Don't some carts also check for pal/ntsc as well as language?
I got ripped on a modded Megadrive II and Mega-CD combo that has the language and frequency switches plus S-Video out on the Megadrive and the Mega-CD has a multi bios switch.
But even when I switch it to NTSC, it sends out a variant PAL signal(but allows play of the U.S. Sega-CD bios), so it's useless to me, since I bought it to record videos and my capture card only does NTSC.
I originally asked to buy a modded japanese combo, but he was out of them, so he sold me the U.K. set saying that it'd still work the same. When it didn't work, he said that I knew what I was buying and would only do another Genesis mod if I paid full price for it.
Since he took so long to do the work, I could get a refund through PayPal. I've been meaning to sell it to someone in the U.K., but I keep forgetting about it.
Ideally, I'd at least like to find someone who can do the region and S-Video mods on a Genesis and keep the multi-bios Mega-CD.