I posted this in its own separate thread before I found this specific thread for the LNTXX71F models. I can't tell if this thread is officially still alive or not.
A friend of my wife started having issues with the subject TV in late 2011 and unfortunately was not made aware of Samsung's offer to pay to have capacitors replaced through mid-March 2012.
The TV somtimes would not power on, but the primary symptom was that there would be no picture and the screen would gradually start to go from black to a light gray, starting in the upper right corner as facing it from the front. For several months, she could eventually get the picture to come on by repeatedly powering the set off and on. Once the picture was displaying, it would have a flicker or a line that would drop out in the picture for an instant fairly frequently. After being on for several days it would typically shut off on its own. It then got to where it didn't want to power on reliably.
Based on information I found regarding issues with this TV, even though the capacitors on the power supply module did not exhibit external signs of failure, I ordered a kit for approximately $15 that contained 8 capacitors.
After installing them, the TV powered on reliably for a while, however, there was no picture and rather than the screen staying black, it faded to a light gray starting in the upper right corner.
I had read about a "flashlight" effect happening with these TVs starting in the corner, however, that sounded more like a lighter spot in that portion of the screen rather than something that consumed the entire display and prevented any of the signal source from ever being displayed.
The audio is playing fine and I have noticed that once the screen is a complete shade of grey, over time it seems to dim and then brighten slightly. I don't know if this is associated with changes in the video source that is being supplied to the TV or not. I have read that the T-Con and main Video board could also be at fault in these TVs. Given that audio is working, it sounds like it could be either of them.
After leaving the set powered on for a while and then going through various power on/off cycles, I got the TV to the point where it would display alternating horizontal and vertical lines or grid patterns with various colors of lines. Later it even got to the point where it displayed light/dark green vertical bars some of which were "hashed" and all of which gave the appearance of rapidly scrolling. Finally, after a couple of hours the set actually began displaying the actual source video with flickering white horizontal lines at random locations on the screen.
Then the TV started powering itself off and on. When it powers off it doesn't produce the descending musical notes that would be the case if you used the remote or power button to power it off. Also note that the blue light at the bottom of the frame that would normally extinguish when you power the set off does not go off when this happens. Sometimes it powers right back on. Other times it will keep cycling on and off numerous times before it ever gets to the point of attempting to ignite the display or respond to the remote control.
I am wondering if there is still an issue with the power control module or if there is something else that is signaling an abnormal condition and resulting in a shutdown. If this was the case I would expect the set to stay off rather than attempt to automatically power back on.
Any assistance is greatly appreciated.