Ответ на "iPhone 4 screen is black after replacement"After replaced the LCD for GSM iphone 4 with iOS 5.x, the screen couldn't turn on. I tested the LCD on another known good iphone 4 with iOS 7.1 and the LCD is working perfectly fine. At this point, I am sure the LCD is good. Therefore I updated the iphone 4 with iOS 5.x to 7.1 and reassemble the LCD to the phone. Then, it solved my screen problem.