Fall 2008 IMAC issues - please help. (freezing, garbled graphics).

Discussion in 'iMac' started by BSF1972, Apr 12, 2010.

  1. BSF1972 macrumors newbie

    Apr 12, 2010
    Last night my 24 inch IMAC seized up. Restarted and for the next 10 reboots had no screen, just the looping BONG sound every 5 seconds. Tried a few suggested key commands at start up. Screen would flicker like a strobe light with some horizontal banding. More reboots and eventually got to the desktop. Now that the Mac has sat overnight it will start up, but shortly after gets static-like garbled pixels and freezes up altogether. The visual problem can best be described as static, not banding, not lines, just scattered visual static dirt and broken graphics. Any advice? I miss my computer already and will have to take her in soon. Thanks for any info in advance.
  2. panzer06 macrumors 68030


    Sep 23, 2006
    Which GPU? If it's the 8800GS there have been a few cases of desktop nvidia cards having the same issues as the mobile 8x00 series. If not, sounds like bad logic board or GPU. Good luck.

  3. Hellhammer Moderator


    Staff Member

    Dec 10, 2008
    See guide, don't so the reinstall if you don't want though, it shouldn't fix this.

    @Panzer: BTW, the 8800GS is indeed the mobility version I think
  4. BSF1972 thread starter macrumors newbie

    Apr 12, 2010
    Okay so it was purchased mid August '08.

    24" iMac - Intel Core 2 Duo 3.06GHz, 2GB Memory, 500GB Hard Drive

    And I do fear it's a graphics issue now that I have read some more. Does this require a full logic board replacement? If so, on average what's that run?

    I have to say this is very discouraging. Been a Mac user for 15 years and this is the first one that has had issues.

    Thanks for your help.
  5. Richdave macrumors member

    Mar 23, 2010
    Outer East - Melbourne, Australia
    I believe your unit does have the 8800GS

    Can you run an ext display? If so does the problem continue on the ext display?

    If so then VGA or logic board are possible causes, if not then LCD may be cause.

    A repeating bong sound at boot may indicate a RAM issue??

    Have you got iStat widget installed? What are the VGA/GPU temps like?

    Are your fans running? what speed?

    If it will boot after sitting overnight then perhaps it is a heat issue...

    Are you willing to take front bezel off and look at the diagnostic LEDs? They may help isolate the problem

    Bezel removal

    LED #1
    • Indicates that the trickle voltage from the power supply has been detected by the main
    logic board. This LED will remain ON whenever the iMac is connected to a working AC power
    source. The LED will remain on even when the computer has been shut down or put to sleep.
    The LED will turn off only if the AC power source is disconnected or the power supply is
    LED #2
    • Indicates that the main logic board has detected proper power from the power supply when
    the computer is turned on. This LED will be ON when the computer is turned on and the
    power supply is working correctly.
    LED #3
    • Indicates that the computer and the video card are communicating. This LED will be ON
    when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON
    and you heard the startup sound, but LED 3 is OFF, then the video card might be installed
    incorrectly or need replacement.
    LED #4
    • Indicates that the computer and the LCD display panel are communicating. This LED will be
    ON when the computer is turned on and video signal is being generated. If the LED is ON
    and there is no image on the LCD display panel, the LCD display panel or inverter might be
    installed incorrectly or need replacement.

    IIRC, The LED's are on the logic board to the right of the memory slots and above the HDD SATA connector. The SATA connector is a right angle connector so may obscure them a little. They are surface mount and look like little white squares when OFF and green when illuminated

    LED location


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