2011 iMac display too dim after replacing defective GPU?

[speedynix]

System: iMac 27 inch 2011, model iMac12,2
GPU: AMD Radeon HD 6770M
Memory: 8GB


This iMac didn't boot properly anymore. It showed the gray boot screen and just stayed there without loading the desktop interface.

My guess was the GPU was defective, so I ordered a replacement from eBay from a trusted seller.
After replacing the defective GPU the iMac booted fine again.

One thing I notice now is that the screens seem to dim at full brightness. I compare to a 2012 27 iMac from which I understand should be about the same brightness.

So I'd like to know how to go about troubleshooting this issue. I already tested target display mode where i used the older iMac as the target display which also gives a dimmer image from the newer iMac. Using the 2012 iMac display from the 2011 iMac gives the right brightness on the 2012 imac's display.

On the images the left is the 2011 iMac.

Any ideas?
Thanks!

 

[Nguyen Duc Hieu]

Idea about what?
It's a 12 year old LCD panel. Might be power-on for a very long hour during its days before reaching your hand.
The LED got aged and could only produce 75% or 50% of its maximum brightness comparing to the new conditions from the factory.

 

[Speedonl]

Idea about what?
It's a 12 year old LCD panel. Might be power-on for a very long hour during its days before reaching your hand.
The LED got aged and could only produce 75% or 50% of its maximum brightness comparing to the new conditions from the factory.

Thanks for your reply. I meant ideas as to how to solve this, but never mind my stupid question..

 

[kirreip]

No stupid question. Are you sure this is an original Apple card? People usually swap their GPUs on their 2011 iMacs to install a better one (metal support) and then they can install the latest OS (Monterey or maybe even Ventura). Often these cards can't make the screen as bright after installation as with the original card. Opencore contains a fix for this though. You could try this as a test, costs nothing (only time, but made easy with OCLP...).

 

[Nguyen Duc Hieu]

Thanks for your reply. I meant ideas as to how to solve this, but never mind my stupid question..

I can suggest several options below, but I can't be sure whether it will work for your iMac. Troublesome as hell

1. Change the LCD backlight powerboard (about 15$ plus shipping) => This is easy.
2. Change the LED bars inside the LCD panel
3. Add an extra PWM module to intercept the control line from the logicboard to the LCD backlight powerboard and control it manually. This is also troublesome and require an oscilloscope to measure frequency.

 

[speedynix]

No stupid question. Are you sure this is an original Apple card? People usually swap their GPUs on their 2011 iMacs to install a better one (metal support) and then they can install the latest OS (Monterey or maybe even Ventura). Often these cards can't make the screen as bright after installation as with the original card. Opencore contains a fix for this though. You could try this as a test, costs nothing (only time, but made easy with OCLP...).

Yes, definitely original Apple card.

 

[speedynix]

I can suggest several options below, but I can't be sure whether it will work for your iMac. Troublesome as hell

1. Change the LCD backlight powerboard (about 15$ plus shipping) => This is easy.
2. Change the LED bars inside the LCD panel
3. Add an extra PWM module to intercept the control line from the logicboard to the LCD backlight powerboard and control it manually. This is also troublesome and require an oscilloscope to measure frequency.

Thanks, I let it as it is and gave back the iMac to the owner. He is happy with it