27' + 24 LED Display and flickering


Original poster
Feb 11, 2008
Hey guys

I finally got my Imac fearing for the worst (delivered BTO option i7)

The imac itself was fine. Got the WD drive so was happy with that, no dead pixels just some dirt behind the screen which i was going to remove when i replaced the DVD drive with a SSD. So physically it was Great and a keeper. It really is a beautiful thing!

Hooked up the 24" LED Apple display and was really happy with the setup. The 24" looks so small next to it.

Though it was too good to be true. About an hour in decided to fire up wow to see how it would play, about 15 min, started to get some artifecting and flickering at first i thought it might just be the game... but then bang blackout! the whole screen went black and back. The big smile i had just turned into a big frown, having read posts of the 27" i knew about the flicking and black screen and f*rk it i just got one!

Some simple findings.

1. The back of the imac top right and left hand corners are very hot - Its fine though i got istat and the CPU sits on about 55-60 C and the GPU is on 80 C, well well within operating temps - fans at this point are on low (as temps are low)
2. The PSU sits on about 72 c
3. Dropping out of wow and back to the desktop where the CPU/GPU temps dropped by alot made no diff, still has flickering
4. flickering/archifecting is random but when it starts you will probably see it every 10 secs, Blackouts are less common, but suck when they happen cause the whole screen goes black like the computer is off
4. All updates are applied, running 10.6.2
5. My 27" mac i7 has the 4850, my mate bought a C2D with a 4670, had the same issues, so that rules out the CPU/GPU.
6. Takes between 30min - 1 hour for the flickering to start.

So I decided to run the 24' LED ACD off the 27' iMac, as I was trying to troubleshoot and work out what could be the cause. As I type, my 27 imac is flickering every 10 second, the 24 hooked upto the imac is absolutely fine. Initially I was thinking that it was might have been a GPU issue, though it is driving the 24 without sweat, the 27 imac will flicker and blackout while the 24 ACD is perfect running as an external display. I have also turned off the wireless and running on ethernet as there have been suggestions it might be causing the flickering. Looks like my new imac is going back. The LED panels might be faulty :( not sure a firmware update will fix this one.

Also if case people are wondering, I have everything hooked up (KB, mouse , external HD off my ACD, and they all run great with the ACD plugged into the Imac.

If you have a 24 ACD, do not worry it will not be affected by the same flickering that happens on the iMac.


macrumors 6502a
Nov 19, 2009

Sorry for your computer, but great you did the additional display test. This is the strongest hint so far that there may be a LED backlit power supply problem on the new iMacs (this is how the PS shutdown and restart would look like).

It is not a good news IF this really is the case, since the PS has likely to be redesigned (recall is in the air?).

You may also want to add your vote to the poll?

Tom B.


Original poster
Feb 11, 2008
Hey Tom,

I think your spot on. I have done some reading into PSU today and it seems that their efficiency drops as heat goes up. That would explain why it takes my machine around 30 min - an hour to develop the issues. As i noted at the time of the blackouts and flickering the PSU was at around 72-75 Celsius .

For anyone reading this and not getting the flickering / blackout issues can you please check your PSU temp.

I used istat


And also for people with flickering blackouts, would be good at what temp ur PSU is sitting at.

If this were to be a PSU issue, that would be quite bad as a recall would have to be in order :(
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