Sorry this is not an answer, but I have the same problem. I get no display and no chime when powering up. I have it on a wired keyboard and I don’t get any lights turning on the keyboard. I removed the display panel and I get LED 1 and 2 lighting up but 3 is not and 4 is not. (1) I can hear the fan going faintly. (2) I get the beep when testing with ram chip removed from board. (3) I also did another test (forgot what I did, read online somewhere) that made my fans go on high speed…. not sure what the test was for lol. (4) I already tried changing the coin battery just to see if it’ll help but still nothing. What could possibly be a fix for me?
the problem could be the IC cover that is on the LCD ribbon is exposing some of the micro caps which is touching the LCD shield when you install the screen. Those caps ground on the LCD shield causing the LCD display to either shut down, over heat or make the display look like its fading out. I experienced this recently with two iPhone 7 plus, both same problem with over heating and LCD display fading out randomly after changing the screen with an aftermarket screen. I checked the new aftermarket screen and those caps were exposed and touching the LCD ribbon shield. I taped it off with a small piece of Kapton tape and i have not experienced the problem since.