Hello, and sorry you're having problems. We are now connected on this website- which saves you money over having a Tech come to your home, and in many cases we can fix the problem you're having.
If the MENU itself is also going dark- this is a huge clue. It tells me without a doubt that the issue is a hardware fault with the TV's backlighting. The LED inverter/power supply board feeds the lights in the screen. Generally there are backlight settings in the MENU, but these do NOT affect the MENU- only external devices.
The fact that the MENU also goes dark means this issue goes beyond any settings or adjustments and is hardware related. I'm sorry for the bad news on this one, but I would suggest immediately contacting the retailer to setup an exchange- as you have 30 days to do so. Beyond that, you're left with a repair through Samsung service- and this could leave you without a TV for weeks.
Ultimately your LED inverters will fail, so I would suggest taking this set back sooner rather than later. I can tell you with certainty that upon the fact the MENU also goes dark- this tells us the problem is internal. In the field we use the MENU test to determine if the issue is "setting/source" related- or TV hardware related.
I'm sorry for the bad news on this one and wish there were a quick fix.
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