SamyGO

If you're sure that you have to replace mainboard, you can fond it on eBay or http://www.euroservice2000.pl

But first I suggest you to make exlink cable and check what's going on terminal window. Might here isn't that bad as you say...

Yes, a lot of micom errorors here... Sounds not well... Some of errors are Samsung-specific and occur even on well operating TV.
But most of errors are critical.

Good thing is that you`re getting any response from TV at all
I guess here is preFEB (old ) firmware on board and if i was you, i tried to repair TV over u-boot and console.

Step by step:
1. access u-boot and reflash kernel (if you can access console -kernel is boting - this we can see in logs, then skip this step)
2. access console over exlink (after success kernel boot) and reflash exe.img and appdata.img
3. switch to reflashed firmware.

I don`t know, perhaps this don`t help at all or maybe you get errors while re-flashing (if here are some hardware problems - errors indicating to this), but we will know that here isn`t anything else to do and mainboard replacement is case of choice.

Mainboard replacement.