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iPad mini possible dead cpu ?

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  • iPad mini possible dead cpu ?

    Would an iPad mini with a static .102a draw when the power button is pressed look like a dead cpu to you ?.

    so a bit of history.....the board was water damaged. I ultrasonic cleaned it and replaced several dodgy looking caps. The board would now power on, but would die on the Apple logo. I found a shorted speaker amp ic and removed that, as the 3.3v line was short to ground. I guessing the pmic was damaged as when the speaker amp was dying the pmic got rather hot.

    replaced the pmic as ipad would not charge. The iPad now charges fine and will charge battery fully.
    all voltages appears to be present apart from a line called spare, (not sure if this is used), acc , (presumably accessory, so probably irrelevant), grape (no screen plugged in), cam (no camera plugged in).

    the Ones I think that are relevant are pp1v2_soc which is supposed to be 1.2v but is 1.01 and pp1v2_cpu which is supposed to be 1.2 and is 1.05.

    all the other voltages appear to be present. The iPad isn't detected by iTunes and nothing is displayed on the screen.

    It wikk charge the battery but that's pretty much all it does.

    I did have have the cpu heat sinked with coins, there is no sign of solder balls around the cpu but the cpu seems to be not very lively according to the power supply.

    diode mode measurement for those lines are
    0.092
    and
    0.160

    the thing is I can change a nand on an iPad mini without frying the cpu, and the nand is vastly closer than the pmic. It's just when I do nand I use a big aluminium block taped down on the cpu, where as the pmic I just added a couple of large coins, so it's possible but there are no signs of solder coming out of the cpu.

    another possibility is a semi shorted cap on the cpu line, but that's as bad as a dead cpu as equally as impossible to find without hot tweezers.
    Last edited by crea2k; 12-11-2017, 12:48 AM.

  • #2
    This is what jumps out at me from your post:
    "It would die at the Apple logo" What does this mean? Was it detected by iTunes? Apple logo then no image is common on the iPad mini. This will happen if you try to boot a mini with no digitizer or a faulty digitizer as a function of the hall effect sensor for smart cover function not being present. Long press the power button to bring back image.

    "3v3 line shorted at speaker amp." From memory, there are two 3v3 lines and I don't think either go to the speaker amp, it goes to NAND power and is a common PMIC short from charger damage. Speaker amp damage itself is common after water damage, but i'm not sure that is related to 3v3---I don't have the mini schematic in front of me.

    "The iPad now charges fine and will charge battery fully." I am not sure about this, but I don't think an iPad mini is capable of pulling normal charging current in the absence of a functional CPU. The batt_swi line for example goes to both the PMIC and CPU and an iPad will not charge at normal charging current without this line present. I doubt that CPU play no role in regulating the permission of charging.

    "all the other voltages appear to be present. The iPad isn't detected by iTunes and nothing is displayed on the screen." What is the current draw sequence on DC power supply exactly at prompt to boot--video?
    Again from memory--I believe that grape voltage is required for image. It may not be detected by iTunes because it is boot looping from software corruption, which you may not recognize if it also has an image line or backlight problem.


    "Would an iPad mini with a static .102a draw when the power button is pressed look like a dead cpu to you ?." No. I would expect an iPad with a dead CPU to be able to go into recovery mode and be detected by iTunes and assuming that the "static 0.102A" draw means current consumption from dc power supply connected to battery with nothing in usb port---I would expect a dead CPU to show either a much lower steady current or a flickering very very low current.

    I think it would be very difficult to kill a CPU with heat, especially if you had a heat sink on it, but anything is possible. If the CPU is dead, it would likely be from either an inappropriate voltage going into it from water damage, or from a bridged line under pmic sending inappropriate voltage into it.

    Most likely case is that PMIC is not making contact on some data line pads. iPad mini pmic can be difficult to not pull important pads along the border and to sufficiently clear underfill entirely under the chip so that the chip can find its way to the pads on all sides. If this iPad were in front of me, I'd put a screen and digitizer on it, then prompt to boot with DC power supply and watch the current consumption exactly. If it shot up to 0.102A out of the gate and stayed there I would expect a short on some minor line somewhere.


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    • #3
      Yes it hits 0.102a out the gate soon as you press the power button (on dtps) , and sits there at that current draw. Il check again for shorts as I didn't see any others last time I checked.

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      • #4
        sounds like a data line is bridged to something else or ground likely under pmic.

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        • #5
          It was doing it before I changed the pmic, it happened after the short, was working then, but no life after short removed so replaced pmic as was getting hot before the short was removed, then stopped getting hot like somethihg had blown out.

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          • Jessa_the_Professa
            Jessa_the_Professa commented
            Editing a comment
            was this the classic 3v3_out short? What line was short at speaker amp?

        • #6
          Iv still got to go back to this yet, the cap that sits in front of the speaker amp nearest to the bottom of the iPad registered short, but was the ic that had shorted. Il go back to it in the next day or so

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          • #7
            By the way what usually shorts out on the 3.3 rail, is it a cap ?.

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