I'm sorry for bumping this old thread, but I've run into some issues with my own YI romhack cart and this is one of the only places I've been able to find where people have been talking about making YI romhack repros.
Here's a video of what the cart does:
https://www.youtube.com/watch?v=DjuRjE43tiUAs you can see the logo at the start shows up, the music plays fine, the text at the bottom appears... but all of the graphics are garbled and it eventually reboots.
Here's what I've done:
I picked up a JPN Yoshi's Island and an M27C160-100 16mbit EPROM. I cleaned the cart and tested it before desoldering the Mask ROM. I patched a valid YI ROM to SMW2+2 and verified that the file worked in higan. I then burned it to the EPROM using a GQ-4x4 programmer. It wrote and verified, and I have re-verified the chip a bunch of times. I wired a 42pin socket to the board using
this diagram which should, according to my understanding, convert the chip to 8bit mode. I've used my multimeter to test all the connections at least twice now, and the socket is connected using kynar that's no longer than about 3 inches.
In my testing, I took another M27C160 EPROM and burned the stock YI ROM to it, and it does the exact same things (logo, music fine, garbled graphics, reboots). So it's likely not a problem in the romhack, but something I've done wrong. I'm really confused about why the music plays just fine and the text shows up, but all the other graphics are junk. Other carts play fine in my SNES, so the connector there is clean, and I've cleaned the pins on this cart a few times in testing, just to make sure I didn't re-dirty them while working on it.
Does anyone have any ideas of what I should test?