Faulty model】：iPhone 7
Fault phenomenon: No 4G letter on water inserted card
The phone is in the water, the plug-in card has a 2G signal, no 4G signal.
The phone can be on 2G signal, not on 4G signal, indicating that the 4G signal part of the problem, the phone is water before this fault, then water parts in the 4G signal channel caused by.
Mobile phone disassembled, take off the motherboard, microscope observation, observation is not out, remove the 4G amplifier chip.
Remove the TDDPA_RF 4G amplifier chip, measure the ground resistance of the pad, found that the 11 pin MLB_PA_VBATT to ground infinity, it seems to break the line.
View the circuit diagram, found that this voltage to TDDPA_RF, MLBPA_RF 2 chip, MLBPA_RF this chip is not installed.
This voltage is PP_PA_VBATT through R7113_RF resistor renamed MLB_PA_VBATT, microscope observation, found that R7113_RF resistor has been corroded.
Check the circuit diagram labeled as 0 ohm resistor, directly shorted it.
Put the chip back on, install the machine test.
Test 4G signal back to normal, water caused by 4G amplifier power supply is insufficient, caused by no 4G signal, to the end of this repair.