TV signal disappear after after a few seconds.

This sounds like a bad FET on the main board, change that board and that should fix it, or if you have the test equipment fix it yourself

dead power board

I had the same problem. all the service reset would only make the display come on for a few second with no "k" on the screen - only the channel would show up. I purchased a complete set of board for the lcd on ebay, and started swapping them one by one. The inverter boards were the culpret. Tele working 100% since that board swap.

You may have either failing 24vdc power supply or the backlight inverter board.
If you are going to DIY and have proper tools, electronics knowledge, and know safety precaution then please read on:

Please see my basic troubleshooting guide here:
http://www.fixya.com/support/r5093881-lcd_flat_panel_tv_troubleshooting_guide

http://s807.photobucket.com/home/budm/allalbums

http://en.wikipedia.org/wiki/Capacitor_plague

Parts: www.shopjimmy.com
Capacitors kit: http://lcdalternatives.auctivacommerce.com/Default.aspx

U still have sound when the picture disappear?Than ur LCD inverter board it dead.No sound picture disappear?Than Ur Tuner signal board it dead.Tries Shopjimmy.com,Ebay.com for those refurbish boards.

It seems that the "backlight" has gone bad. "Backlight" is the device that allows to see the picture in an LCD TV, when they are about to go bad the symptom that you describe is very usual. It depends on the tv manufacturers and model to expect how long the "backlight" is going to last in a tv.You can shop around for a replacement part and try to do it yourself or take it to an especialized technician. Good luck..

Hi,
Symptom shows its weak Lamp problem in TV and you need to replace lamp. Replacement procedure is easy and written in instruction manual of TV.

Always buy Sony Original Lamp from Sony Accessories for longer life of TV.

Sounds like the blue is disappearing from your picture. A common cause of this is bad solder connections which can cause the blue signal to be lost at several places. It could also be due to picture tube failure. Sadly, I'm leaning toward tube failure because of your description. Bad solder connections usually take some time to start acting up after you turn on the set, but you say this happens every time within a few seconds. This is typical of a picture tube failure where an internal connection is okay until the tube heats up, and that takes only a short time. A service shop could check the set to make a firm diagnosis, but if the tube is the culprit you should replace the TV. It's not economical to consider tube replacement.

You need to do self diagnosis test first to see what area it is in.

All symptoms indicate a problem in the vertical circuit,the first thing to look for will be loose conection on the vertical output IC,sometimes this IC goes bad completly and it needs to be replaced.If you are familiar with soldering electronics you can take care of it but if you are not too familiar you can play it safe and take it to a servicer ,it will be a lot less expensive than if they come to your house.Let Us know how it goes or if you need more information.