crwdns2934241:0crwdne2934241:0 RebTech

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Here's a link to a schematic. Hope it helps. https://github.com/schenkzoola/NES/blob/main/NES-001%20Console/NES-001.pdf

I agree, sounds like a mobo issue. Try following the traces from the pins to their destinations and check for continuity. Inspect under magnification. Could be an old/weak solder joint, loose leg, trace rot,  so many possibilities.

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