So i ordered a new power supply board for my LG 55la7400 television set...part was a perfect match..it went on smoothly with no issues whatsoever. Still just comes on and then black screen

Chances are it's bad backlights if you can see the image with a flashlight

if voltage constant from board to screen bad screen if voltage fluttering bad board

No 24 volts mean a bad power board. You can get a new one from here: 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://electronicshelponline.blogspot.com/
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.

John have you tryed the t.con board it sounds to be the problem B.O.Luck

If the remote control is the original one for this TV; a firmware update might be necessary after the main board replacement. If the remote control is a universal type, you might have to set-it up with the set-up codes stated for LG again.
If you wish to get more details; check the sites linked here. It has set-up code list & set-up procedure to many popular brand remote controls. Common codes to check devices with different brand TVs are given, in the order of the device's brand name list in alphabetical order. Pull up older posts. Surf the site with patience.
http://remotesearch.blogspot.com/
http://electrohelponline.blogspot.com/

Unplug the tv for about 5 minutes. If it comes back on and plays properly then you are likely to have a bad power supply which has bulged capacitors on it. If you are proficient in soldering and unsoldering you can save buying the entire power supply. The bad capacitors will be domed or bulged on the top. They are cylinder shaped components and good ones are flat on top (usually). You can usually get capacitors at Radio Shack and are the radial type and not axial. Radial is the two pins at the same end.. Do a search with your model number in Ebay or Google or shopjimmy.com to find a power supply should you need one. Be sure to match the picture for picture with your power supply and also match the numbers

possible power supply issue

Looking at it from the back, the board on the left is the ysus. You should see a big red cable coming from the power supply board in the middle of the set going to the ysus. Near the red cable plug on the ysus should be a fuse. Most likely the fuse has blown. If it is you need a ysus PCB. Check out encompassparts.com. If you get a new board be sure to set all the controls on the new board to match the old board. If you don't you will have little sparkles all over the screen when it comes back on. I hope this helps. You may want to check all fuses on the power supply and the zsus which is on the right side of the set. I have seen both the ysus and zsus be bad together. Good luck and thanks.

maybe the inverter board for the screen issue- the power issue seems to be LG has power board design flaw- basically last time I looked the power board is manufactured NOT by LG but by H&E ...who designs this stuff??