TV sound no picture? Can see picture with a flashlight..

Here is a video on how to test the voltage and troubleshoot the power supply and LEDs.

https://www.youtube.com/watch?v=UxR8-xztdGs

T con board issue. TV Parts Universal TV Stands DLP Chips ShopJimmy

Inverter board responsible for backlighting on set.

The led strip is faulty. Normally it's comes with the panel.
If you wish to know bout the Woking of back light LED the check the below added link.
www.electro-medical.blogspot.com

Inverter board powers the backlights.

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.

Measure all the voltages coming off the power supply board first. Then move to the board that controls the picture. This board has one or two wide flat flexible cables running form it to the bottom of the screen.
TV Parts Universal TV Stands DLP Chips ShopJimmy You can get all the parts you need here.

Fault to its main board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. http://electronicshelponline.blogspot.com/ . In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

check TV settings with the TV remote look for display or picture settings.