My tv was cycling on and off by itself. every now and again it would come back on but only for a short period of time and just start cycling on and off again. i replaced the power supply board as that seemed to be the majority of answers to the problem on forums but it is still doing it. Any thoughts? Maybe motherboard?

Hi,

I am sorry to hear regarding the trouble. This is a common problem on Samsung plasma TV's. Though without being there, i cannot provide legitimate 2nd opinion, however i could add my input to the diagnosis. The major probability is always power supply board and main board. As you have eliminated the power supply board, the next major cause would be main board. Though, either sustain board or scan driver board could cause this issue, however on almost all occasions, when the power supply board is fine, main board is the culprit.Make sure that the controller board buttons are not sticky and remote buttons are not sticky.You need to check the voltage across these. The suspected part would have the power missing in it.

Let me know,if needed further assistance.

Hope i helped you.

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This issue seems to be dealing with the start up circuit, each design is different and it could be a comprised of a diode array or a IC chip not sure as to what since the schematic will need to be present physically to be sure.
Also a choke circuit is the culprit for this instances which again will take a schematic to involve your self in the circuit analysis.
I suggest a qualified LCD television tech to look at the unit to be sure.