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Author Topic: iMac 24" Model ID# 8,1 - 9800m GT Successful!!!  (Read 10404 times)
alaskastyle
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« on: May 17, 2015, 10:56:05 PM »

I couldn't just be finished with this project and not share my experience  Grin
Reading through these forums helped a lot!

By trade I'm a network technician at a ski resort, but I've also worked in the past at an apple self-servicing authorized school district as a certified ACMT, ACSP, etc.
Aside from my day job I do a lot of side work for the employees here.

One such side job happened to be this 24" iMac that belongs to one of our dining room servers.
It obviously had graphics card issues, it wouldn't get to the efi boot screen 97% of the time, and when it did manage to boot: artifacts everywhere!

So, initially the obvious thing to do was to start shopping for the direct replacement card... Sadly, it's hard to find an Apple 8800GS nowadays.
After making several (unsuccessful) attempts to purchase an 8800 on eBay, it was time to look into alternatives as no one actually had the card on hand.

That's when I started reading up on netkas.org, among other sites.

After determining the physical form factor of the card was an MXM type HE/2.1
I started looking into potential replacements.

I came up with three different GPUs for the owner to pick from:

9600m GT - ($70 on eBay) Figured it would work since the efi rom from the 2008-2009 MacBook Pro should be readily available
9800m GT - ($130 on eBay) I was mistakenly under the impression that this GPU was featured on another model
ATI 4870m - ($300 on eBay) I owned an iMac with this GPU, so I knew I could get the efi rom.

When presented with these options, my buddy ended up ordering the 9800m GT.

You could imagine my dismay when I realized my mistake about the GPU and didn't find any custom efi roms for it.
The more I looked into it, the more I started doubting that this GPU would be compatible at all.
I found all sorts of info about eeprom chip size vs rom size, using nvflash, etc.

By the time I'd read everything I could possibly find that pertained to this computer, I was absolutely convinced I'd f*cked up.
The only thing I'd read that gave me any hope was a particular bit on another thread (I'll have to post a link when I find it again)

Another netkas user had suggested using the 8800 GS rom on a 9800m GT could possibly work and claimed the initial batches of 9800s were just rebranded 8800s.
(I don't know if the individual had made an actual attempt at this, but I sure as hell did)

So, I dug a little deeper into my research and did find that the initial 9800m GT's used the same G92 core.
They were for all intents and purposes exactly the same as an 8800 GS aside from a dozen or so shader cores.
(Later revisions of the 9800m GT use a G94 core)

The graphics card eventually came in, and aside from the green vs blue PCB looked exactly the same as the original GPU.
So, armed with the slightest glimmer of hope I went ahead with the replacement.

(I have previous experience with making these sort of repairs, but if you don't use this:
https://www.ifixit.com/Guide/iMac+Intel+24-Inch+EMC+2267+Graphics+Card+Replacement/13765)

The hardware portion went swimmingly as expected.
I used Antec formula 5 for the GPU when applying the heatsink, and generic silicone thermal compound for the VRAM

Now, my research had led me to believe that the machine should at least be able to boot Windows from which I should be able to use NvFlash.
So I threw in a blank hard drive and a Windows 7 Pro install CD, and to my pleasant surprise it booted! (Albeit the brightness/contrast/native white point was screwed up and super bright)

I went ahead with installing Windows, NvFlash (v5.165 I think?), and downloaded the 8800 GS rom (http://gilles.aurejac.free.fr/imac_mxm/8800gs-imac81.rom.zip)
Also installed GPU-Z to verify the card was in fact a G92 core

After a little bit of hesitation I figured it was a do or die thing at this point.
I ran the "nvflash -check" command to verify the eeprom chip would be the appropriate size, to my relief it was in fact a 1mb chip. (smaller ones supposedly won't work)

A little bit of trial and error, about 20 minutes later I'd managed to flash the rom. (I was using a bad rom dump at first, came back with a "PCI block error - size mismatch")

The command that managed to do the trick was "nvflash -4 -5 -6 -j filename.rom"

I rebooted holding option, efi bootloader came up!
Thew in the original hard drive, put the aluminum trim and top glass back on, booted up.
OS X reports the card as an 8800 GS and everything works perfectly!
« Last Edit: May 19, 2015, 04:05:10 PM by alaskastyle » Logged
Rominator
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« Reply #1 on: May 19, 2015, 01:07:42 AM »

You are one lucky devil

All really about the device id matching

looks like it did

did you dump the original PC BIOS off the 9800 before flashing by chance?

Thank you for sharing
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lion10
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« Reply #2 on: August 18, 2015, 08:32:56 PM »

My replay here comes somewhat late…

However, that report is just AWESOME! Smiley Many thanks alaskastyle to make this public. That’s another superb demonstration that crossflashing of PC MXM to Apple EFI MXM is possible.

Exactly that was always my goal!

Well, one fundamental point seems to be the EEPROM chip size, - all my attempts are failed on that. I tried this with several PC MXM Radeon HD 2400 / 2600 series MXM cards. Unfortunately they had a too small EEPROM chip size. Only workaround seems then to „transplant“ the Apple EFI MXM EEPROM chip to the functional PC MXM card. This only works if the card have almost an identical PCB layout. Furthermore this way is only recommended for people with very good electronic technical skills...

Whatever, will now also try to crossflash a PC MXM Geforce 9800M GT to Apple MXM Geforce 8800GS. There are also some PC MXM Geforce 8800M GTX cards available, - it will be interesting to know if these are also compatible.

Also from my side, many thanks for sharing this!
« Last Edit: August 18, 2015, 08:34:44 PM by lion10 » Logged
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