Resolved [SOLVED]Two iMacs 2009 - black screen, backlight OK, GPU OK, four LEDs on - any ideas?

[Tom.Tom]

Hello,

I've recently got two 2009 Imacs with the video issue I have never come across before. The symptoms are as described briefly in the topic:

• both boot with chime and load the OS (Live Linux from USB),
• four LEDs are on,
• the LCDs get the backlight, but no video data (black screen with just the backlight),
• no video on the external monitor,
• GPUs are 100% OK,
• LCDs and LVDS cables are 100% OK.

I suspect motherboards, but two iMacs with the same issue? Maybe this is something common for these models and somebody has solved that before? I've got the schematics, boardview and skills to do SMD soldering, but I'm not an expert in finding such issues on the motherboards.

Any help would be very appreciated.

Thanks in advance.
Tom

 

[rpmurray]

both boot with chime and load the OS (Live Linux from USB),

How did you verify that the OS is loaded and running?

 

[Tom.Tom]

How did you verify that the OS is loaded and running?

ssh-ed to it remotely

 

[Mactech20]

Is the video problem still present when running macOS?

 

[Tom.Tom]

Is the video problem still present when running macOS?

Yes, I've just configured the OSX to allow screen sharing, put the SSD into one of these iMacs and I'm able to connect to it and normally remotely work on it. The screen remains black - just with the backlight.
I can even put the screen to sleep and the backlight goes off. When I wake the screen, the backlight goes on.

 

[rpmurray]

I've recently got two 2009 Imacs with the video issue I have never come across before.

How did you acquire these iMacs? If they were sold as salvage it might be that the reason is that they don't have working video.

 

[Mactech20]

Honestly it sounds like the Backlight is plugged in but the display plug is not. It is seperate on these models.

 

[Tom.Tom]

How did you acquire these iMacs? If they were sold as salvage it might be that the reason is that they don't have working video.

Yes, they were sold as with failed GPU, which is 80% of failures in these macs and in 99% replacing the GPU solves this problem, but apparently this is that 1% where it's not the case...

 

[Tom.Tom]

Honestly it sounds like the Backlight is plugged in but the display plug is not. It is seperate on these models.

If the display plug was not plugged, there would be no backlight and no LED#4.

 

[Nguyen Duc Hieu]

If the display plug was not plugged, there would be no backlight and no LED#4.

LCD panel tested already? Jumping to the conclusion that the GPU is dead of half dead is easy but it seems to me that you want to eliminate all other possibilities first. I've got 2 dead HD 4850 with the same symptom (boot halfway to Mac OS). Replacing the GPU solved the problems.

Probably you don't have another working GPU at hand to try.
GPU is the first thing I would try to replace on these 2009 ~ 2011 model. Save a lot of time wondering and testing.

 

[Tom.Tom]

LCD panel tested already? Jumping to the conclusion that the GPU is dead of half dead is easy but it seems to me that you want to eliminate all other possibilities first. I've got 2 dead HD 4850 with the same symptom (boot halfway to Mac OS). Replacing the GPU solved the problems.

Probably you don't have another working GPU at hand to try.
GPU is the first thing I would try to replace on these 2009 ~ 2011 model. Save a lot of time wondering and testing.

As I wrote in my first post, the GPU is first thing i suspected and first thing I checked and it's 100% ok. I've got several GPUs and the whole 27" 2009 c2d and have exchanged all the parts except the Mobo.

My suspicion is that there's some problem in the circuit between the GPU and display port. Unfortunately I do not have working 820-2733-A Mobo to replace but I guess it would work fine.

If there was the video on the external display, I would suspect partially damaged lvds socket, but there's no external video signal as well...

 

[Nguyen Duc Hieu]

Check the transistor (mosfet) Q9001 to see if it was knocked off its original position.

https://forums.macrumors.com/attachments/mosfet-jpg.1784207/

 

[Tom.Tom]

Check the transistor (mosfet) Q9001 to see if it was knocked off its original position.

https://forums.macrumors.com/attachments/mosfet-jpg.1784207/

Yeah - have checked that one already. Once I had the Mobo where I had to replace this one, but I think it's responsible for switching on the BACKLIGHT of the LCD when there is video signal available... or does the LCD_PANEL_PWR* mean powering the t-con and not the backlight, and that may be it?

But I have another finding - it seems like those two iMacs do not have the exact same problem.
Now I connected one of them again to the external display and the video is there!

If I connect the external display only (no internal one) the main OSX screen is there (with login prompt). If I connect the internal display, I see only blank desktop wallpaper on the external one - this means the main screen is on the internal display although I do not see it (like before - just the backlight).

About the Q9001 - @Nguyen Duc Hieu - do you know what's the exact role of this component? It look's ok on the board, but I have couple of these and can replace it. The question is - isn't it responsible for the backlight which I have?

 

[Nguyen Duc Hieu]

About the Q9001 - @Nguyen Duc Hieu - do you know what's the exact role of this component? It look's ok on the board, but I have couple of these and can replace it. The question is - isn't it responsible for the backlight which I have?

I don't know anything about it role.
I just know that, on my imac 2009 and 2010, when I knocked them off because of my recklessness during the disassembling of the GPU, the screen went black.
I had to bring both iMac to a repair shop to have it fix. After that the internal screen were back again.

 

[Tom.Tom]

SUCCESS!!!

That L9000 inductor-bastard was the source of the problem - it's got infinite resistance. Replaced it from the donnor board and voila - the video on internal display is back 💪💪💪

 

[wolf1734]

Hello, on a 27" iMac the screen stays on a bit but nothing is visible, however it works on an external screen. Could it be from the video card or something else? Thank you for your answer.

 

[Nguyen Duc Hieu]

Hello, on a 27" iMac the screen stays on a bit but nothing is visible, however it works on an external screen. Could it be from the video card or something else? Thank you for your answer.

It could be anything or all things below:
- Video card
- Logicboard components (like what you have read in this thread)
- Bad LVDS socket on the logicboard
- Bad LVDS cable (connecting the logicboard and the LCD panel)
- Bad LCD backlight board (LCD panel got no power for the LED strips)
- Bad LCD panel.

 

[wolf1734]

hello the screen works on another iMac, the backlight card has been changed, the power supply changed, I am waiting for the LVDS tablecloth to change it. thank you for your answer

 

[wolf1734]

the video card must work because the iMac works on an external screen.

 

[Nguyen Duc Hieu]

the video card must work because the iMac works on an external screen.

Then the causes are narrowed down to 2 factors:

LVDS socket on the logicboard, or
Bad component on the logicboard.

 

[wolf1734]

hello and thank you for the answer I'm waiting for the lvds tablecloth I'll change it and I'll let you know if it works.

 

[Nguyen Duc Hieu]

hello and thank you for the answer I'm waiting for the lvds tablecloth I'll change it and I'll let you know if it works.

So you found out that the LVDS cable is the issue, not the LVDS socket on the logicboard?
Replacing LVDS cable is easy.
Re-soldering the LVDS socket is a very difficult job.

 

[wolf1734]

I don't know if the LVDS cable is the culprit I'm doing a test to eliminate the cause of the failure