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820-3476 Cycles on and off

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  • 820-3476 Cycles on and off

    Hi,

    I received this 820-3476. They said it was liquid damaged and they cleaned it.
    I found rice in the machine. I can't see any corrosion. They cleaned it very well.
    Well, they said it powered on and there was nothing on the screen.
    After they tried it to power on few times it started to beeping.

    I don't know what's true or what's not, but here it just cycles on and off.
    Fan spins up stays on for 1-2 sec and then turns off and on again.

    I ultrasonic cleaned the board.
    Actually there is no visible corrosion on the board except LVDS connector which I already removed.

    There is CPU Vcore which is 1.68V.
    I checked all the Pins on the SPI rom. I can see there is communication on Pin 1,2,3,5,6 and there is 3.3V on Pin 7 and 8.

    Y1905 oscillates when the board cycles on. There is 1V on the other side of Y1905.

    U1900 I've 3.4V on Pin 1 and I've the right clock signal on Pin 12.

    U1950 looks fine.

    Could this be a ROM problem?

    Thank you.

    Edit:

    Checked U7100 area too.

    Pin 17/18 4 Ohm
    Pin 27/28 21.8 Ohm
    Last edited by bighead; 01-03-2017, 09:31 PM.

  • #2
    As you probably know by now this could be anything. ROM will be my last suspect in this case. More likely will be corroded resistor in the SPI lines to CPU or SMC or crap under the SMC.

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    • #3
      Yes, unfortunately I know . The on/off cycle not that fast, because the CPU gets hot with the heat sink on.
      It "feels" like it restarts right when it should chime. Well, I'll check those resistors in the SPI lines.
      I'll try to heat up the SMC and if it it still does not work it will be good for parts.

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