XERO, you can buy AC adapters that have multiple voltage settings (i.e. 7 V, 7.5 V, 10 V) and come with several interchangeable "tips" (the part that you stick into the SuperGrafx). The different tips vary by size and (sometimes) polarity, so you have to look at the diagram etched on supergrafx by the AC IN (i'm pretty certain there is a diagram). It will indicate the polarity (+ / -) and voltage.
Now, with that said, it still might turn out that a generic AC adapter won't work with supergrafx (i.e. if SGX uses a wacky voltage...)
therefore, since you don't want to fry your SGX, check out the TG-16 hardware faq (it's at gamefaqs, among other places)... hopefully there is a section about SGX AC in it (there may not be). Regardless, there is an informative discussion of how to CHOOSE a replacement AC adapter... follow those guidelines.