I have good news for you, this is one of the few models for which service manuals can be obtained free of charge.
From this webpage specifically:
where you have to use a specific technique to get the manual, free:
open that page then
wait for at least 45 seconds
and then after that tiem, scroll down to below the Preview image of the file,
and look for a link like this:
and click on that link, and only that link.
This link will not appear before the 45 seconds is up, so you have to wait to see it.
All other links on the page are junk, don't bother with them. The 'Get Manual' link is the one that actually provides the service manual - free of charge.
Please tell me whether you need help interpreting the service manual in any respect, and whether it helps you with the task of identifying (hopefully, "for sure") the type of problem.
I myself would try measuring power supply voltage outputs first, comparing them with specifications in the service manual, and proceeding from there... there may be a shorted component in the TV, e.g. one that "holds down" the voltage from the power supply.