Electronics > Repair

LCD Tv repair- sounds like standard capacitor issue, but not

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dbdb2000:
Hi, I'm a long time reader of the forums and watched most of Dave's Videos and just thought i would post this for any Jedi Electronic experts who like a challenge.
I have a broken Sharp LCD TV-

Fault-
-turns on fine everytime (backlight and picture)
Then after around 20seconds picture goes black (channel/Av no difference, but backlight still on
Turning off and instantly on-works again repeating until maybe 5th ->10th time stays on all night

as the tv goes black the sound crackles leading me to believe it was a capacitor power issue, once opened the tv, very few power capacitors

Tested so far.
-The tv has separate 12v 5A power pack and have checked with spare 12v supply- still same fault
-have checked with esr meter, capacitors on board- all seem fine
-most capacitors are low esr Nichican so cannot check easily- have unsoldered a few and checked capacitance outside circuit and seem within limits
- with backlight invertor disconnected, picture visible with torch but still turns off.
-Have changed the k2503 D N-channel Mosfet (near Yellow sticker) as there was some heat marking on the board but no change and still not sure how to check mosfets correctly.

This leads me to believe either a transistor/mosfet or Ic problem- most likely a dry joint as once the tv has warmed up it works fine.

Any good ideas what I could check quickly to narrow down problem, otherwise i will use the hot air reflow 858D i recently bought on Dave's recommendation (really useful) and just heat the whole board to try and solve any possible dry joints, before dumping.

Thanks and will keep you posted on any changes, I just like a challenge to try and get it going again due to what is probably a 5c piece breaking.
Thanks for any suggestions

Edit:
It is always a variable time to failure (sometimes 2 secs othertimes 30secs) but always when cold, today with it on and working, I cooled different areas with a freezer spray but couldn't get it to trip until i had turned it off for several hours.
Normally it goes cold- 5sec turn off and on 10 secs working off and on then in progressively longer spurts until after 30 seconds never a problem 

Further edit to prevent bumping. When turning it on and off fairly frequently it suddenly released the magic blue genie smoke and two surface mount resistors disappeared. As i don't have the time at the moment i will leave it for the moment and come back to it. Dave released a very good video on lcd tv repair showing a similar fault and once i replace the fuse i will check the flex cables and repost. 

daveshah:
Is it the same exact amount of time each time, or does it vary (for example, what happens if start it up again while it is still warm)?

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