Need Help. Replaced main board and now half screen is darker

I sent my Samsung LN46C750 for repair a couple of weeks ago since the TV did not accept any HDMI input at all. The service shop (which Samsung itself recommended through costumer support) replaced the main board and then returned my TV. Upon firing up my TV everything was fine, every HDMI input was working again and I could finally watch movies and cable again.

The problem started moments where in a dark scene of a movie I was watching I could clearly see half of the screen was brighter than the other half, by as much as two steps in the brightness control setup. At first I thought it was that particular input with that particular source, but when I changed the input and changed the source the problem prevailed. It's something that's clearly visible in any instance where the screen is dark or completely black. It's a straight division down the middle of the screen in which the left side is a couple of notches brighter that the right half.

The next day I immediately called the repair center and returned my TV. A couple of days after that they call me back saying that the issue is not on the board they replaced and that they don't know what the issue is and how to repair it.

So now I turn to the boards to see if anyone knows and can help me with this issue.

Please, any tips or suggestions are really appreciated.

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Clarification Request
Is this a LN46C750R2FXZA ?
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Yes it is.

But I don't know what the cause is. So I wouldn't know which part to replace.

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In service manuals I usually see

A diagnostic flow chart and you follow that to repair the set. As I have no reason to buy this manual but the shop does, they should consult that. Since shops have to make money on such work, they rarely discuss what it could be since, well, I hope you can figure out why.

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