Other than proving that the TV power cord is faulty or not by either disconnecting it from the outlet and the TV and testing it with an Ohmmeter or trying a compatible replacement cord, there are no other options except than to take down the TV from the wall and remove the back cover so that the power board can be inspected and tested.
Here’s an image of the power board, showing the slow blow fuse (white glass fuse) and the surge suppressor (green MOV). These are the components that usually fail if there has been a power surge due to a storm or outage. If the fuse has blown replace it with an equivalent slow blow fuse (rating marked on board near fuse and also on fuse end caps e.g. T5A 250V. The T=Timed which means slow blow. If you insert a standard fuse of the same rating it will blow straight due to the high inrush currents when the power is first connected, whereas the slow blow will hold until this current has subsided. We are talking 1-2 milliseconds here for it to subside.
If the fuse and MOV test OK then connect power to the TV and check if the Standby voltage is appearing on the connector test point as highlighted i.e. 7.5V DC. If it is then it may be a mainboard/power board cable problem or a mainboard problem as the mainboard turns on the red standby power light if it is receives the correct voltage signal from the power board.
If there is no 7.5V DC standby power at the test point then it is a power board problem.
Be safety aware when working in the back of a TV when the power is connected as there is exposed lethal AC voltage on the power board. If you don’t know what you’re doing, then don’t do it. Call an expert to fix the problem.
(click on image to enlarge for better viewing)
I’m assuming that this image of the power board is the correct one for your TV as it is the one that turns up consistently when searching online for Samsung un40mu6290f power board. Just something that you might want to double check if you determine that it is necessary to order a replacement power board.
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