LG 42LN5400 Power but no backlight, picture or sound

[dhoff024]

My neighbor was getting rid of this LG Tv and I have a problem seeing things get thrown away that I feel should be able to be fixed. I plugged it in, red light comes on and when you hit the power button red light goes out but nothing happens, no sound, no picture, and no backlight. I dont know exactly where to start but I have fixed some TV's in the past with the help and knowledge from the great people on this forum. Hopefully you can help me bring this one back from the dead.

[budm]

Re: LG 42LN5400 Power but no backlight, picture or sound

https://www.badcaps.net/forum/attach...3&d=1505188482
First, you need to report the DC Voltages of all the pin of the power supply connector that goes back to the main board.
The exposed pins on the top side of the board correspond to the left column (GND, 12V, 12V GND, 24V, GND, 3.5V, etc....) of the pins' chart printed on the board, the right column (GND, N.C, etc...) is for the pins that you cannot see from the top side of the board so you will have to probe them from the back side of the connector.
You MUST REPORT PIN NAME AND FOLLOW BY THE DC VOLTAGE READING FOR EACH PIN when TV is plugged in but not turned on, then the readings after the power switch is activated.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

Ok so I took some readings tonight. This is what i got

top pins: plugged in but power off

GND- 0
12V - 0
12V- 0
GND - 0
2.4V - 0
GND - 0
3.5V - 3.5V
3.5V - 3.5V
PWR ON - 0

Bottom Pins: Plugged in but power off

GND- 0
NC- 0
12V - 0
GND - 0
2.4V- 0
GND - 0
PD1M2 - 0
PD1M1 - 0
DRV ON - 0

When I press the power button nothing changes. I did notice that the standby light is off when I unplug the cable and it comes back on when I plug it back in. I also noticed that the power on pin on the board is broken off in the cable connector. I dont know where to turn next but I would have to say there is a power supply problem or that pin being broken is effecting it. I dont mind buying another power supply and cable but I dont want to do that if that isnt what is wrong with it.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

Ok so I kept messing with this TV tonight and I think I was taking those voltage readings incorrectly the first time so I apologize for that. I was able to get the pins straightened out and I plugged the connector back in and was able to get correct readings. So this is what I found. Everything on the top row of pins and bottom row of pins matches the voltages printed on the board except the 2.4V pin on the top row was reading 24V I don't know if that is correct or not. The other thing I have now that I was able to get the pins straightened out is I have sound and picture but no backlight. I can see everything on the screen with a flashlight and I can hear the static sound. I am guessing there is an LED problem but don't know for sure.

[budm]

Re: LG 42LN5400 Power but no backlight, picture or sound

That is 24V pin not 2.4V pin.
"PD1M2 - 0 (Backlights diming control)
PD1M1 - 0 (Backlights dimming control)
DRV ON - 0" (Backlights ON control)
What do get on those pin now?

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

that is 24v pin not 2.4v pin.
"pd1m2 - 0 (backlights diming control)
pd1m1 - 0 (backlights dimming control)
drv on - 0" (backlights on control)
what do get on those pin now?

pd1m2 - 0.8
pd1m1 - 0.8
drv on - 2.8

Also when the tv is turned on the backlight flashes real quick for like a second but doesn't stay on or come back on at all after that.

[budm]

Re: LG 42LN5400 Power but no backlight, picture or sound

The dimming controls look to be on the low side, but we need to verify the Voltage at the LED connector first.
Keep the black lead of the meter on the chassis then on the LED connector P801 (connector in place) you will
1) Put Red probe on pin 1 (LED -) and then turn on the TV while keeping an eye on the meter to see how high the Voltage jumps up to and then settle down after 10 seconds, report the reading. Turn the TV OFF.
2) Move Red probe on pin 3 (LED +) and then turn on the TV while keeping an eye on the meter to see how high the Voltage jumps up to and then settle down after 10 seconds, report the reading. Turn the TV OFF.
3) Move Red probe on pin 5 (LED -) and then turn on the TV while keeping an eye on the meter to see how high the Voltage jumps up to and then settle down after 10 seconds, report the reading. Turn the TV OFF.
4) Move Red probe on pin 7 (LED +) and then turn on the TV while keeping an eye on the meter to see how high the Voltage jumps up to and then settle down after 10 seconds, report the reading. Turn the TV OFF.
I will not be surprised if you have bad LEDs.

http://www.shopjimmy.com/catalogsear...?q=LG+42LN5400

http://www.shopjimmy.com/lg-eay62810...-led-board.htm
LG EAY62810601 (EAX64905401(1.5)) Power Supply / LED Board

Your screen has two sets of LED strings with two boost converter and controller circuits.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

I appreciate all your help. I will test the voltage at the LED connector tomorrow and report back with what I find. Could it be both the board and the LED strips or is it most likely 1 or the other?

