What is the service mode for a supersonic led model;slm4001

Do a factory reset to this TV and then check. There are two methods to reset TVs. The first is simple to do; a Hard Reset. The other is somewhat skilled; is a Factory Reset (System Reset). To make it a hard reset, just unplug the TV from AC mains wall socket, and re-plug it back after 15 minutes or so. If you do this by overnight is better. This procedure will make it a hard reset. If you want to make it a system reset [Factory Reset]; you have to enter its service mode option, and select the factory reset option. If you wish to get some details; check the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/
Service mode entry, adjust and exit procedure to many popular brand TVs are given. Never forget to exit the service mode, after all adjustments have been done.

thanks,
ding

Sounds like 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://electro-medical.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.

www.ask.com/mfg.name/make/model/operational/code/and/universal_codes/

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If it shows only a blank white screen, means, there is no video signal. Just try it with external Ariel and see whether there is picture tunes in. If not, the main board to this set is likely faulty. Contact any service technician. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" or "Magazine" will make surf easy. Pull up older posts. Surf the site with patience. Do a search by type in the brand name of your device at its search box.
http://electronicshelponline.blogspot.com/

There is no place that sells LCD screens that big because of the expense and liability for shipping. You have to go to a repair center for that make.