55LN5700 intermittent led failure

[rustytierod]

What is the deal with intermittent issues? I have logo flash then off, taking apart I find the bottom 3 strip do not light. So I reseated the connector going to them and then they all stay on. Then after I get it to fail again it doesn't work.

Coincidence?

Power supply intermittently not supplying enough current?

[chrismurphy]

Re: 55LN5700 intermittent led failure

cut the plug and hard wire and jumper the center connector also

[rustytierod]

Re: 55LN5700 intermittent led failure

Ok, if it was one burnt out led or short I'd see it right? Or it wouldn't be intermittent, right? Can I reseat connector while it's on without arc hazard?

When it does stay on they ALL light up very well. Jabbing that connector (first connection from power supply) doesn't change anything.

[budm]

Re: 55LN5700 intermittent led failure

I would jump the SMD connectors with wires needs or not so you will not have to take the panel apart again due to fail connectors.
Are the LED strips are connected in series to form one large LED string or it has more then one LED driver circuits?
Pictures of what we are dealing with will help.
No sing of any over heated LEDS? These LG LN series are know for bad LEDS and they burn up badly.
https://www.badcaps.net/forum/showth...hlight=latw%2A
I am surprised they are not recalled by LG.

[rustytierod]

Re: 55LN5700 intermittent led failure

Ouch definitely no burnt up ones.

There are two pairs of leads from the PS feeding the top 4 and bottom 3 rows, respectively. Both sections are fed in series. I guess I'm going to have to suspect the driver. Saw someone with model close to this on youtube fix by reflowing transformer joints.

[rustytierod]

Re: 55LN5700 intermittent led failure

After initial flash, I get 88v from both led+ to chassis ground, Vmax 10v from string that doesn't light led+ to led-. And Vmax 20v from string that flashes led+ to led-. Sometimes the back lights come on and stay on for a while until restart.

Everything coming in from main board good.

Tcon is disconnected and removed as it doesn't affect anything and the screen is off

[rustytierod]

Re: 55LN5700 intermittent led failure

[rustytierod]

Re: 55LN5700 intermittent led failure

I'm sorry I guess I should edit that other post to say they're wired in parallel?

[rustytierod]

Re: 55LN5700 intermittent led failure

Anyone have the service manual for this? Looked on elektronica but did not find.

[budm]

Re: 55LN5700 intermittent led failure

Can we see the whole pictures of all the LED strips instead of partial pictures?
Also need straight shot pictures of the bottom side of that power supply/LED driver board.

[rustytierod]

Re: 55LN5700 intermittent led failure

[vinceroger69]

Re: 55LN5700 intermittent led failure

can you also remove the power supply and take a picture of the underside of it so we can see the traces etc

[budm]

Re: 55LN5700 intermittent led failure

https://www.badcaps.net/forum/showth...695#post786695
The 88V reading is the feeding Voltage to the boost converter circuit.
You have two LED string and two LED driver circuits. The LED+ of each string is connected together and fed by the Voltage by the single boost converter circuit.
The LED - for each string is the Cathode return through GND through the PWM Controlled MOSFET (2) and the current sensing resistors to maintain constant current.
Have all the connectors and all the boards connected then:
1) Leave the black probe of the meter on the chassis, RED probe on the either one of the LED+ then turn on the TV and watch the meter to see how high the Voltage goes up to before it goes back down to 88VDC.
2) Turn the TV off and move the red probe to one of the LED- then turn on the TV and see how high the Voltage goes up to and wait 5 seconds to see how low it goes down to.
3) Repeat step 2 for the second LED -.
Report your finding.
BTW, those SMD connectors on those LED strips are known for poor solder joints which will cause intermittent backlights. https://www.badcaps.net/forum/showth...124#post690124
This looks to be your board:
http://www.shopjimmy.com/lg-eay62810...upply-unit.htm
LG EAY62810701 (3PAGC10124A-R) Power Supply / LED Board.
Pictures provided by shopjimmy.