Hi, recently on my twin famicom only the A button would work on the controller, which I thought was an issue with the built in controller. To get around this I thought I'd get an adapter for the extension port to plug in an NES controller. The same thing still happens where only the A button works. Any help would be appreciated.
I actually had a twin famicom with a problem I think was almost the same. We had to solder a wire underneath the motherboard. there some some contact going to the CPU that was corroded.
Let me try to diagnose the issue. Obviously the problem is with the CLOCK signal for the gamepad shift register.
If only PLAYER 1 gamepads are faulty - check the red traces on the schematics. If BOTH PLAYERS are faulty - the issue is with the blue connection.
(https://i.imgur.com/P4OusYm.jpg)