Display black screen but backlight works, replacement LCD also OK

I have read posts about iMac 21.5” A1311 black screens, and none of the posts describes the issue I am facing. My iMac is a EMC 2389, but the issues here could I think affect any A1311.

After replacing the hard disk in the iMac and putting the screen back on the iMac, the screen backlight works, but no image at all is shown. Even examining the screen with a torch shows no image. All 4 diagnostic LEDs are on.

So it’s a loose cable?

No. I have even replaced the LVDS cable, AND even borrowed the LCD panel from a friend’s iMac and that works just fine. External monitors work just fine as well.

This leads me to conclude that there is something faulty with the LCD panel itself - it is a LM215WF3 (SD)(B1). Is there some fuse in the panel that could have blown? Or any other way to diagnose a panel fault?

Please note my problem is not After HD changed, fuse (wall) tripped, no internal video, but external, or Why is screen black after replacing hard drive?, as using a replacement LCD panel is fine.

Any assistance would be most welcome!

EDIT

In light of Dan’s answer below, here are some pictures of the T-CON board. There is no obvious sign of any burnout. The darkening on the plastic above the main chip is I assume simply due to heat over the years?