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It could be possible that you ended up with some cheap knock-off transistors. I found this out the hard way, back during the CRT TV days. Many of the mail order parts places that I ordered from carried parts that were very cheap; so, not knowing any better, I ordered the cheap parts. Once, I ordered a bunch of horizontal output transistors and every one of them blew within seconds of turn-on. Once they were replaced with decent transistors, the TV's worked fine.
I ordered a batch of cheap STR53041 regulator IC's, the ones used in those horrible '90's era Zenith TV's. Every one of those were inoperative. On the first one I used, I pulled my hair out trying to figure out why the TV wouldn't power up. After all, the new part couldn't be bad, right? WRONG! After sticking a known good IC in there, the set powered up.
I've also had other experiences of cheap generic semiconductors and flyback transformers failing after short time periods. Now, I always order name brand parts from reputable suppliers.
Besides that, I'd look at bad capacitors in the power supply. Most of these cheap LCD TV's use cheap capacitors and don't be surprised to find more than a few bad ones.
Otherwise, you may be better off replacing the whole board (if one is available at a reasonable price).
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