Black screen iMac icons visible with torch

[Ljd]

Hi
I'm new to asking for help on forums but I'm stuck and could do with a bit of advice.
I have a 2012 a1419 iMac 27. I bought it with a black screen, but the icons are visible with a torch. Plugging in an external monitor works fine.
I've tried replacing the whole LCD unit -no good.
I'm about to try a new display cable, but the costs are mounting......
I know it must be a backlight issue, but I can't see any separate module/board on this machine for that.
What could it be, and how can I fix it? Fuse, cable, chip...... im willing to try anything (as the wife is pretty unhappy with the costs......).
Everything else seems fine.
Help please...
Thanks
L

 

[CWallace]

Might not be the backlight, but the cable that connects the backlight to the LCD. Doing a Google search returned results that the cold solder used can crack with age. Re-soldering the connector fixes the issue.

 

[Ljd]

Might not be the backlight, but the cable that connects the backlight to the LCD. Doing a Google search returned results that the cold solder used can crack with age. Re-soldering the connector fixes the issue.

Hi thanks for this. Are there any videos etc anywhere where I could see/follow how to try this? Would I see the crack under a microscope, or is it too difficult to see. Would I need to unsolder the whole thing to start again?
I'm trying the new cable itself first (when it arrives). Someone else suggested a fuse, but I have no schematics so see which it is. There are posts elsewhere about other machines' fuses, but I can't see on my board which it is (white spot on top)...