Status: Successfully repaired on 2/22/25
My buddy asked me to look into an issue with his Nintendo AV Famicom with NESRGB V1.4 mod installed.
The modded console had worked fine for years, but it recently developed an issue where the Vertical Sync signal would become intermittent as the console warmed up, until finally dropping out completely after the console had been powered on for approximately 30 – 40 minutes.
I put the oscilloscope on the V-Sync line, and sure enough, the signal looked healthy at first with the expected holding High and occasionally pulsing Low behavior:
But after 30 minutes the signal would eventually hold High all the time:
I reached out to Tim Worthington, the creator of the NESRGB mod, and he shared a schematic and board layout files with me. He said that the most likely components that could be causing the problem are either U10 (MCP6561T-E/OT comparator used as sync separator) or U12 (SGM8051XN5 used as composite video buffer)
I ordered replacement parts from Mouser. Swapping them both in fixed the problem completely. Thank you for your support Tim!
