Mysterious Imac screen goes 25% darker 2 seconds after start up?

Here is the deal ... A few months ago i noticed that my screen goes darker after start up. It happens round about the time when all my hard drives and documents show up on my desktop. It is so dark that watching movies and using iphoto is not really an option anymore. I have tried to calibrate the screen, I have reset the pram and the nvram. It seems to my a mystery. The screen is fine for a second!.. Please if anyone can shine a light on my ordeal it would be much appreciated.

2.1 ghz powerpc g5 imac / OSX 10.4.11

It could be caused by several things, however, in order to test the display on your iMac try this:

If you have access to another monitor, attach it to your iMac and see if the same occurs with the external monitor. If it doesn't, you have then eliminated software or any other operating system trouble. If the screen does the same thing on the external, post back and we'll go from there.

Your main display getting darker could be caused by a weakened or even bad backlight or the inverter (provides power to backlight) and possibly a defective GPU (graphics processor). It could also be caused by other hardware defects.

You didn't mention how old your G5 was or how much use it has had, but there is a good chance it may turn out to be a defective backlight or inverter.

Regards.

Hello .. great Thanks .. ok as far as I can tell the computer is from 2006 .. but i bought it refurbished and yes i have use it alot... also ..last summer it overheated a few times and I had to clean the ventilation hole... .. ok am now on the mission of an external screen to do the test .. . . ..

ok so i connected another screen (mirror) and my imac screens brightness popped up ...but then when i restart without the extra mirror screen it's dark again ... hmm .. well i am happy coz it seems it's a software problem and not a hardware problem ..but how to fix this?

Let me see if I can understand what you did....

When you attached the external monitor, both screens light up normally? Then when you removed the external, the problem came back?

According to MacTracker your machine does video mirroring only so that kind of narrows us down to perhaps it being a graphics adapter problem. Your machine should have an ATI Radeon X600 XT graphics adapter. (Again according to MacTracker which is not always correct.)

Open the G5 up and blow all the dust out of it. Make sure the GPU card and fan are clear of dust. Pull the card from its socket to clean it and then reseat it carefully. Post back.

Regards.

This strange thing happened to me too!

I bought an iMac 20 aluminum 1 year ago and suddenly the same thing happens. When it starts, for 2 seconds the display is perfect, then it blinks and become very dark! I use a second monitor Samsung 19" and both become dark. The most strange is that if I disconnect the miniDVI-DVI adapter it blinks and become perfect. After that, I can connect again the second monitor and it still perfect. The problem is that if I restart it comes again with the same problem. I've already tried the oposite, leaving it disconnected and reestarting and the same problem happens. Then if I connect the second monitor it becomes perfect again!!! Mistery!!!! It looks like it is a driver problem..., because if I boot from the original DVD the problem doesnt happen. I also tried to reinstall the system form the DVD and it doesnt resolve the problem. But I didnt try the complete new installation... HELP! Please!!! Sorry about my english, I'm from Brazil.
Thank's for your help!

chscag said:

Open the G5 up and blow all the dust out of it. Make sure the GPU card and fan are clear of dust. Pull the card from its socket to clean it and then reseat it carefully. Post back.

Rega

Click to expand...

Ok .. i will do this .. later on tonight ... sounds like a good plan somehow
thank you and I will post back ...

ok i am about to open the computer... blow of dust from the fan and disconect the gpu, clean it and gently put it back ... But i have now been looking for a map of the inside of the imac.. I can't find the gpu.. What and where is it ...

by the way .. i am very thankful ..

so thanks again .

loop1984 said:

ok i am about to open the computer... blow of dust from the fan and disconect the gpu, clean it and gently put it back ... But i have now been looking for a map of the inside of the imac.. I can't find the gpu.. What and where is it ...

by the way .. i am very thankful .. so thanks again .

Click to expand...

Your graphics card should be easy to identify by looking at the card(s) which are in your Power Mac G5. Look for the one that says "ATI" on the chipset. It may even have a small fan on it (not sure about that particular card). Let me know if you still can't find it. BTW here's a picture description of your machine (somewhat) from the Wiki. Link

Regards.

ok .. great .. so i started.. to open but got stuck right away .. I need some special small tool to open it up with ... also I have an g5 isight... i haven't really found any confirmation that I myself can open this "tricky" computer.. but.. I am gonna try ... ok i am off to find that tool ... if you have any tips for me regarding opening this baby up .. yes .. i need it .. other wise .. I'll let you know how it all went

cheers !!!!

Please, let me know if this works, and then I can try myself in my mac. I'm afraid it should be much more difficult to open an iMac 20 than a G5. Thanks for now...

The OP says his is an IMac G5 2.1Ghz. That is the last iSight model 20" iMac G5. I own that machine and the service manual and been in there. There is no plug in graphics card. It's all on the main board. The ATI Chipset for the video is on the main board. Just a heads up.

If he does not own an iMac G5, there were no G5 2.1Ghz Macs except that last iMac G5 that I own.

The Flat Panel Display plugs into the main board on a connector to toward the bottom left of the board.

loop1984 said:

ok .. great .. so i started.. to open but got stuck right away .. I need some special small tool to open it up with ... also I have an g5 isight... i haven't really found any confirmation that I myself can open this "tricky" computer.. but.. I am gonna try ... ok i am off to find that tool ... if you have any tips for me regarding opening this baby up .. yes .. i need it .. other wise .. I'll let you know how it all went cheers !!!!

Click to expand...

Be sure to read the post by dtravis7, your machine does not have a separate graphics card as I thought but instead uses an integrated graphics chipset by ATI. So it looks like we're back to square one again. If it turns out the GPU is defective the entire main board will have to be replaced. You can try running the Apple Hardware Test and see if it picks up on it, but in truth, the AHT is not very accurate. Try anyway. Apple Hardware Test Link

Regards.

I have tried running the AHT and the simple test do not show any error. The extended test gives an error messsage:

This is the only error I get.

I have also tried running OSX from the CD and then there is no error when starting up.

This definitely point to a driver/software error. Also, hotplugging a second monitor fixes the problem until you have to reboot. Seems like some software is not detecting the monitor correctly until it get a "kick".

Sorry about the late reply. The Apple Hardware error codes are only available to Apple technicians and the genius bar at the Apple store. I have no idea what that code means . It might be worth your while to give Apple a call and run that code by them. Could be your problem.

Regards.

I don't think the problem is anything related with dust or any hardware, because if I boot from the original DVD the problem doesn't happens.

Ok .. my screen is normal if I connect the external monitor cable (no need to connect an actual monitor... just the cable works fine)... I can live with this minor annoyance .. so I wont be opening up this (isight) imac g5... Thanks for all the input and suggestions ...

Finally solved the display problem...

I still don't know what exactly happened with my display driver, but I finally solved the problem making a clean Install of Mac OSX 10.6 Snow Leopard. First I made a backup of everything using the Time Machine, then installed the Snow Leopard. After booting from the DVD, I used Disk Utility and erased the hard disc before clicking in "Install" button.

I think that a Clean Install with Mac OSX 10.5.8 should work too.

Thank you everybody for the help!!

Best Regards,

Julio