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ITiPhone 8 no boot after repairing by another repair shop. Compare with the water-damaged phones, this kind of second repaired phone is even more difficult to repair!
When receiving the phone, first connect with the DC power supply to check the boot process.
Boot current is abnormal, the current loop is between 250mA and 0mA.
As per previous repair experience, this kind of current loop with a high chance caused by USB IC.
The structure of the iPhone 8 is similar to iPhone 7, can be disassembled easily. After teardown, found the components around the charging part get resoldered.
The USB IC is just located around the NAND chip.
We need to replace the USB IC with a new one with the same specification.
Tip: from the iPhone 8, USB IC is different. Not compatible with iPhone 7 and before models.
Remove the U6300 and measure the diode value on the PCB pads, all pads with normal diode value.
And test the boot current again without USB IC installed, there is no loop again, the phone can enter into the system normally.
Then replace the USB IC with a new one.
Then assemble the phone and try to trigger the boot with charging, but the phone cannot be booted up, also there is no charging voltage. So there must also with charging problem!
Replace another iPhone 8 charging port to test, the issue remains the same.
After that, we can draw a conclusion, the issue might come from the charging-related rails.
Find out the charging IC and related components on the Refox bitmap.
Apply with DC power and measure the voltage value, the basic working conditions (the power supply for charging IC) like the PP_VBUS1_E75, around 5V voltage can be measured on C3301.
So the issue might be caused by charging IC U3300 getting damaged.
*. Different from iPhone 7, there is no charging tube (MOSFET). So we go to charging replacement directly.
Replace a new charging IC then test, the boot current becomes normal and there’s a 4.2V voltage can be measured on the battery connector.
Assemble the phone and test again. Everything is OK, the phone can also be charged normally.
Summary
- The phone gets fixed by USB IC and charging IC replacement.
- For the no boot issue caused by charging relative parts, we can start from the battery and charging dock replacement first. After that, if still unresolved, check the USB and charging IC relative rails.
