My t.v starts well then after a short while, the picture starts reducing its horizontal until one horizontal line is left ,then goes off. What is the problem?

Check and replace damaged component/s at its horizontal scan output section circuit. Check the LOT (Line Output Transformer) too for damage. Replace the horizontal scan output transistor. Check the input voltage to LOT. If the voltage is absent check the feeder resistor; if any. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electronicshelponline.blogspot.com/
Type in the brand name of your device, in the search box [at the top right] of the display window and the Search will start at once as you type in, and will show you all the related posts. It will make surf easy.

The vertical sweep output section circuit to your TV has the fault. Might be an IC failure, or with some loose solder points at the solder terminals to this IC. As it will be very difficult to detect the exact dry-solder point, it will be best to resolder all the suspected solder terminals to this vertical output section, by applying a little more solder to each terminals, without making any solder bridge short in between adjacent solder terminals.

I had the power board capacitors on my Samsung 46LNA650 checked, and the out-of spec/leaking ones (13 in all) replaced. The horizontal lines disappeared; but, the left side of the panel was dark for about 20 minutes (until warmed-up). I then found this forum/link that solved the problem! This simple fix was probably all that was ever needed - IT WORKS!!!!!!

http://forums.cnet.com/7723-13973_102-391250.html

My Samsung now has a perfect picture again.

this will continue to degrade, almost any failure that can be affected or temporarily cleared up by hitting the set is caused by bad solder joints.

A good shop should be able to find and fix the bad solder connections for probably a little under $100
if you bring it to them and pick it up.

Picture is reduced to a straight horizontal line across the center of the screen.

Bad solder connection in the horizontal pincussion circuit.have someone resolder.God luck.4tune

sounds like the vertical, more info ctechnicians.com

get new batteries, if recharging your current batteries does not work (to stop it shorting out).

reduce your shutter exposure, by changing to Manual Mode (M), selecting +/-, and then the left button (flash) around the Menu OK button. Mine works fine at 1s shutter speed, but was showing horizontal lines before that ...

now only visible in a thin horizontal line. first of all you reduce brightness to monimum. it just to protect your tv from crt demage. your problem is in horizontal output section as you have mentioned that week or so ago the right and left bottom edges of the picture began to move in like a sideways triangle filled with black so this sympton is due to hor. section problem. your flyback transformer or horizontal output might be defective. but remember check for power supply is ok from surge protector hope this will help.

Sounds like something is getting hot causing voltage loss or drive pulse loss to the vertical IC. Maybe you have a bad solder joint. Try tapping the circuit board around the vertical section. A capacitor could be heating and opening up. Stargazer Also you might try using circuit cooler spray and make the problem show up.