Replaced Motherboard in late 2011 27inch iMac Problems

[fraunpetri]

Hey,

So I recently replaced a motherboard in my late 2011 27 inch iMac as the system was constantly rebooting and I had only one thunderbolt port that worked for the last few years. I wanted this to serve as a backup system and potentially a 2nd monitor for a late 2015 27 inch iMac but I'm not able to get it to boot and don't know where to turn for advice.

I used my old graphics card and connected it to the replacement motherboard. I am able to boot into target disk mode, select between my OS options as I've got 3 installed, load the hardware utility which finds no problems on the system and can download the recovery software from apple's site through control-R on startup but have black vertical lines on the screen and it will only boot to a white screen when attempting to start the OS. I can't get to a login screen.

Any thoughts on ways to troubleshoot?

-Fraunpetri

 

[Nguyen Duc Hieu]

You can try to replace the GPU with a known-to-be-working one.
GPU failure is a well-known issue with this iMac model.

If you don't think it's worthy to replace the GPU, then convert your iMac to a monitor. The whole premium kit will cost 75$. Basic kit costs 25~30$.

 

[fraunpetri]

You can try to replace the GPU with a known-to-be-working one.
GPU failure is a well-known issue with this iMac model.

If you don't think it's worthy to replace the GPU, then convert your iMac to a monitor. The whole premium kit will cost 75$. Basic kit costs 25~30$.

Thanks, I was hoping I'd missed something when putting it back together that would be an easy fix and someone could tell what was wrong by the screenshots. Seems like a new GPU or converted monitor are my only options.