OMG, do you think that the new ROM is being mapped over RAM?, I'm busted then...
Well, _ROMEN will only become active when the system attempts to read anything in the 512k of ROM space, starting from $F80000 (or $000000 at power on). So just because you have a larger EPROM, it doesn't magically change the address decoder's range and make it stomp over everything else.
So I don't think that's the problem. The red screen indicates that the system is at least starting to read the ROM correctly, and it's obviously failing the checksum test, meaning it isn't reading the entire 512kB of ROM space reliably/correctly for some reason.
A simple check would be to put the EPROM in your favourite programmer, bend A18 and A19 away so they don't contact the socket, and link them both to ground. Tell your EPROM programmer it's a 27C400, then read out the entire device. The 512kB output should compare correctly to the standard Amiga ROM. If that's OK, it means the correct data is available to the Amiga. If it doesn't verify, you've got some programming problem.
You are using a known good ROM image, right? i.e. the ROM dump you did is actually OK.
Another guess - the Amiga permanently holds CE active and _ROMEN simply gates the Output Enable line. There's a slim chance the device might not like this, perhaps it's using the falling edge of CE to do address setup or something silly. Would seem unlikely, but try removing bending the CE pin so it's not contacting the Amiga ROM socket and tie it directly to OE so that CE and OE get activated together with _ROMEN.
As for using a larger than 512kB ROM, that's another subject for a another thread. Get it working as a standard ROM to begin with.
Interesting find by the way, I've not seen this part before. Where'd you source it from?
