I have a sinotec STL3-42P38 LED FLAT SCREEN .ON THE LEFT HAND THE WORDS" LOCK PANEL KEYS"APPEAR AND MY REMOTE CONTROL IS INACTIVE HOW DO I SWITCH IT OF AND ALLOW MY REMOTE TO CONTROL THE TV

Some help here, but it appears to be a South African television, and we are in North America. The search results may be better for you.

https://www.google.com/search?q=sinotec+STL+-43E3000G+remove+system+lock+password

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If you wish to get some details; check the site linked here. Pull up older posts. http://electronicshelponline.blogspot.com/ Unlock procedure and to override the password lock of many popular brand TVs and other devices are given. Search the site by type in this key phrase "How to unlock television"; in the search box at the top left of the display window, to get gathered related posts.

default would be to unplug for 12 hours or sometimes there is a little pin hole reset hole type button very small that you have to stick a pin or paperclip end into on the front by the power button, hold it in for 5 sconds sometimes this will reset if not get a new remote a universal one.

Remove Mains Plug and press Standby switch from LED Panel for 30 sec.
Press Input & Ch Down Switch from LED panel and plug in and start LED
LED will start and "K" will appear on screen
Press Ch Down & Vol Down switch at same time
You will get 1/10 page on screen
Go to 9/10 page by pressing left & right button from remote
Select EEP and press Ok button from remote
After 30Sec remove main plug and restart LED.

Remove Mains Plug and press Standby switch from LED Panel for 30 sec. Press Input & Ch Down Switch from LED panel and plug in and start LED LED will start and "K" will appear on screen Press Ch Down & Vol Down switch at same time You will get 1/10 page on screen Go to 9/10 page by pressing left & right button from remote Select EEP and press Ok button from remote After 30Sec remove main plug and restart LED.

Press the 'Display' button on the remote control once. It must make it vanish. Gopakumar Gopalan Google

Key lock function is now ON with your TV, and side panel key won't work any more, till it has been unlocked. But, if you want to unlock the key-lock function, your remote control must be OK. First check the remote control for its infra-red emission. Surf the site linked below. It has details about how to check a remote control. If remote control is found OK, the remote sensor inside the TV might be faulty, and should be replaced. Anyway, at present condition, you've to make the remote control to work with TV, to proceed any more. If you're sound in electronics, and wish to get more details, check this link. http://electronicshelponline.blogspot.com/ Selecting the view option, either to Mosaic / Flip card, can be selected by the drop-down list, by the top left of the display window, will make surf easy. You can also do a search, by type in the brand name / model number / IC used, in the search box at the top right of the display window.

Be assured that fault does not lie with the remote handset itself. In that case repair or replace it.Remote control system has two parts. One is the remote hand set and the other lies within the TV or other device for which remote controller is being used. The hand set transmits commands in shape of singnals of particular frequency and pulses per second depending on the key pressed. The other part of the remote system lying within the TV receives, decodes and responds according to the commands received in shape of signals of particular frequency and pulses of paricular width and sequence. The remote controller uses invisible infra red light as the carrier of the signal. As the light moves in straight path, no obstacles should block straight line of sight path between the remote hand set (the transmitter) and IR sensor window (part of the receiver). Hence remove the blockage from the front side of the TV.Check if the IR sensor is not defective. Check if the associated circuit and components inside the TV are not defective.Check if the TV has any switch for selecting only manual mode and locking the remote mode.Hope, some sytematic exercise will bear the successful result.

Helo
Assumes that you have its orginal remote control handset with your now, and posts this solution to your Lock Probelm. If you haven't it now with you, just try to buy a new one, or borrow it from any of your friends or relatives, who own the same model of TV, just for ulock it. After unlock, you can use any other universal remote control to control your TV.
Sinotec:
If you have the original remote (RC-A07) for Sinotec tv, then press and hold down the Music button for about 10 seconds. The tV will be unlocked. After unlock, you have ot set a new password. OK.