@samuelluca19941
Just wondering, did you try cleaning the eye sensor?
The eye sensor detects if you're looking through the viewfinder and switches the screen off. Not sure how it works but if it works on amount of light then when your eye is near/on the viewfinder I assume it would be darker there then if it weren't, so being greasy (due to skin touching it) etc would be the same i.e. it thinks it's darker than it should be.
An ordinary DMM (digital multimetr) is sufficient but when testing small components the meter's test lead probe tips may be too fat. I have a pair of [https://www.altronics.com.au/p/p9184-set-of-red-black-croc-clip-test-probe-adapters/?gad_source=1&gad_campaignid=22400591978&gclid=EAIaIQobChMI4OaxjLHIjgMVoS-DAx2haRgJEAQYASABEgL19vD_BwE|adapters] (example only to show what they look like) that I slip onto the probes and then clip a pin into making for a narrower tip. A steady hand is also useful ;-)
I would try to isolate the problem a bit more first i.e. board problem or screen problem
If it is only the backlighting, have you still got the original screen?
Don't know what type of backlighting is used (presumably LED but maybe still fluorescent??) so if you have, disconnect the backlight cable from the board and then use a DMM's (digital multimeter) Ohmmeter function to measure between pins 1 & 4 ''on the screen backlight cable'' to see if there's a resistance reading at all.
Do this for both for both screens.
If they measure the same then most likely the problem is on a board. If not then it's either the original screen is OK and it's a board problem or the replacement screen is faulty and the board is OK, not conclusive but.....
If it is the board check for backlight voltage as in the answer above. If there's no voltage on CN3000 50 then check IC5300 pin6 and pin 5 each one to earth.