[Macpartners] iMac Hangs with Black Screen During Startup

[Andrew Munchbach]

Hi John--

The blinking light usually indicates that the Mac is not passing its boot
test -- called POST (power-on self test).

You could have RAM or Firmware issues.

http://support.apple.com/kb/HT2538?viewlocale=en_US

I would look at the following article and check to see what the blinking
lights indicate and try testing appropriately from there.

Best--
Andrew

On 7/21/11 12:41 PM, "John Canfield" <canfield at MIT.EDU> wrote:

>Hello,
>
>I can't find the solution to this problem on the regular online forums,
>but perhaps someone here has the answer.
>It's a late, 2006, 24" iMac (Core 2 Duo) that I'm trying to completely
>refresh by re-partitioning/re-formatting the HDD and re-installing the
>operating system. Unfortunately, it now hangs with a black screen during
>startup. I can start it in Target Disk more with no trouble, and holding
>down the Option key during boot brings up the expected startup disk
>selection screen. But regardless of the chosen disk (or even booting from
>a CD), after a short while with the grey Apple screen and rotating
>progress indicator, it goes completely black. I can hear the disk working
>for a while, but it never recovers from this state.
>
>The Apple site does mention that this particular model can exhibit such
>behavior, particularly when it uses Boot Camp and a Windows partition.
>Indeed, this machine did have such a configuration in the past (but I
>want it to be just OS X now). Apple suggests the following steps to fix
>the problem:
>
>1) Reset the PRAM. (Cmd-Opt-P-R) - did this a few times.
>2) Reset the SMC (Unplug power cord for 15 seconds) - also tried a few
>times.
>3) Restore the firmware to factory default - I can't get this to work:
>
>On another machine, I downloaded the correct Firmware Restoration disk
>image for this model (CD 1.4) and burned it onto a CD. The instructions
>at http://support.apple.com/kb/HT2213 say to do this:
>
>"Turn on the computer and continue holding the Power button. The sleep
>LED will blink rapidly, then slowly, then rapidly (3 quick blinks, 3 slow
>blinks, 3 quick blinks).
>
>On Macs that don't have a sleep LED, tones are used instead. Hold down
>the power button until you hear 3 long tones, then 3 short tones, then 3
>long tones. 
>
>Insert the Firmware Restoration CD while the lights are blinking or tones
>sounding, then release the Power button. A long tone is played once the
>recovery process has started."
>
>In my case, the sleep LED starts flashing rapidly and a single long tone
>sounds. But then, the LED changes to steady and the machine tries to boot
>off its hard drive. This happens regardless of whether the Restoration CD
>is inserted yet. It sounds like it tries to read the CD, but instead of
>displaying the firmware update progress bar, it just continues as before,
>ultimately going to the black screen again.
>
>I've tried booting from a Snow Leopard install DVD, and also tried
>putting the SL DVD on a USB drive and booting from that. The USB drive
>appears as expected in the startup drive screen, but when selected it
>just does the same thing and I get the black screen again.
>
>There's one more fix for this problem I found on the Apple site:
>http://support.apple.com/downloads/iMac_MXM_Update_1_0 however, the
>system apparently needs to be running OS X to install this update and I
>can't get there from here.
>
>Prior to starting this refresh, the machine was working fine. This is
>what I did to mess it up:
>
>1) Booted the iMac into Target Disk mode and copied any useful data to
>another machine.
>2) While still in Target Disk mode, used Disk Utility on another iMac
>running SL to re-partition the drive.
>3) Ran the Lion-GM installer from that other iMac to install the new OS
>on the Target Disk.
>4) The installer seemed to run correctly, and completed to the point
>where I was able to set up the initial user account, etc. (still using
>the working iMac booting from the other's Target disk).
>5) The very first time the iMac was booted from its own disk, it started
>up with a "patchwork" of textures and shading rather than a completely
>black screen, but every subsequent attempt goes to the black screen as
>explained at the beginning of my story.
>
>In hindsight, I probably shouldn't have tried to re-partition and install
>via Target Disk mode (though that method has worked for me in the past).
>
>Thank you for reading the lengthy details about my problem. I'd sure
>appreciate any clues to solving it you may have to offer.
>
>Thanks,
>
>John-Elmer