I just got a used Retro Duo. The cartridges boot right up and the NES half works great. However, when I put a game in the SNES slot, the controller either goes nuts or doesn't respond at all. The Retro Duo only has two controller ports and they are both the SNES plug, so I don't think the port is broken - it works fine for NES games. 

I want to keep the console but does anyone have any idea what the problem is or how to fix it? If it's simple soldering I could do that, or hopefully it's a loose cable or something. I'll probably unscrew it and have a look tonight. If anyone has ideas or experience with this unit that would be great!

Are you using Retro Duo controllers, or official SNES ones? I've had problems in the past with clone machines not fully working right with official controllers.

Same problem no matter what controller. I actually have another Retro Duo that used to work but now it has the same problem. Bizzare. I opened it and from little I can see nothing is out of place. What's weird is the same controller and same port work on NES games, but not SNES games. And I tried multiple games too..