LG 47LG6000 most often no backlight

[PowerAmpFreak]

Hello!
This LG LCD-TV model 47LG6000 does most often not light up the backlight.
There are drive waveforms on the primary side of the 2 HV transformers, which looks ok. 400V peak to peak driven from the PFC.
Wonder if those transformers often fails? (see picture)

Thanks

[PowerAmpFreak]

Re: LG 47LG6000 most often no backlight

The first time the set is started the backlight does most often come on. Second try it won't ... Any advice?

[tom66]

Re: LG 47LG6000 most often no backlight

Locate the inverter controller IC, see if it has an time-out capacitor and disable it temporarily by blobbing it with solder. Then you will see if it is an aged bulb problem, the bulbs will start after a couple more seconds, or another fault.

[PowerAmpFreak]

Re: LG 47LG6000 most often no backlight

Hi,
The psu board use the OZ9966 inverter controller.
https://www.google.se/search?q=oz996...ml%3B448%3B306

Could you see what pin is the time-out cap? I would assume pin 14 or 15?

[tom66]

Re: LG 47LG6000 most often no backlight

Pin 15.

Beware, do not use this as a permanent fix.

It can burn out the transformer if left in for any significant amount of time.

This can cause the transformer to physically burn and melt, which can cause heat damage. (It shouldn't cause a fire though as it's a safety-rated device, but you can't be absolutely certain.)

30 seconds = usually OK, any more, possible transformer damage.

[PowerAmpFreak]

Re: LG 47LG6000 most often no backlight

There is no simple cap on pin 15 so I suspect there is another circuit..
BUT, probing the primary waveform with my scope. There is 400V peak to peak about 50kHz, trying to fire up the backlight.
Probing across the backlight connectors (yes, with my HV probe) there is virtually nothing there. It flashes to about +2kV for a very short time, and goes down to zero.
Could the inverter/psu board be at fault.

[tom66]

Re: LG 47LG6000 most often no backlight

Try it without bulb connectors in - possible intermittent short?

[PowerAmpFreak]

Re: LG 47LG6000 most often no backlight

Won't the overvoltage protection kick in then?

[tom66]

Re: LG 47LG6000 most often no backlight

It will, but maybe you will see a longer start up time, normally about 2sec?

[PowerAmpFreak]

Re: LG 47LG6000 most often no backlight

I see no difference with bulbs unplugged. Maybe there are internal shorts inside the HV transformers or HV caps.
Obviously the bulbs does not recieve proper drive voltage