Hisense 50h7gb1 black screen

In this case and on this brand set it's best to replace all the strips, if you only replace the bad one. or ones the anothers will fail in a few weeks to months so it's back to square one with needing to open the set up and lift the panel and replacing the bad ones again, also it's cheaper to replace them all than one at a time in most cases, you most likely will find several strips that have shorted diodes that will need replacing, it's seldom just one diode that opens up making the picture black. Also when handling the screen use extreme caution, it's VERY fragile and sensitive to static that will damage the panel boards, making the set not work once it's back together. You will get backlights but have what's called the blue screen of *****, then the money was waisted attempting to repair the TV with new lamps. Hope this helps, I apologize for the answer bots on this site that give generic non helpful answers, wish their tech department could do something to stop them but so far it hasn't happened.

Based on the symptoms you described, it sounds like the problem is likely with the LED backlight strips on your Samsung TV model DF400BGLV1H. When you disconnect the LED connector, the standby voltage comes back, and the T-CON board blue LED lights go on and the boot sound. Also, the fact that the power supply board measures 225 volts on pin 1 and 2 of the D+ on the connector when disconnected and toggles off when connected to the LED, suggest a short on the LED side.
It is possible that one or more of the LED backlight strips may have failed, causing a short and preventing the TV from powering on properly. In this case, replacing the LED backlight strips would be the most likely solution to the problem.
Keep in mind that it is always a good idea to double-check all connections and components for proper function and continuity, before replacing any parts.
It's recommended to seek the help of a professional technician or contact Samsung customer service for further assistance, especially if you are not familiar with TVs. Also, make sure you get the right replacement parts and follow the instructions.

I have no idea why you would change the power board 3 times, this is the one board that can be tested with a simple multi meter, voltages are marked on the board, ifor future reference you know the 5 volt standby is working, next thing to test is the PS_ON pin coming from the main board, there should be about 3 volts there when the set is turned on, if no voltage replace the main board, if normal power-on voltage test the other rails, if all good the power board is working. Also a tcon board will never give the issues you have, a bad tcon will give distorted picture or blue screen without a picture. Now for what's wrong, according to service lit the blink code you have is bad inverter-backlight issue, since this set is LED it doesn't have an inverter so it's a LED backlight issue. Replacing the backlight strips should correct your issue, unfortunately on a set this size and with it being edge lit replacing the strips isn't a real easy job, this set has a diffusion panel behind the panel (screen) and it has to be replaced correctly or the picture won't look right, also with this size panel extreme care must be taken when handling the panel, suction cups work the best for removing and replacing it. Good luck!

This is explains the problem.

https://www.fixya.com/support/r29790007-repairing_own_television

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Are you able to get some voltage measurements?

Chances are your T-con may be bad . I would check the backlight by shining a flashlight by the screen to look for an image on the screen . If an image is present, then the backlight is the problem. Your Tcon and main board could be ruled out, if this is the case . At this point check the power board for voltage to the led driver. The leds could have issues.

T con board issue.

Power Issues Troubleshooting
*NOTE* LG & Vizio TVs must be plugged in for 5 minutes before it will turn on*
1. Inspect ALL Ribbons and connectors. Any tears or bad connections between boards must be replaced FIRST
2. If you have any of these boards, it should be the first board to be replaced
A) T-Con Board - Video Problems (No Video with standby light on, audio but no video, grainy, distorted, or color problems in screen)
B) X or Y Buffer Board (Plasma TVs only) - Standby light is on but no power
C) Backlight Inverter / LED Driver - No Video with standby light on
3. Main Video Board
- Standby light is on but no power
- Either video or audio does not work (if no T-Con in TV. If audio but no video it is T-Con)
4. Power Board - No power AND standby light does NOT turn on. If standby light is on try other boards above
5. Panel - No power, distorted picture, lines, slow motion, half screen, faded in / out. Panel problems are usually not worth replacing the panel.

usually bad power supply, Where did you source your part? return/exchange it. Bad LEDs/ possible but not as likely, Tcon working if you can see images with flashlight. You must also have backlight on signal from main board ,check for 3 to 5v on labeled connecter on power supply. UPDATE---47" & 50" LN series. This model has had LED problems if your TV acts as described by Mr. Hughes in is answer to this problem you have bad LEDs