[budm]

Re: LG 42LN5400 Power but no backlight, picture or sound

More likely to be bad LED/s in one of the LED string.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

Ok so I was able to take some readings tonight. and this is what i have.

PIN 1 LED - Peak 33 and settles to about 16
PIN 3 LED + Peak 75 and settles to about 58
PIN 5 LED - Peak 84 and settles to about 2
PIN 7 LED +Peak 166 and settles to about 58

Those numbers may be slightly off but they are pretty close.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

I think I let those voltage numbers settle to much. I took new readings and I waited until each pin hit peak voltage and then started counting to 10 as it fell and here are the new numbers I have. The low numbers are the reading after I hit 10.

PIN 1 LED - Peak 33 after 10 secs 25
PIN 3 LED + Peak 75 after 10 secs 62
PIN 5 LED - Peak 84 after 10 secs 20
PIN 7 LED + Peak 166 after 10 secs 80

I noticed that PIN 5 and PIN 7 fell pretty rapidly after they hit peak voltage and that didnt happen as much with PIN 1 and PIN 3.

[budm]

Re: LG 42LN5400 Power but no backlight, picture or sound

Are you 100% sure that you did not see the backlights come on for seconds when you first turn on the TV? May be turn to TV in the dark room and see. Right now it looks like you have problem with one of the LED strings.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

Are you 100% sure that you did not see the backlights come on for seconds when you first turn on the TV? May be turn to TV in the dark room and see. Right now it looks like you have problem with one of the LED strings.

Yes the backlight does flash for a second when the TV turns on right before I get sound the backlight flashes 1 time and then is dark and doesnt come back on.

[budm]

Re: LG 42LN5400 Power but no backlight, picture or sound

Yes the backlight does flash for a second when the TV turns on right before I get sound the backlight flashes 1 time and then is dark and doesnt come back on.

OK, that is important information.
Now disconnect wire from pin 1 (LED-) then turn on the TV in the dark room to see if the backlights will still flash on for seconds.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

OK, that is important information.
Now disconnect wire from pin 1 (LED-) then turn on the TV in the dark room to see if the backlights will still flash on for seconds.

This may be a dumb question but how do I pull just the wire connected to pin 1? Since it's a connector that is plugged in. Do I pull that wire out of the connector? Or do I try to unplug that side enough to get the pin to not make contact with the connector.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

Ok so I was able to get the 1st wire disconnected from the connector. I tested the tv with it unconnected and the backlight does not flash when it is unconnected. once you put it back together like it was originally and turn on the TV you get the backlight to flash again.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

Is it easier to just replace all the LED strands with new ones? Or should I find the strand or strands that are bad and just replace those?

[budm]

Re: LG 42LN5400 Power but no backlight, picture or sound

Ok so I was able to get the 1st wire disconnected from the connector. I tested the tv with it unconnected and the backlight does not flash when it is unconnected. once you put it back together like it was originally and turn on the TV you get the backlight to flash again.

So you have bad LED string that is connected to pin 5/7 that is why the Voltage jumps up over 150V due to no load.
Replacing each LED can be done if you have good proper tool, in your case you may be better of replacing the LED strips, you need to get the correct one for your model of your LCD panel, check out shojimmy.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

Thanks for your help I will get it torn apart and get the correct LED's ordered.

[dhoff024]

Re: LG 42LN5400 Power but no backlight, picture or sound

So if you look at the attached photos I believe I have found the bad LED strand. My next question is should I just replace that strand or replace all of them just to be safe? Also what is the proper way to get them disconnected and taken off the board without causing any damage? I dont see a tab to press and take them apart or do they just push together and pull apart? Also how do I tell what panel I have? I know that I need the panel number to order the correct LED's.