Malfunction Phenomenon
iPhone 7 Turns On With Black Screen, but the boot current was normal.
Trouble Shooting
First, check the appearance of the motherboard, there is no trace of deformation or water damage.
Then assemble the screen and connected it with the DC power supply, turned on the phone, the current value changed normally, but the screen still cannot be lighted up, which means the problem is from the display circuit.
Disconnect the power and measure the diode value on display connector J4502, all the diode values are normal.
Then connected with the DC power supply to provide the power, measure the voltage values on the display power supply related rails, the power PP5V7 and PN5V7 on pin1 and pin5 are abnormal. Diode values are normal but voltages are abnormal, most of the time the issue was from the chip side.
Remove the display PMU U3700 and measure the diode values on the pads under the chip, the measured diode value on pad B2 was OL ( or infinite) which was abnormal.
According to the schematics, we can see the pad B2 is connected with the inductor L3704, continue to measure on the top side of C3702 (which was connected with L3704) and B2, it was disconnected, so we can conclude that the problem was from L3704.
Replace L3740 with a new one and measure the diode value on pad B2 again, the measured value is 252, which is normal.
After that, solder back the U3703, then assemble the phone and test the function again.
Display function back to normal, issue resolved.
iPhone 7 Motherboard Repair Course
