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iPad 4, not booting, strange charging pattern.

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  • iPad 4, not booting, strange charging pattern.

    We received an iPad 4 for battery replacement, but there is more going on with this tablet..
    • If the battery is connected for the first time, and charging cable plugged in it draws normal charging current and charges the battery.
    • When the charging cables is disconected, and reconnected it only charges at 0.010 Amp.
    • The pattern is the same with the original battery and the new battery.
    • The tablet does not boot in any of those cases, and iTune does not detect it, nor did it ever show any image on the lcd.

    Prior to Repair
    Upon receiving I tried to turn it on, but the owner said the battery was drained.
    I've plugged in the USB ammeter and only had a 0.010 Amp draw. I left it on the charge for a while, but no change.

    I've proceeded with replacing the battery. After installing the new battery, it appeared to be charging right 1.80 Amp on the usb meter, but the tablet never booted.
    I've disconnected the charging cable and reconnected it. Now it only draws 0.010 Amp again.

    • If I disconnect the battery, then connect it again, then plug the charging cable, it will charge at around 1.80 Amp, and the battery voltage does increase over time. That being said, the tablet does not boot; it does not show any image on the lcd; iTune does not reconize it.
    • If I disconnect the charging cable and reconnect it it draws 0.010 Amp again.
    • This is consistent with the old battery and the new battery
    • Pack_Pos (Positive terminal rail to the battery) has battery voltage when the battery is connected.
    • When I plug the charging cable for the first time and usb meter is reading 1.80 Amp, we get a slightly higher voltage reading on that rail which indicated that it is in fact charging.
    • When I unplug the charging cable and reconnect it, the usb meter reads 0.010 Amp, and the voltage reading is now the same as the battery voltage again.

    Beside doing fpc replacement, water damage repairs, and charging port installation, this is my first time doing board repairs on a iPad. I'm not familiar with it's symptoms or signature faults.

    I will be looking at the following this afternoon:
    • Check if any abnormalities can be found on the other battery lines
    • Check if PMIC is creating basic voltages such as PP3v0, and 1v8
    • Replace Tristar
    Your help:
    I will be working on this today but being that this board is somewhat unfamiliar to me, I would appreciate any feedback, or suggestion as to where I might want to focus my attention.

  • #2
    I think replacing the battery in an iPad that doesn't boot is not a good idea---native iPad batteries are usually better than replacement batteries which are highly questionable.

    Other than that, it is like any other no power device. Connect to DC power supply with alligator clamps and see why it won't boot and follow rule number 1---known good housing, known good charge port etc.

    iPad 4 has the 1608 tristar, and that goes bad a lot, remember that 1610 tristar is not compatible. It also gets weird faults that are often pmic. Not much else goes wrong with ipad 4. oh, one more--speaker amp area shorts.


    • #3
      Thanks Jessa for the quick reply.


      • #4

        Ok, so I've put the board in a known good assembly, and it has the same behavior.
        Bad parts ruled out.

        I've compared a diode reading of the positive rail terminal of the iPad to a known good.
        I have a reading of 0.280V (bad board) vs 0.388V (known good)

        So next up is looking for a shorted line subsequent to Pack_Pos.


        • #5
          I've check VCC_main, and the reading is normal.
          Changed Tristar to rule it out, but it still behaves in the same way.
          Removed Q8104 connecting PPBATT and VCC_main, and I still have the same low reading of 0.280 on the line.
          On DCPS it does not draw any current when prompting to boot via usb.

          The only two other IC on the line are the PMIC and a RF IC. I'm leaning towards that the PMIC is the problem.
          Unfortunately we don't have that PMIC in stock, and it would take a while before I can get one because of the Chinese holidays.

          So this is where this one is going to end.

          Thank you Jessa for your help.


          • #6
            You can get the pmic on eBay locally


            • Sly
              Sly commented
              Editing a comment
              All I'm finding is from China.