Störungsphänomen
When we receive a phone with a dark screen, the first thing that we need to do is to judge whether it’s no boot-up, no display, or no backlight issue.
After carefully checking, the screen can’t be lighted on but we can see a faint apple logo.
So we can draw a conclusion this is iPhone 7 Plus no backlight issue.
Störungsbeseitigung
Disassemble the phone and connected the motherboard with the DC power supply, the current reading on the ammeter is normal. So we can confirm that the phone has been turned on normally but the backlight can’t work.
After replacing another screen to test, we confirmed that the problem is from the backlight circuit on the motherboard. From the schematics, we could see that there are 6 lines related to the backlight.
First, let’s check the diode value on pin37 to pin47 of connector J4502 first, and the diode value on pin 37 was found to be OL, which was abnormal.
Following the abnormal pin 37, we found the filter FL3902 was on the line between the backlight IC U3701 and the pin37. Then check the FL3902 to see if it’s broken or not, after checking, it’s OK.
So we can conclude that the problem might come from U3701.
Then replace U3701 with a new one, and assemble the phone to test, problem resolved.
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