Why is my picture flashing on my 55LB5900?

freespirit8534  could be an issue with the power board as well as the backlight array. Measure the output voltage to the backlight (connector P801)and let us know how many volts you get and if they are consistant. If the pictures only flashes it is most likely the backlight array that has failed.

To replace the backlight this video will help. Slightly different model same idea

Of course it will help if you can post some pictures of all of your boards with your question. That way we see what you see. Use this guide Adding images to an existing question for that.