Hi @jmorton6 ,
Seems as though it’s a faulty mainboard or faulty cable/cable connection between the mainboard and the power board
Turn off the TV if on and disconnect the power from the TV.
Remove the back cover from the TV and then disconnect the cable that goes between the power board and the mainboard.
Reconnect the power to the TV and switch on the TV.
The TV backlights should come on and stay on. There will be no picture because the mainboard is disconnected.
If the backlights continue to blink and since you have replaced the power board there is a problem with the power supply circuit for the LED backlight array somewhere, from the power board LED cable onwards into and through the LED array, loose connection, dry joint connection, faulty LEDs etc.
If the backlights stay on and since you have replaced the power board, there must be a problem with either the cable/cable connection between the power board and the mainboard or the mainboard.
Also the red light not coming on when the mainboard is connected to the power board, indicates that the mainboard is not responding to the standby voltage signal sent from the power board
As you say maybe two problems.
If the power board is a BN44-00774A (I think that is is the right one, check what is says on the board) then here’s an image showing where to test for the standby voltage.
(click on image to enlarge for better viewing)
To uncomplicate the matter, disconnect the power from theTV and make sure that the power board is connected to the mainboard and disconnect the LED power cable from the power board, reconnect the power to the TV and check for the standby voltage at the test point on the power board.
I can’t find a schematic for the board so you may just have to check the voltages as what is described on the board and make sure that they get to the mainboard. If the voltages are all there then there is a problem with the mainboard
If there is no standby voltage on the new board, disconnect power etc and replace the old board and check. Hopefully the new board hasn’t also been damaged.
Here’s is a link that may help with testing the LED strips. There are also videos on YouTube that describe what to do
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