Som ebrave soul needs to work on this stuff.
So much potential.....
Hi to everyone,
I'm new to this forum, but willing to do some work on the iMac MXM hack (Apple will stop to supply parts for this mac during september 2012).
So here are my guess, from what I know, and what I tried :
1- the white, core2duo iMac 24" (iMac 6.1) uses either a Nvidia Geforce 7300GT (128MB) or 7600GT (256MB).
2 - Those originals apple GPU are both, plain regular MXMII form factor cards (41mm separated holes), with 1.3 or 2.0 specifications (this is not clear).
What follows is only for the 7600GT :
3 - the original card use GDDR3 ram (K4J52324QC-BC14 - 512Mbit chips)
4 - it has a regular 128KB eeprom (Atmel AT25F1024AN)
5 - It seems to be a desktop GPU, it has a 0391 device ID.
While booted with extensions off (the card was defective), "Zeus" software did told me the 7600Gt worked at 200.475 Mhz speed (with multiplier ?). I was able to get a dump of the apple rom.
6 - There seems to be hope for finding a replacement.
I found those cards to have a nearly same design than the original 7600GT :
http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=271010547585Those cards have a design very closed to the iMac 6.1 7600GT cards :
- same chips, transistors, selfs, condensators, in the same place
- GPU is a G73M which seems to be the same as the G73 with different timings ? (do i make errors on this ?)
- they have a 128 KB eeprom which is great
- but they have a device ID 0x0398 and use DDRII.
I manage to make a try, and I bought and install this card in a defective 24" white iMac.
In fact, it fitted perfectly with the original heater.
So I did make a try : while it has of course shown no video in MacOS X, I managed to get working video on an external display with Bootcamp (I did not tried with onboard display, sorry...) :

I then did try to flash the firmware of the card with the iMac 7600GT firmware, and, to my surprise, it worked and guess what, I did have video on EFI boot...
Of course, it was scrambled video, but with using GDDR3 timings on DDR2, it did not surprised me :

So, where to go from this ?
It seems I'm not far from having a working replacement car for my iMac 24".
I have no knowledge at all on how to mod the EFI rom on this card, so that it work with DDR2, but I'm able to give some work, and to send my rom dump to anyone who want to make a try...
I have the iMac laying next to me, and I'm able to burn bootcamp CDroms with nvflash, and try any EFI rom that someone with the knowledges will be king enough to give me to try.
I will also, if all this works, be pleased to make a how-to...