Repairing a TOSHIBA LCD TV 19W331DG

A customer brought to me a TOSHIBA LCD TV, model 19W331DG, with some strange problems.

For example, sometimes after turning it on the backlights didn’t light on.
I immediately found the problem: on the power board there where two blown capacitors, one was a high capacity value, like 25V 1000uF, and it was clearly visible. The other, was a 50V 220uF that I found as defective just after measuring the ESR value. Externally it seemed ok.

So, after replacing these two capacitors, the TV worked perfectly.

But another problem came out.

After something like 2 hours working, the backlights turned out again. This time wasn’t a capacitor problem, and everything looked ok.
So, what could stop working after 2 hours, and why?

TOSHIBA TV LCD 19W331DG power board repair
TOSHIBA TV LCD 19W331DG power board repair

Note the heat skins added to main the power regulator, the main power diode and the main transistor.

Simply, the semiconductors regulating the backlights main power shut down after overheating.