Remote sensor No remotes seem to work on it... bought 2 different kinds of universal and they are flashing, but tv doesnt seem to respond.

Return the tv universal remote where u bought it,u only used a universal remote when u must have too,because universal remotes are very strangely and react differencely each models of tvs u tries to programming it codes to it CPU chip.Norcent tvs are recent tvs in the market,have many spare original remote.Try website like Remotes.com to buy the tv original remote.

Hi, I have a few things for you to try.
1. Your TV "6424TF" seems to have problems with its IR "Infra-red" sensor going bad. This is a common problem with your model TV. If this is the case no remotes will work on your TV.
2. Because I do not know the model of your universal RCA remote I cannot tell you how to program it.. please supply the model number of it. Try looking under the battery cover. might be an "RCR..." number. I need to know this because all universal remotes have different codes and programming methods.
3. To know 100% if its your TV that's bad you will need the original remote for your TV. I have already looked up your original remote and you can purchase a brand new replacement that will work right out of the package (assuming your TV IR sensor isn't broke) here: RR0108UD for all sylvania TVs . no programming is necessary for this remote.

main board problem need replaced

is it the factory remote? if it is check to see if something is blocking the sensor on the tv, clean the sensors, if it is a universal remote alot of new tvs have codes that even some of the new universal remotes dont support. if thats the case you will just have to make do.

I'm sure you've tried this, remote batteries is most common cause.
Make sure sensors on TV and remote are unobstructed(remote only works if line of sight to sensor is clear) and clean. Windex and Qtip. Damaged/dirty/defective remote, try different if possible (universal). If none of above works(99% chance), extreme longshot is TV circuitry.

Your problem is not with the remote but rather the circuitry designed to respond to the signals transmitted by the remote(s)

Find out what board has the circuitry that responds to the remote signals and replace that board. Got a better idea?.
If very expensive then you have 2 choices - deal with a remote less set or get a new one.

you can easy check if you existing remote really work just look on the IR led with a digital cam and u will be able to see the led flashing

Remote problem could arise from either tv or remote. First check your universal remote on some other tv and if it is okay your tv's infrared sensor has stopped working. You can get it from the local component shops, take the existing sensor with you. This problem could be from the micro processor in rare cases. Best of luck.

it seems that there is problem with the remote receiver circuit of your tv since your remote is working alright with the other tv. inspect for possible cold solder on that circuit or failing component, but if you are not inlined with kind of job, better bring it to a qualified tv technician. good luck!