Question SOLVED How to diagnose if a dead screen is due to LCD, display cable, mobo or bad karma?

[lowep]

Like many other users I got thousands of kilometers and many good memories out of my trusty mid-2012 Macbook Pro 13" unibody - until the LCD screen died. However the old nag continues to work just fine with an external monitor and correctly reports: "Intel HD Graphics 1024MB graphics" as well as the model number and resolution of the external monitor. So I got a replacement LCD screen from a donor laptop that was supposed to work perfectly. But it didn't.

So now I am trying to figure out a way to diagnose if the problem causing the dead LCD screen is either:
(1) the original LCD screen that I thought was the problem
(2) the replacement LCD screen that I thought would be the solution, but wasn't (maybe dead on arrival?)
(3) a faulty LCD cable socket on the motherboard
(4) some other problem for example with power supply or graphics components etc on the motherboard (even though the computer works fine with an external display),
a combination of some or all of the above,
or something else that I haven't thought about yet?

If I had another Macbook Pro I could try the original and replacement LCD screens on it to see if they work, or I could try the LCD screen from that Macbook Pro on my original Macbook Pro body, but I don't have another Macbook Pro, at least not yet.

Is there any other way to check or test a Macbook Pro to be sure the graphics and other critical motherboard components are working properly (apart from connecting it to an external monitor. If that works fine, can I fairly safely assume the motherboard is fine?)

Is there any other way to check a Macbook Pro LCD screen (apart from hooking it up to a Macbook Pro motherboard that is known to be working correctly to test if the LCD works as it should or not. Maybe if the LCD screen looks completely dead, as both of mine are, then it is almost certain the screen is dead, or at least not alive enough to even flicker, as if there is no power).

What would you do - tip it all in the trash and start over with another working Macbook Pro, or...

 

[lvt]

It seems like bad karma to me because you don't know if the replacement screen is working or not.

 

[lowep]

Exactly, thanks for your reply. This is why I am reluctant to invest in another replacement screen!

Instead I got it out and put the original display back on again that was not a bad idea, as I found out that with a bit of fiddling around the backlight in the original display started to work - but not the backlight in the replacement display.

So now I know the replacement display is not working as well as the original one.

But there is no graphics on the original display - just a glowing grey screen that can be adjusted via the keyboard controls from black to bright grey. Plus a glowing white light in the cutout apple logo on the top of the display. Plus white spots of light wherever I press my pinkie on the display. Trying to boot in safe mode and reinstalling the os makes no difference.

So now I definitely think what happened is bad karma!

I figure some graphics components on the circuit board may have been damaged by a technician who I had replace the magsafe power socket on the circuit board, as suggested by this video:

View: https://youtu.be/GQwSGI1kNKg

 

[lowep]

SOLVED!

turns out the problem was that the original LCD display was a dead duck that did not work though the backlight did, the replacement secondhand LCD in "perfect working condition" that I got online was just as dead but the (third time lucky) LCD cannibalised from a dead donor Macbook works fine.

So to answer my own question : how to diagnose a dead screen?

throw everything up in the air and see what lands upright