Figured I’d start out with a simple one, just so a reader can get a feel for the process. Got in an iPhone 7 with a cracked screen, didn’t show any display but it did show signs of being alive, such as responding to home button press and being recognized in iTunes when connected to a computer.
First step is ALWAYS to see if the problem is parts or if it’s the motherboard. Rule 1. So, put in a good screen- still nothing displaying. Also tested the cracked screen & discovered it was functional.
Next step in the process is to start measuring with a multimeter, so pin by pin I looked at all the display related signals at the connector that goes to the screen. Discovered that the line for driving the screen backlight was open.
On the backside of the board there’s a “filter” (a small component) that connects between the backlight driving chip and the screen connector. Discovered it was burned out- replaced it. Problem solved.