Malfunction Phenomenon
The phone turns on normally, but the touchscreen unresponsive.
Trouble Shooting
For the iPhone X No Touch Issue. First, we should take apart the phone and run a cosmetic inspection of the motherboard, there is a sign of water damage around the J5800.
Then clean the place where had been water damaged with PCB cleaner. And get the display assembly installed and connect the battery connector with DC power supplies to test, the current reading on the ammeter is normal.
Then measure the diode value of J5800, the measured diode value of pin 28 (PP5V25_TOUCH_VDDH) is OL, which is abnormal. According to the circuit diagram, the 5.25V is outputted from U5600 to pin 28 of J5800 by way of C5891.
Below block diagram for reference.
Or you can also check the schematics as below to know the connection relationship.
Then we clean up the glue around J5800 and find that FL5891 is completely corroded. The bonding pad is oxidized and blackened.
So we scrape the pad with a knife and put some tins with soldering iron, then short connected pad. After that, we measure pin 28 of J5800 is 620, which is normal. Got the motherboard mounted and test again, the touchscreen is back to normal, problem resolved.
