Not sure if I responded on this thread as it is an old thread that won't die.
I also have a Samsung 50" that was given to me years ago (CFL backlight) due to this same issue.
I was able to crack it open and replace 4-5 capacitors that were visibly bulged. The unit turned one 100% normal and worked great for about 3 years until now. It started doing the same thing over the past 4-5 months, slowly getting worse. Sometimes it would only take it 2 minutes to come on, now it takes it about 10. Once "warmed up" it can come in fine if turned off and back on.
It seems like a bad circuit design that allows for the capacitors and possibly other components to become damaged over time. But in my experience they can be replaced with a soldering iron, caps and a little know how. 30-40 minutes of your time and 5 bucks worth of caps.
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