Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
When primary supply voltages are at minus voltage levels or at zero voltage, there is either a short circuit or a defective power supply, I think it is safe to say.
Also the pattern of what you say does somewhat indicate a hard short circuit in the loads on the power supply and the subsequent shutting Off of the power in some respects as a protective measure against burning out worse.
Do you have a service manual to consult for this model of TV? they can be very useful in preventing the spending of money by mistake on parts that turn out not to be needed.
If not, I can recommend some sources (which I have no affiliation with) for such a manual for this manual, I think, upon research and if you are interested.
As for common means of locating the shorted circuit or board - looking closely at all the boards and finding any brown spots beneath components, or burnt or slightly melted components, is one approach.