Standby 12V power (permanent power coming from power supply and present as long as AC cable is connected, even if computer is o ) corresponds to LED #1īacklight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup)ġ2V Run-Mode power to logic board (coming from power supply, present as long as system is on or asleep) corresponds to LED #2 Note: If present, the 12V power will come directly from the Power Supply to the Hard Drive thru the power harness cable. If you need to play with your multimeter, here is where to use it:ĥV power for Hard Drive(s). If external display does not work verify/replace the logic board If external display works then verify/replace the LED backlight board -If external display works then verify/replace the LCD panel Verify external video functionality, and according to result check the following items:.Inspect LCD display cables for cable damage.Verify cable connections between LCD panel and logic board. If fourth LED is not visible after power on: LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card found, and internal LCD found. If POST boot chime is heard, go to No Video symptom ow.If POST boot chime is not heard, go to Won’t Start Up symptom ow,.Verify if boot chime is present and fans are running when powered ON (reset SMCĪnd PRAM, check backup battery if necessary for proper boot up):.Verify that the MXM video card is seated properly.If third LED is not visible after power on: LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found. Verify power button connection to logic board. If second LED is not visible when power button is pressed: LED #1 + LED #2 = Power available, and system is powered on.
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