LG 75UH6550 Wont Power On

Im stumped. The TV wont power on. Standby light is on. When I hit the power button the backlight flashes one time and then goes off. No sound picture anything.. I put a new mainboard in and that didn’t work. I put a tester on all the lights and they all worked. I got a used power supply that allegedly worked and that didn’t fix it. Any ideas? Im not an electronic repair person so I am limited on my testing skills.