Question Defective backlight or something else?

Feb 26, 2019
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I have a Samsung 50KU6000 4K TV model 2017 with this rare problem of backlight.

Everything works ok, all backlight leds work but in bright parts of every image appears those white rings.
I attached a image of a plain white picture; in darker movies is much less noticeable. Some rings are more noticeable than others, but almost every led produce those rings.
I tested different levels of backlight setting from 0 to 20.... same results
Opened the TV and it have 6 rows with 13 leds each. The power supply have 2 output lines with 120Vcc each, each for 39 leds of 3V....everything seems ok
The led's lens are good attached to each led.

Have you ever see this problem? Do you have any suggestion?
I don’t want to buy a whole new backlight and maybe I spend that money and not resolve the problem

I appreciate any input in advance
DQt9JL1a_o.jpg
 
Is the firmware up to date?

I have no real clue either, but could be
a filter screen is missing
a filter screen was installed wrong sided
the backlight was not installed correctly
the backlight is burned into the screen