on DAC chip
- Verified all traces on RAM and nearby ICs
- Verified relevant traces on ASIC
None of these fixed the issue. Next steps:
Replace PCM IC - No fix
Test PCM RAM - Verified good
Replace logic - Fixed
Fix: 74HC86 @ IC16 had failed. This chip is responsible for clock for the DAC and IO board, and also write signal for the PCM (I believe this IC decides which audio signal plays at any given time)
Success! The PCM audio is now working correctly alongside the CD audio
Booted and played Silpheed to verify fix
Fix: Bad 74HC86 @ IC16
Region: Japan
Fault: Damaged traces
Received a Mega CD with trace damage from a BIOS removalremoval/socket installation. Traces were repaired and console now boots to the normal BIOS screen, boots and runs games, but PCM audio is not working
Fault: PCM audio not working
This took a long time to resolve because the issue was inconsistent. Console would boot and have partial PCM sound, then if console is reset would have clicking, another reset and better sound than the first boot, then another reset would kill all PCM sound
I did the following in order:
- Tested with another main board to confirm
- Recap main board
- Verified all traces on BIOS
- Verified traces on edge connector to Mega Drive console
- Verified connections between CD drive and IO/power board and main board
- Verified connections from CN1/CN3 (ribbon cable connectors) to main board components
- Verified all traces on PCM chip
- Verified all traces on DAC chip
- Verified all traces on RAM and nearby ICs
- Verified relevant traces on ASIC
None of these fixed the issue. Next steps:
Replace PCM IC - No fix
Test PCM RAM - Verified good
Replace logic - Fixed
Fix: 74HC86 @ IC16 had failed. This chip is responsible for clock for the DAC and IO board, and also write signal for the PCM (I believe this IC decides which audio signal plays at any given time)
Success! The PCM audio is now working correctly alongside the CD audio
Booted and played Silpheed to verify fix
Fix: Bad 74HC86 @ IC16