Why your iPhone won’t Turn ON and How to Solve It_Logic Board Repair

In today’s guide, I want to take you through various procedures on how to troubleshoot and solve circumstances where iPhone won’t turn ON. Normally, there are 5 possible scenarios where an iPhone may not turn ON. I will take you through every stage and how to solve it.Whether your iPhone:

i. Fails to turn ON after falling

ii. Does not turn ON after pressing the power button

iii. Fails to turn ON because current exceeds the normal rating, or

iv. Does not turn ON even after ammeter response

v. Does not turn ON but it could work when connecting with computer

I will show you how to go about every procedure here.Keep reading to learn more:

iPhone won't Turn On Example 1:

The iPhone 6S cannot power on after falling

Failure Phenomenon

The iPhone 6S cannot power on after falling. As shown in Pic 2-40.

Why your iPhone won’t Turn ON and How to Solve It

iPhone 6S after falling Pic 2-40

Maintenance Procedures:

  • After connecting the motherboard with the DC power supply, no current was shown and there was no obvious short circuit on the motherboard itself. It was found that the current is a bit large after short connecting the boot pin, which indicated that there was the short circuit of a voltage on this iPhone’s booting circuit.
  • Measuring the ground impedance values of each major power supply on the motherboard by multimeter, it was found that the ground impedance values of the voltage PP_CPU are 0.002, which is obviously too small. As shown in Pic 2-41.

Why your iPhone won’t Turn ON and How to Solve It

Short circuit of the voltage PP_CPU Pic 2-41

  • Since there were so many components connecting with PP_CPU voltage, it was a time-consuming job to eliminate them one by one. So we chose to use fumigation method that is to dip the soldering iron into the rosin then evenly spread them on the surface of each component.
  • After starting up with short connecting the booting pin, we noticed that the rosin on the surface of the C2006 melted faster. The C2006 is on the edge of CPU. The position of the C2006 capacitor on the motherboard is shown as Pic 2-42. The circuit of the C2006 is shown in Pic 2-43.

Why your iPhone won’t Turn ON and How to Solve It

The position of the C2006 capacitor on the motherboard Pic 2-42

Why your iPhone won’t Turn ON and How to Solve It

The circuit of the C2006 Pic 2-43

  • After directly removing the C2006 capacitor, we measured the ground impedance values of the voltage PP_CPU again and it was back to normal. The iPhone could turn on as normal after connecting the screen. The problem solved. As shown in the Pic 2-44.

Why your iPhone won’t Turn ON and How to Solve It

The iPhone could turn on as normal after connecting the screen Pic 2-44

iPhone won't Turn On Example 2:

The iPhone 6 Plus no reactivity when pressing the power on the bottom, unable to start up

Failure Phenomenon

There was no reactivity when pressing the power on the bottom and the iPhone failed to start up.

Maintenance Procedures:

  • Disassembling the phone and taking out the motherboard. Charging the main board with DC power supply but there was no reaction when triggered the boot pin with tweezers.
  • Measuring the power supply of boot pin by multimeter and the ground impedance values of each power supply. The boot pin had 1.8V and we noticed that PP1V8_SDRAM, PP1V2, P1V2_SDRAM were all short circuit to ground.
  • Founding out the parts using these voltages according to the circuit diagram, then taking them down one by one to eliminate the short circuit, as shown in Pic 2-14. It was found that the ground impedance values of PP1V2 were normal again after we removed the power supply chipset.

Why your iPhone won’t Turn ON and How to Solve It

The soldering pad after removing the power supply chipset Pic 2-14

  • Continuing to eliminate the short circuit of other parts. After we disconnected the power supply of U5411_RF (As shown in Pic 2-15), the ground impedance values of voltage PP1V8_SDRAM was normal.

Why your iPhone won’t Turn ON and How to Solve It

The position of U5411_RF on the motherboard Pic 2-15

  • After removing the CPU (As shown in Pic 2-16), the ground impedance values of voltage PP1V2_SDRAM was normal. But since this voltage was the power supply to CPU and temporary storage, so we still not sure if it was the damage of CPU or temporary storage, we could only repair by eliminating.

