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Thread: iMac 21.5" EMC 2428 doesn't turn on, no fans. Only 1st LED.

  1. #1
    Join Date
    Nov 2016
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    Default iMac 21.5" EMC 2428 doesn't turn on, no fans. Only 1st LED.

    iMac 21.5" EMC 2428 doesn't turn on, no fans. Only 1st LED.

    My iMac doesn't turn on.

    On the logic board is only the first 1 Diagnostic LED turned on. No chime, no fans, no hdd. The power Button is okey.

    What i find is:

    The 4st PIN on power supply output connector shows 12,05V

    The 6st PIN on power supply output connector shows 4,95V

    The 1st PIN of J600 shows 12,05V

    The PIN 2 and PIN 3 shows 0 - 0,2V

    PIN 4 of power supply output connector is railed to PIN 2 and 3 of J600 but they doesn't have 12,05V

    It is correct? Or should they have also 12,05V?

    Is the power supply fail?

    Do I understand it right? PP12V_G3H generated PP12V_S0? Which transistor is responsible for this? Through PP12V_G3H flows 12.05V. What does PM_SLP_S3 have to do with it? Can someone tell me which measurements I have to perform?

  2. #2
    Join Date
    Nov 2016
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    2

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    I checked some rails:

    PP3V3_S5_REG = 3.3V OK!
    PP3V42_G3H_REG = 3.4 V OK!
    PP12V_S5 = 12.04V OK!
    PP5V_S3_REG = 0.1 V NOT OK!

    PP5V_S3_REG is shorted to the GRD.

    What could it be?

  3. #3
    Join Date
    Aug 2001
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    Grangeville, ID USA
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    Default

    Might give David at www.galaxyhp.com a call. He has a better path on understanding signs of a power supply failure and can provide you with a replacement power supply as well.
    molṑn labe'
    "I am a mortal enemy to arbitrary government and unlimited power. I am naturally very jealous for the rights and liberties of my country, and the least encroachment of those invaluable privileges is apt to make my blood boil."
--Ben Franklin

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