Malfunction Phenomenon
iPhone 7 Boot Loop and No Connection With Computer
Trouble Shooting
Because the phone can’t be connected to the computer, we can conclude that the problem might be from the USB circuit. Take out the motherboard and check if there is any corrosion or deformation, the motherboard looks good.
Continue to measure the diode values on the capacitors around USB IC U4001, measured result as below
- The diode value on capacitor C4004 is 274
- The diode value on C4002 is 393
- The diode value on L4021 and L4022 is the same value 693
So all the diode values are normal
At last, we checked the capacitor C4005, the value was 0, which was abnormal.
Remove the capacitor and measured the diode value again, the value was still 0. It means the problem is not from C4005.
And this capacitor was connected with the USB IC, after checking the bitmap, the problem is from the USB IC U4001.
Remove the USB IC and checked the diode value on the related pad E6, the short is gone. So we can replace another USB IC and tested again, with no boot loop and it also can be connected with the computer, problem resolved.
Summary
For phones with more than 1 issue, we can focus on checking one problem first. For example here, for the issue we can focus on checking the USB connection issue first, by measuring the USB relevant working conditions and checking the diode value on the PCB pads, finally, we find out the root cause which causes the no connection issue.
