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trying to repair imac motherboard model A1311 (late 2009)

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    #1

    trying to repair imac motherboard model A1311 (late 2009)

    Hello everyone,
    Some time ago I acquired an imac computer model A1311 that does not boot (motherboard ref. 820-2494-A).
    The power supply is working properly (I have two), it delivers the 12V on pin 4 of the power connector (PP12V_S5).
    As I have the schematic diagram and the boardview, I have made a series of measurements, obtaining the following results:

    PP12V_S5 -> 12V
    3V3REG_VCC -> 3.12V (something low?)
    PP5V_S5_LDO -> 0.02V
    3V3REG_VCC -> 0.0066V

    I have also measured the resistance to ground on the coils, and I can't find any shorts.

    It is not clear in the schematic where the 5V voltage is obtained, since the reference PP5V_S5 only appears in two parts of the schematic, in the input of a 4.7uf capacitor and in the input of a regulator (SMC AVREF Supply), which enters PP5V_S5_AVREF and obtains PP_3V3_S5_AVREF_SMC.

    The 4 status LEDs remain off at all times, the power switch is not powered either.
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    #2
    I'd check the details on Page 69 that shows power rail sequencing.
    Do you have 3.3v on PP3V3_S5 and 1,05v on 1V05_S5?

    Comment

      #3
      Originally posted by kevingill View Post
      I'd check the details on Page 69 that shows power rail sequencing.
      Do you have 3.3v on PP3V3_S5 and 1,05v on 1V05_S5?
      I have found that PP3V3_S5_REG delivers 0.0075V, and 1V05 practically 0v.
      When examining the VIN input on U700 (ISL6237) I check that 3.1V arrives, which does not make sense, since the datasheet indicates that the input voltage range is between 5.5V and 25V.
      As the power supply is received from PP12V_S5_REG, it passes first through the resistor R7650 (2.2 ohm), I remove this resistor and it measures more than 100 ohm, so I replace it with another one of the correct value.
      At this point, now the output voltage is no longer 3.1V, but 11.8V, but then I notice that the ISL6237 circuit heats up and delivers nothing at the outputs.
      Maybe it is this integrated that is broken?

      Comment

        #4
        It will be U7600 or the FETs on the outputs. Check for any short on PP3V3_S5.

        Comment

          #5
          Originally posted by kevingill View Post
          It will be U7600 or the FETs on the outputs. Check for any short on PP3V3_S5.
          I have not located any shorts and the Q7660 and Q7610 mosfets seem to be fine. I then tried to remove the U7600 controller, but I'm afraid my tools are no good.
          After a long time with the heat gun at its maximum temperature (theoretically 480 degrees), I have not been able to desolder it.

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