Screen blacking out! In need of help with 27" iMac please?

[T3C]

Hi everyone,

I hoping that somebody with much more Apple experience than me can help please?

My friend has a really nice iMac 27" late 2009
https://forums.macrumors.com/attachment.php?attachmentid=450338&stc=1&d=1386247434
that has developed a very strange issue. He can be working away with a large Photoshop file and boom the screen will go dark (see vid). Sometimes he can recover by pressing sleep and then touching the space bar brings everything back but sometimes not. It can last half an hour or maybe only 5 mins but it will start doing this annoying thing at some time or another.
Here's a video of the problem:
https://vimeo.com/80889318#

I have asked in a couple of other forums where I spend some time but I've been advised that this forum is probably my best bet for getting some help. Here are a couple of links to those forums, if people don't mind reading them to get a clearer picture of what is going on and what I've done to troubleshoot the issue.

http://www.avforums.com/threads/apple-27-imac-late-2009-is-it-dying.1836684/#post-20014678

and

http://forum.osxlatitude.com/index....ckingtosher-in-need-of-help-with-imac-please/

I'm no expert in iMacs but I'm leaning towards hardware issue, maybe the LVDS cable or maybe the panel itself but who knows. Only having 3 leds showing on the logic board is also strange too. Could it be the logic board all along?

Help please and many thanks in advance.

Update: I've just gone into Disk Utility and I've noticed that SMART is reporting impending disk failure....please backup your data! Could that be having an impact on the display or is that something else going wrong too?

 

[pastrychef]

My guess is that it might be the inverter or cable.

 

[T3C]

My guess is that it might be the inverter or cable.

Thanks Pastrychef. That is my best guess too from reading the entire 27" iMac (late 2009) Apple Technician's Manual pdf, especially page 36 or was it 37? Anyway, I'm guessing that the cable has a defect or is breaking down somewhere and that is having a knock on effect to the nvram or something. Since I've cleared both the smc and the nvram I can not get the damn thing to show the problem again!!

Any other ideas on how to confirm this besides new cable?

 

[pastrychef]

Other than trial and error, I don't know any other method. Visual inspection of the cable may reveal something. I once saw where a cable was damaged and was able to solder it back together. I don't know if visually inspecting an inverter board will reveal anything.

 

[T3C]

Thanks again

I've been stress testing all day with gpu tests and running loads of different video files simultaneously with PS open with a couple of big files and nothing seems to kick the problem off.

I'm still trying to understand why I have only 3 led's showing, the fourth not being light says: Video controller unable to communicate with LCD panel. But it is displaying (at the moment anyway). Guessing I can rule out logic board and video card completely now. Or can I?

Is it fair to say that the clearing of nvram is a work around that will provide, extend or allow my friend to continue using the iMac for his work?

 

[pastrychef]

The following is what the Apple Technician Guide says if 4th LED is not lit:

 

[DeltaMac]

The SMC reset may be a good fix…
Unless you have tried that before, and the problem comes back.

You could try plugging in an external display, to see if the video also goes black on that second display at the same time as the internal (likely the graphics card, then)
If the external video remains good, even though the internal goes black, then - yes - the internal DisplayPort cable is a good next step.

Does the #4 LED EVER come on (even briefly)?
Is LED #4 OFF even while video appears on the LCD screen?

Is this just a backlight issue? Can you shine a bright light into the screen when the video appears to be off, just to look for video elements, such as folders or menus? Or, is the screen completely black (and blank)
Again, looking at the repair guide - that all points to the DisplayPort cable - or, ultimately, the LCD panel

 

[T3C]

The following is what the Apple Technician Guide says if 4th LED is not lit:

Thanks for that and I've got the same as a guide already.

The SMC reset may be a good fix…
Unless you have tried that before, and the problem comes back.

You could try plugging in an external display, to see if the video also goes black on that second display at the same time as the internal (likely the graphics card, then)
If the external video remains good, even though the internal goes black, then - yes - the internal DisplayPort cable is a good next step.

Does the #4 LED EVER come on (even briefly)?
Is LED #4 OFF even while video appears on the LCD screen?

Is this just a backlight issue? Can you shine a bright light into the screen when the video appears to be off, just to look for video elements, such as folders or menus? Or, is the screen completely black (and blank)
Again, looking at the repair guide - that all points to the DisplayPort cable - or, ultimately, the LCD panel

Thank you too...I'm trying really hard to get the problem to show itself again so I can test to an external display. I have a mini dp to hdmi cable ready and waiting to use. When it finally first started displaying (or not displaying) the issue on my test bench I didn't shine a light on the screen to test. I was mainly working on testing how the machine would boot up and still be running while the screen went dead. Once I was happy the system (and the screen briefly came to life) once the power button was pressed I did a SMC reset and a NVRAM reset too. This brought everything back to normal and hasn't gone off since and it's not for the want of trying to invoke the issue either. I never seen the fourth one light up since I've found out about them not even a little twinkle.

 

[Twimfy]

Are you 100% sure all of the cables are seated correctly. I took my iMac to bits to give it a good clean a while back and shortly after I got the same thing happening to me and I freaked out thinking my GPU was on the way out.

I ordered a new Inverter cable in the hope that it was all it needed, upon taking it apart the original cable was ever so slightly unseated, a quick push and that never happened again.

I know it sounds really obvious but it could have been easily overlooked.

 

[T3C]

Are you 100% sure all of the cables are seated correctly. I took my iMac to bits to give it a good clean a while back and shortly after I got the same thing happening to me and I freaked out thinking my GPU was on the way out.

I ordered a new Inverter cable in the hope that it was all it needed, upon taking it apart the original cable was ever so slightly unseated, a quick push and that never happened again.

I know it sounds really obvious but it could have been easily overlooked.

Thanks Twimfy, I don't really want to go inside again unless I really have to and when I first put it together from giving it a huge dust clean I had to do a double check of connections because I'm a bit paranoid like that from years of putting pc's together.

I've been tapping it, moving it, shouting at it, gpu and cpu stress testing it to get a huge heat build up into it and visiting lots of naughty sites (as the owners history points towards him doing lots of that) to get the thing to black screen but to no avail...maybe just maybe the combination of all my work has somehow cleared the issue?