Why your iPhone won’t Turn ON and How to Solve It

The CPU soldering pad Pic 2-16

  • The disassembled CPU has slight copper dew, coating the exposed copper position with the green oil and replanting the ball. The CPU after finishing the ball planting is shown in Pic 2-17.

Why your iPhone won’t Turn ON and How to Solve It

The CPU after finishing the ball planting Pic 2-17

  • After soldering the CPU, we could sure that the fault was caused by temporary storage because the motherboard could be powered on. After replacing with a good temporary storage, the phone could be started up. This problem solved.

iPhone won't Turn On Example 3:

iPhone 6 won’t turn on, the electrical current is higher than normal

Failure Phenomenon:

iPhone 6 won’t turn on, the DC power shows the current beat normal, but it is slightly higher than the current value of the normal mobile phone.

Maintenance Procedures:

Test the ground impedance value of every display screen pin is normal, and then find out that there is no voltage output when measuring the power supply of display screen at ±5.7V. Replace the power chip U1501 directly and the same failure still occurred, in this case, we can figure out that there are problems in the resistance capacity element of the peripheral circuit.

After measuring carefully, the C1554 capacitor is found to be the short circuit. The position of C1554 in the bitmap is shown in pic 2-3, and the physical location is shown in pic 2-4.

After replacing the defective capacitor, the voltage output of the main board is normal, and the problem has been solved.

(The C1554 capacitor is without 5.7v voltage output indicates that the display touch IC is damaged or the starting condition is insufficient.)

Why your iPhone won’t Turn ON and How to Solve It

Pic 2-3 Location of C1554 in The Point Bitmap

Summary:

The iPhone 6 won’t turn on is caused by the defective C1554 capacitor. Replace the defective capacitor to solve the problem.


Why your iPhone won’t Turn ON and How to Solve It

Pic 2-4 The Physical Location of The C1554 Capacitor

iPhone won't Turn On Example 4:

iPhone 6S plus won't turn on while there is the response when connecting to the computer.

Failure Phenomenon:

There is nothing displayed on the screen after turn on the iPhone 6S plus while the computer prompts for a password when iPhone 6S plus connected to it.

Maintenance Procedures:

Disassemble the iPhone 6S plus and remove the motherboard, use the power cord to connect the DC power and power on the main board, the DC power shows the current beat normal, but nothing shown on the screen, most of these failures are caused by the abnormal power supply of the display screen.

Use a multimeter to measure the ground impedance value of every display screen pin, no short circuit or open circuit has been found, then the test is the power supply of the screen display. The power supply circuit of screen display is shown in pic 2-20.

●  The normal voltage of C4002 is 6V, but it’s only 2V now which is obviously lower than normal standard.

●  During analyzing the problem, we find that the working conditions of the U4000 are normal, so it is estimated that the damage is caused by U4000, the problem has been solved by replacing U4000. The physical location of U4000 is shown in pic 2-21.

Why your iPhone won’t Turn ON and How to Solve It

Pic 2-20 Display Power Supply Chip U4000 Circuit

Why your iPhone won’t Turn ON and How to Solve It

Pic 2-21 U4000 Chip Physical Location

iPhone won't Turn On Example 5:

iPhone 5S won’t turn on while the ammeter responds during testing the iPhone 5S.

Failure Phenomenon:

The ammeter up and down when power on the iPhone 5S, but nothing displayed on the screen. After changing a new screen display, the iPhone 5S shows the white screen and then turned to the blue screen, after that the iPhone 5S restart.

Maintenance Procedures:

Firstly, disassemble the iPhone 5S and remove the motherboard, do not connect the power button ribbon, the mobile phone can be switched on normally.

Secondly, replace a new power button ribbon, and find the problem still occurred.

Thirdly, the iPhone 5S white screen, blue screen, and restart problem appear when switching the mute button to the mute state, while the iPhone 5S could be turned on normally if no switching to the mute state.

●  Fourthly, according to the maintenance ideas, it’s better to analyze if it’s the software problem first, otherwise to consider if it’s the hardware problem. Enter the DFU mode first and flash the iPhone 5S, after flashing, everything is fine, the problem has been solved.


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