Got a cheap RetroDuo, only 1/2 works

I think it's why they ended up at a salvation army store for only $20. Only the NES slot works. The SNES slot always displayed blank screen regardless of which games I plugged in.

Any idea what I can look for? It does not appear to be an easy hack to make it NES only since I'd still need power switch reset, and controller ports which are all on SNES board. I could probably cut out SNES video and audio so it'd affect NES video and audio less.

At a quick glance, it's a rev 2 with no spot for 7805 or PAL component like the first version had. I haven't had a close look yet as it's close to bed time for me and I need to sleep.