Screen is very dim,

I have a 50” 4k Sharp UHD  TV.  Model # LC-50N7000U

When you first turn it on you can see a faint Sharp logo. But then the screen goes black. If you get the perfect angle and the perfect lighting you can barely make out a menu when you hit the home key or settings.  I was told it was the backlight so I replaced the power board. This did not work at all.

When I turn off all the lights and power on the TV I can see the white backlight turn on for a second but then it goes off when the sharp logo disappears.

I was thinking it was the ast-con  board. But if anyone has any other ideas please let me know.

Also I have tried holding the power key while unplugged and also I have navigated the barley visible menu and reset all the settings including a factory reset.

Thank you