Lg 47ln5750 led tv

[tech2014]

I have an led tv with sound but no picture. When the power comes on, there is picture for about 2 seconds then goes black. I did the flashlight test and there is video. I tested all the voltages and they were all there. I noticed that at connector P801, the voltages jump to about 150V for the short time the backlight is on, then drops to 80v and stays consistent at 80V. This connector P801 has 2 (LED+) pins and 2 (LED-) pins .I assume that P801 is connected to the LED strips in the panel. When power is applied, I get 80v on LED+ pins and around 22-25V on the LED- pins. With the voltage starting high then dropping, would this be a bad driver circuit in the power supply, or is the panel the problem.

Here are 2 pics of the tv. Pic 1 is the connector P801 and pic 2 is of the back of the tv with circuit boards. I also included a schematic of the power supply below

[ReeceyBurger123]

Re: Lg 47ln5750 led tv

https://m.youtube.com/watch?v=Bf5T2wwtydY

[ReeceyBurger123]

Re: Lg 47ln5750 led tv

Watch this sounds like a similar issue.

[tech2014]

Re: Lg 47ln5750 led tv

When I turn on the tv in the dark it appears that all the LEDs are lit. Is there any way to force the LEDs to stay on ?

[budm]

Re: Lg 47ln5750 led tv

You have open circuit in one of the LED string that is why it goes up 150VDC.

[tech2014]

Re: Lg 47ln5750 led tv

ok. How come the LEDs all seem to light up at power up? So its for sure a panel problem then?

[mattstolte]

Re: Lg 47ln5750 led tv

Take the panel out and watch the leds light up there may even be one out you wont tell unless you have panel out and watch them light up these 47 lg led are common for bad leds. I have some led strips for sale for it. I have done about 5 repairs on these models within last 3 months

[tech2014]

Re: Lg 47ln5750 led tv

ok so its a common problem on these TVs. I will take out the panel then. Thanks for the help.

[budm]

Re: Lg 47ln5750 led tv

ok. How come the LEDs all seem to light up at power up? So its for sure a panel problem then?

You have two set of LEDs with two driver circuits as you can see the schematic, so one set will still lights up but another one did not so the protection circuits kick in.

[tech2014]

Re: Lg 47ln5750 led tv

I forgot to mention that when I disconnected the connector that goes from the power supply to the LED panel, the voltage still jumps up to about 160V then settles down at 80V. Wouldn't this be a power supply problem then?

[budm]

Re: Lg 47ln5750 led tv

"I forgot to mention that when I disconnected the connector that goes from the power supply to the LED panel, the voltage still jumps up to about 160V then settles down at 80V. Wouldn't this be a power supply problem then?" Both LED+? OR just one of the two?

There are TWO LED + outputs, check both of them one at a time every time you turn on the TV and report back what you read on BOTH LED +, then do the same again with the LED connector NOT in place and report the readings again at each LED+ pins, you must do that with the probe connected to the LED+.
The Voltage drops down when the error is detected OVER VOLTAGE CONDITION DUE TO NO LOAD so the boost converter is shutdown that is why you only see 80VDC. It is constant power supply.
Did you not look at the schematic to see what is going on? Look at PDF page 20.
"Wouldn't this be a power supply problem then?"
You can replace the power supply board if you want because I do not know how much more explanation you want.

[tech2014]

Re: Lg 47ln5750 led tv

both LED+ voltages jump up to around 160v then drop back to 80V. I am going to test the LED strips today. thanks for the help.

[ReeceyBurger123]

Re: Lg 47ln5750 led tv

Have you watched the video I sent the link to ?

[tech2014]

Re: Lg 47ln5750 led tv

yup I watched it and it was very helpful, thank you very much.

[ReeceyBurger123]

Re: Lg 47ln5750 led tv

Ok no worries does your meter have a min max mode ?

[tech2014]

Re: Lg 47ln5750 led tv

yes it does

[ReeceyBurger123]

Re: Lg 47ln5750 led tv

Did you do a similar test to what Norcal did in the video and compared the two connectors ?

[tech2014]

Re: Lg 47ln5750 led tv

yup I got about 50V on each return.

[ReeceyBurger123]

Re: Lg 47ln5750 led tv

Hmm its possible that you don't have a shorted led but there are several that have failed which is tripping the protection circuit due to change in current, seen it on a 40" toshiba.

[budm]

Re: Lg 47ln5750 led tv

50V with LED connector in place at the LED -? 50V means that BOTH the PWM POWER MOSFETs are barely turned on.
Also you are saying you are getting 160VDC on one of the LED+ or both LED+ WITH the LED connector in place?

"I get 80v on LED+ pins and around 22-25V on the LED- pins." so the peak hold shows 50V then?

"both LED+ voltages jump up to around 160v then drop back to 80V. I am going to test the LED strips today. thanks for the help." With or without the connector in place?
The key point I am looking for is the Voltage with load and no load.