Samsung LE26B455C4W backlight problem

[dick_barton]

Re: Samsung LE26B455C4W backlight problem

You said there was 18 volts on the VCC pin of U801S.
Does this voltage alter when you open the jumper link that goes to the Vcc pin for the FAN7530 (pin 8)?

I would check QB801. What voltages do you have on the 3 legs when in and out of standby.

[thth]

Re: Samsung LE26B455C4W backlight problem

I opened the jumper and was measuring the VCC of U801S (actually UM801S) when the backlight suddenly came on. What could have happened? And why would the light be on when the supply for FAN7530 is cut?

EDIT: I just wanted to add that the light came on when I was actually doing the measurement (I saw the light from the back through some holes in the metal back plate). Also it _might_ be that I accidentally poked the probe between the Vcc and ground shorting them momentarily (but I'm not 100% sure about that).

[thth]

Re: Samsung LE26B455C4W backlight problem

I left the TV overnight and when I tested it today the backlight wouldn't come on but putting it on standby and on again made it work. The jumper was still open. I tried to reconnct it but the backlight wouldn't turn on. Opening the jumper resumed the operation of the backlight.

What is the function of the FAN7530? I'm guessing that it is not wise to leave the jumper open permanently?

[thth]

Re: Samsung LE26B455C4W backlight problem

Would it be safe to do a longer test with the jumper left open? I haven't kept the tv on longer than a minute or so.

[thth]

Re: Samsung LE26B455C4W backlight problem

I wasn't until now that I undestood how this circuit works. Sorry for being so thick.

It was mentioned here before but I didn't understand it then and I only got the schematic recently. The backlight is now working because it is getting rectified mains voltage but the voltage is not boosted as it should be by FAN7530 IC. So the backlight is not getting the full power. Presumably there is some circuit in the FAN7530 that cuts the backlight if the boost is not working. By removing the jumper I disabled that circuit.

I shall investigate more.

[thth]

Re: Samsung LE26B455C4W backlight problem

I finally found the fault. It was CP810 0.22nF 1KV capacitor. It was cracked but I didn't see that earlier. Now the TV works. Thanks for the help.