I found those on the web, split already, but apparently it is the same image ^_^;
Did you try the 4.15 device driver or the ROM image? or both? I dumped the ROM on my HC+8 a while ago, it is labeled 4.13
http://www.hyakushiki.net/GVP-U34interestingly, it is a 32KB ROM but appears to have 8KB of something, followed by 8KB blank space, and finally 16KB of something else. Don't know if that is of any use on your card.
Can you post your original ROMs? Someone who knows about the structure of Amiga expansion ROMs might be able to find a clue as to why the 3.15 aren't working.
