No picture turns off after 15 min - Samsung LNT5265FX 52 in. LCD Television

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horizontal sync missing check defflection circuit for bad filters

they dont last forever... time for replacement as most are not user-serviceable..

Its a older set and I have fixed and even owned one---bad solder connections and or convergence problems---can you describe better what it looks like when cold?

You have the television on sleep mode. You'll have to disengage the sleep function! Most tv have this option so that you can go ahead and snooze for a while and the tv will turn itself off!

it probablt needs power supply capacitors, if you can take it apart and inspect the power supply, the bad capacitors usually are expanded and have a bulge at the top, you would need to solder these parts in, so some skill is required.

Bad flyback transformer. It has a short in the winding making the Horizontal output transistor heat up so when its too hot the set goes into protection.

Your loosing the color wheel or have lost it

Samsung LN40A550 LCD TV

This is probably the LCD Controller (T-CON) board connections. This is a small board with 2 flat cables connected to it which run to the LCD screen. The connections with the flat ribbons needs to be reseated. The heat makes the connections misaligned. This board is usually covered by a metal shield.
When you reseat the connections, run the set for 15 minutes to see if it works before replacing the back cover. You may have to reseat it again.
Remember, handle electronics boards by the edges, no static electricity, and do not touch the connection contacts.
At worst you would need a new board. (Shopjimmy.com has it for ~$90.00usd)

cheak cables what cables uusing ? thx nick