Hi carlajhill,
Sorry that you, too, have been a victim of a garbage bound TV. I do believe that after a year of searching, I have pinpointed the problem. It is one or more of the LED strips in the these TVs. And I took the time to find a video on how to change out the LEDs but let me tell you, IT IS A ROYAL PAIN TO FIX!
There are several tools that one needs to even access the LEDs, including (but not limited to) a large, dust-free area to do the work, suction cups for separating the very thin screens in order to get to the LEDs, and then you need the new LEDs themselves. Though all of the tools and LEDs can be bought, IT IS A VERY LABOR INTENSIVE PROJECT because the LEDs are buried deep within the televisions sandwiched between very thin, flimsy screens, accessible only from the back to the front.
First are the many screws, then removal of the boards, then removal of the bezel around the TV, then the lifting of the back screen with the suction cups (oils from your fingers and dust can have a negative effect on these screens). Only THEN do you reach the LED's which are BOTH GLUED AND SNAPPED IN. If you don't want to spend the money to replace all of the LED strips, you must check each strip to find the ones that are causing the problem, remove them and put in the new ones. And if you choose to replace only the ones that have gone out, you take the chance that others will go out in short order, resulting in having to do this tedious process all over again. So all in all, what it comes down to is that by the time you buy everything you need to change out all of the LEDs, you are quickly reaching the cost of a new television. Factor in what your time is worth, and you very likely rise up and over that cost.
By the time I got halfway through the video, it was apparent to me that my big screen Sharp/Roku was junk. If you have a television service center near where you live, you might give them a call and ask if they would be interested in purchasing your old one for parts. The screen, boards and other pieces may bring you a few bucks. Keep in mind that if you purchase a television with a credit card, some cards have extended warranties for electronics as a perk. Also, many outlets offer an extended warranty plan for a modest $45 fee that covers the cost of repair or replacement for 3 years.