Backlight broken after screen replacement. Soldered filter no fix.

Backlight doens't worker after change of screen

Tried changing cracked screen on my own. Did not turn turn off battery before starting.

Changed the screen. No picture. Used a flashlight and saw that the screen and phone was working, but with no backlight.

I sent the phone to a repair store in Norway. The guy said to me that I broke the backlight filter. He soldered a new one. Backlight still doesn't work. He says that he didn't find any other errors than the short-circuiting of the backlight filter. He has confirmed that it gets the voltage it should get.

He also said that the the backlight went on a couple of times when he tried booting the phone a second or two before turning off.

Do you know what the problem could be?