Malfunction Phenomenon
The phone can be turned on and the touchscreen can work, but the 3D touch does not work at all.
Trouble Shooting
For the iPhone 7 Plus 3D Touch Not Working issue. Disassemble the phone and check the motherboard, especially the area around the 3D touch connector J3801.
We found the capacitor C3828 and the relevant pads on the PCB were lost. This capacitor is a filtering capacitor on the PP_VDD_BOOST line to the LDO chip U3801.
Continue to check the diode value on the capacitor C3724, the value was 379.
Then we continue to the connection pad of U3701, the value was 521, the two pads were connected, the diode value should be the same.
So we can conclude that the rail between C3724 and Pad 4 of U3801 was open.
Jump a wire from C3724 to the pad of U3801 and tested again, the diode value was back to the same as the value on C3724, and the 3D function was also back to normal, problem resolved.
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