Samsung 50 smart LED tv has no backlight but has audio and sound.

Replace the backlight driver power supply or driver board.

Use a voltmeter to confirm all the voltages are present when the TV is powered on. If they are all correct then replace the led driver.

Do flashlight test - if you can see image you need t con board. If no images seen, inverter board. TV Parts Universal TV Stands DLP Chips ShopJimmy

Look, enough is enough, bite the bullet and replace the TV, they are so cheap right now.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electronicshelponline.blogspot.com/ To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Most likely the power supply board needs to be replaced. It could also be the backlighting. A few TV's have a separate power board just for backlighting, but not many.

If you don't know what you're doing, DON'T OPEN THE TV UP TO FIX IT, THERE ARE HAZARDOUS VOLTAGES ON THE POWER SUPPLY BOARD, EVEN WEEKS AFTER THE TV IS UNPLUGGED!

If you don't know how to do it yourself, find some local Electronic repair shops, get quotes, try to find reviews on the shops.

Check and replace the back-light LED strip. Contact any service technician. http://electronicshelponline.blogspot.com/

Your power supply board is working if you get audio and imgaes.

You test AC only the AC side of the transformer before the Rectifier Diodes. Everything after the Diodes will be in DC.

did you get to see any spec of backlight as shown in
LCD LED TV Repair No Image Blank Black Screen Flashlight Test How to Fix...

Here is what to check when your TV is turned on, power light is ON, but no picture is seen or sound is heard:

1 - Be sure the component you are using (Xbox, Cable, SAT, ROKU) is turned ON.
2 - Check to be sure all cable and cords are securely connected to the TV inputs and component outputs.
3 - Make sure you are on the correct input as some TV's will show a black screen if there is nothing connected to that input.
4 - Unplug the input you are trying to use from the TV (such as an HDMI Cable) and plug it back in.
5 - Check for signs of damage on all cords and cables from the TV to all components.
6 - Make sure the TV is not in ENERGY SAVE MODE, press the button on the remote control to go back to normal mode.
7 - Unplug the TV for 10 minutes to see if there was a temporary one time issue with the TV.
8 - Disconnect the TV from power for 2 minutes, then when plugging it back in PRESS AND HOLD THE POWER BUTTON ON THE TV FOR 10 to 20 SECONDS to try and force the TV to reset.
9 - If after checking all these different issues, you may have a Faulty Power Supply Board, A Bad Capacitor on the Power Supply Board, Loose Wire Harness, or Loose Ribbon Connector in the TV itself.
10 - Most likely when a TV has the power light ON but NO PICTURE and NO SOUND, it is the Power Supply Board that has failed.

BTW, here are solutions for you to play iTunes movies on Samsung smart TV

replace backlight inverter