I have an LG 55 inch TV, model no.# 55LB5550-UY. When I plug TV into the outlet the screen flashes on and off rapidly. I have changed the power supply and the main board. Neither resolved the issue.

First off, a main board will never cause a backlight issue like this, the main either turns them on or it doesn't. The inverter for the backlights on this and a few other LG sets have the current tolerance extremely sensitive, there are 2 supplies driving the backlights, one supply for 4 strips the other for 6 and each is independent where one can work and the other not work. Where did you buy the LED lights? Were they some off brand knock off lamp strips or did you get them from a reputable place like shopjimmy? A gut feeling is you have bad lamps, the power board seldom gives issues on this model, if you get them from shopjimmy have them send you another set. Also it's possible for the lamps to work on a LED tester but not work in the TV, I've seen it many times.

The problem is neither the lights or the power board, but is the light board, or a bad connection with a cable.

How many blinks

If you wish to get some details; visit the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/ Circuit diagrams of many popular brand TVs and other devices are given.
Schematics can't be uploaded by this window.

Sounds like the T-Con board is toast. You can get a replacement at : TV Parts Universal TV Stands DLP Chips ShopJimmy

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electro-medical.blogspot.com/
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced.

Hi,
Try to change the capacitors first. Normally lost of lcd tv have problems with the capacitors and which lead to powers problems. May you will not find visually that the capacitor is fault. So go for changing the capacitor also check the following video. Which will help you understand how to check the power supply broad. Also take care while working with the power board there is a risk or electric shock Also one of the capacitor in the power supply holds charge even when you have remove the power supply.
Testing LCD TV Power Supply BoardsSTEP BY STEP

www.electro-medical.blogspot.com

Have the outlet checked by a qualified electrician. Sounds like either a power issue or ground. You should not get a buzz from an outlet for any reason.
If you plug something else into the outlet does it make the same noise?
If yes, something going on with the power. If not, then the TV power supply might be doing it, but the noise would be behind the TV and not the outlet.

Check whether the product is turned on. I Try another channel. The problem may be with the broadcast. I Is the power cord inserted into wall power outlet? I Check your antenna direction and/or location. I Test the wall power outlet, plug another product's power cord into the outlet where the product's power cord was plugged in. From the LG manual kolbrun

my 55613 tv wont turn on just blinking green light