Last week, a man sent his iPad Air to me, and told his iPad Air doesn’t work, because it displays white Apple logo when power on and then back to black screen. But later white Apple come back again. It always has been in this cycle, as shown in picture 1-1. This phenomenon makes him very confused. So, let’s see what happen to his iPad Air.
Above all, disassemble the iPad Air, remove shielding case and press the power button. When iPad shows white Apple logo, use hand to touch each main chip and feel power chip U8100 hot. The location of U8100 is shown in picture 1-2. Without connecting display screen, press power button. Touch U8100 and now it is not hot.
Then, analyze according to display circuit working principle. Without display screen , the backlight bootstrap circuit of display screen does not work. With display screen, the circuit will work. Thus, the problem happens on the display screen backlight bootstrap circuit. Continue to analyze the backlight bootstrap circuit (picture 1-3 and 1-4). IPad Air’s backlight bootstrap circuit has two voltages, PPLED_OUT_A and PPLED_OUT_B.
Third, after measuring, the voltage drop of D8228 on motherboard is 1.9v, which is close to the state of an open circuit. While the normal voltage drop should be about 0.2v. The relevant circuit of D8228 is shown in picture 1-5. Measure the negative pole voltage of D8228 is 0.5v when getting electricity on but do not press the power button. But the normal data should be battery power supply voltage (3.7-4.2v).
Fourth,Based on the data gained above, we know D8228 is damaged. The location of D8228 is shown in picture 1-6. After replacing it and measuring, the voltage of negative pole rises to 16.7v and the chip is not hot. But when enter system, iPad Air still reboots. First root it to exclude the problems from software.
Finally, under the stare of power off, press HOME button and then insert USB cable to enter recovery mode to root, as shown in picture 1-7.
After using iTunes to install the newest firmware, machine enters system normally, as shown in picture 1-8. The iPad Air is back to normal.