UN60KU6270f not powering on.

Make sure that with the new main you change the build code and SN from the old board to the new board, you need a factory repair remote for that. Saying that you have replaced everything I would suggest a new lcd panel. They’re pretty easy to crack when replacing the lights on a Samsung the way they are glued down. Before going that route ensure that power is being fed to the panel from the new power board with a multimeter. GND and a power circuit.

There is a potential your tv is still under warranty and you could reach out to Samsung Care. They would find you an authorized service provider in your area.

@joemontgome2230 let's go through the information you are giving us:

"on reassembly the red light will turn on" your power board supplies standby voltage. Good

"if I hit the power switch the leds will turn on and off" error on Bklight circuit

"sometimes the on sound will play sometimes it wont" could be caused by the TV going into error protection

"If the main board is disconnected the leds will remain on" SMPS working. It;'s not your power board that is the issue

The original issue were loose diffuser on your LED backlights. You said you replaced them all. Does that mean you replaced all of the backlight strips? We will also need to know if your Power LED light changes. it is important to know what happens after the backlight blinks.

I would also not count out the backlight strips. When you disconnect the main board from the powerboard and your back lights come on, do all of them come on. You will need to have a uniform backlight. Check all the wiring and unplug the wires and plug them back in, make sure they are properly seated. Post some images of all of your boards with your question. Use this guide Adding images to an existing question for that so we can see what you see.

If everything is checking out perfectly, then the issue is a possible short or failed component on your driver boards. Again, we need to see those. Those are the long skinny boards that connect right to the LCD panel. That could possibly be helped by attempting a Tape-Off.