Screen Stage light Theatre light - 27" 5K Late 2015

[PeacefulMan]

I bought a Late 2015 27 inch 5K iMac without an HDD. At this stage, I was able to make an external SSD boot disk and it's working. My plan is to install it inside later. But, as you can see in the pictures, it has some periodic lights on and off at the bottom edge of the screen, all the time. I'm new to Mac by the way. My understanding that this could be due to the LED strip(s)/bulbs or maybe the cable or maybe the board?. There is practically nothing online for videos on this matter and model that shows clearly what's happening.
I'm not even sure if it takes 1 or 2 LED strips? what part numbers?
The cable that powers the LED strip splits into two sides, right and left. I'm thinking it could be something due to the cable or the signal/electronics, because the theatre lights are symmetric all along the edge and not shining in one side of the screen, the fault could be coming through both sides of the cable, it could originate from the root cable or before that ?!

Anyway, any more resources please? I can't even confirm the parts.

This is my Model:

iMac "Core i5" 3.3 27" (5K, Late 2015) Specs (Retina 5K, Late 2015, MK482LL/A, iMac17,1, A1419, 2834): EveryMac.com

Technical specifications for the iMac "Core i5" 3.3 27" (5K, Late 2015). Dates sold, processor type, memory info, hard drive details, price and more.

everymac.com

Edit: this video is for a 2017 smaller screen but kind of gives some indications of the guts.

Is this maybe what they call backlight bleeding?! or is that related to the screen pixels and not the LED strip?

 

[TwoH]

Honestly you might just have to reseat the backlight cable going into the logic board. I have had it before where it was not plugged in all the way and causing this

 

[PeacefulMan]

Honestly you might just have to reseat the backlight cable going into the logic board. I have had it before where it was not plugged in all the way and causing this

That makes sense and I hope it's the case.
So there were cables keeping the screen from coming off the chassis. One with a wide socket (or wider, and flatter) and the other slightly narrower and that's the one that splits and runs down to the backlight.

 

[PeacefulMan]

Here is an interesting comment that seems to be same as you described:
https://www.ifixit.com/Answers/View/620180/Faulty+LCD+panel+or+chassis-seal

"The stage lighting effect you see at the bottom of the screen is the backlight LED’s are not all on. The strip of LED’s is setup so every other is lit by a different power feed.
The backlight LED’s within the panel is not likely failing. I have seen the connector at the end of the strip has become disconnected from the wires, while not easy its repairable! The other area I’ve seen is the power cable from the LED driver board has been damaged.
So I would start off by checking the voltage at the LED driver to make sure the LED driver is OK iMac Intel 27" EMC 2429 LED Driver Board Replacement Review Step 6 making sure the cable is not damaged and then test the voltages of the power leads."