LG 47LN5400 TV Backlight problem

[abduct]

I have recently come into a bunch of TV's for cheap and they all need repair. I've had success with getting one working, in an odd ball way so I am here for advice or recommendations.

First the conditions of the TV's:

I have 3 LG 47LN5400, one LG 55LB5900 (broken panel) that I am working with (I have a few others 50" and 42" but they have broken panels that I am not going to bother with besides for parts.

The 3 47" LG's I am trying to fix all have the same problem. When powering on, the backlight comes on for a split second and turns back off. When the backlight is off, I've done the flashlight test and the image is still there. This tells me the main board is likely fine. All the caps look in mint condition and there isn't any obvious damage such as arcing or black burn marks. I've been reading some threads and have done some measurements and I will post those later.

In order to test the 3 47" LG's I took the powersupply from the 55" LG with the broken screen and tried each one, and the TV's work flawlessly so that limits the problem to the power supplies of the 47" TV's.

In my searching I cam across this image, where someone kindly layed out the parts of my PSU

https://www.badcaps.net/forum/attach...1&d=1430269466

I did some measurements like the thread suggested at Filter Caps 1, and Filter CAPs 2, and here are the results.

Filter Caps 1: 78v (peaks at 103v when backlight turns on then off)
Filter Caps 2: 75v (peaks at 182v when backlight turns on then off)

In another thread a similar issue arrised and they said the problem is the LED drive controller IC, which makes sense since when I swapped the other larger power supply in from the 55" TV, everything worked fine.

Here is some other info:

47" TV model number: 47LN5400
47" power supply model/serial number: EAX64905501 (2.0) Rev 1.0

55" TV model number: 55LB5900
55" power supply model/serial number: EAX 65423901 (2.1) Rev 2.0

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So I guess the real question is, is it possible to purchase these IC's to solder them on, or is there a cheap place to buy new power supplies?

Also is it safe to use the 55" LG TV power supply in the 47" TV? The PCB physically doesn't fit so I will have to make my own mounts and cut a hole in the back plastic casing, but other than that is there any risks?

Like wise I have a 42" TV with a smashed panel but the backlights stay on so I assume the PSU is good, but it only supports power to 1 LED string. Can I remove the LED controller from this and use it on a 47" LG PSU?

Thanks!

Edit: I am not 100% positive that it is the LED controller, but from what I read, it sounds like it could possibly be it. Any testing/tips will be appreciated.

[CapLeaker]

Re: LG 47LN5400 TV Backlight problem

could be also a bad LED inside the panel. Measure the voltages on the LED power connectors on the PSU. See what you get. Does it ramp up and fall off or what?

[newtothis]

Re: LG 47LN5400 TV Backlight problem

Did you measure the PFC Caps using VDC and the negative leg as ground?

If so the PFC circuit is dead

*EDIT* Then again you said you can still see the image.... *EDIT*

[abduct]

Re: LG 47LN5400 TV Backlight problem

could be also a bad LED inside the panel. Measure the voltages on the LED power connectors on the PSU. See what you get. Does it ramp up and fall off or what?

These are the readings for the original PSU (Note I don't officially know which set of outputs are which I just gave them channel names to help analyses the data):

-LED + CH1 to ground goes to 76v then when the backlight attempts to turn on goes to 103v and ramps down to 76v
-LED + CH2 to ground goes to 76v and then 171v when the backlight attempts to turn on, then ramps down to 76v.
-LED - CH1 to ground goes to 20v, then 46volts when the backlight attempts to turn on, then ramps down
-LED - CH2 to ground stays at around 28 volts, and then ramps down after backlights attempt to turn on.

The backlights turn on/off for less then a second.

These are the readings from another PSU from a 55" LG which makes the 47" TV work when used with it:

-LED + CH1 to ground goes to 93v and then the backlight STAYS on and slowly lowers until stabilizing at 56.9V
-LED + CH2 to ground 0v at all times
-LED - CH1 to ground 0v at all times
-LED - CH2 to ground -1v at all times (might of had probes backwards, red was on LED -, and black was on grounded part of the PCB)

The backlights turned on and stayed on and the TV functioned as normal, no noticeable dark spots or other damage.

Did you measure the PFC Caps using VDC and the negative leg as ground?

If so the PFC circuit is dead

*EDIT* Then again you said you can still see the image.... *EDIT*

Which power filter caps are you talking about. I tried measuring the power filter caps closest to the LED output (as marked by the link in the original post) and they showed similar VDC results as measuring from the LED output pins. (When I measured the caps, I put the red probe on the positive side, and black probe on a grounded part of the PCB.

When I test positive side to negitive side of the cap, I get DC voltage with similar results as in the table above for the 47" LG PSU.

With the original PSU, I can still see image with via the flashlight test and go through the menus, it's just the backlights fail to stay on for more than a split second.

I find it weird that what I think is channel 2 on the LED power output is putting out 0 volts on the 55" LG PSU when placed on my 47" TV. Might this be a feature that when it notices damage, it tries to keep on working, but closes off voltage to the damaged strip? Then again I don't see any fluctuation in voltage at all when measuring on that channel or noticeable dark spots on the screen where no LEDs might be shining.

Thanks for the help guys. If you want high res images of the power supplies front/back let me know.

[CapLeaker]

Re: LG 47LN5400 TV Backlight problem

you have a bad led string.

[abduct]

Re: LG 47LN5400 TV Backlight problem

you have a bad led string.

Is there a way to bypass this? Why does one power supply successfully make the TV work, and the original one does not?

Is it possible to cut the traces leading to the broken LED strip and solder a resistor onto the PCB tricking the IC into believing it is functioning properly?

I've gotten around to my other LG 47" which had similar symptoms and placing the 55" PSU in it as well fixed the problem, so that makes 2 TV's.

Is there steps to test all the outputs to the mainboard + LED strings? What kind of voltages should I be seeing on them. Can I test the power supplies without the main board plugged in, or does it need to be plugged in?

[newtothis]

Re: LG 47LN5400 TV Backlight problem

55" may give out different voltages for the LED's as there is probably more in the 55". So what would shut down the 47" PSU may not shut down the 55" PSU.

[abduct]

Re: LG 47LN5400 TV Backlight problem

55" may give out different voltages for the LED's as there is probably more in the 55". So what would shut down the 47" PSU may not shut down the 55" PSU.

That makes sense, but why would the normal 47" PSU be failing. These were all returned to the store, but I can't see all the TV's having a bad PSU causing similar or the same symptoms.

Is there a certain process I should take to confirm all the output voltages from the PSU as well as specific parts? I have checked the fuses and visually inspected for damage, but beyond that I haven't checked many of the parts.

What confuses me is if a LED did short or become open, which is causing the 47" PSU to shut down due to safety settings, why is the 55" PSU not doing the same.

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Edit::

I just measured the resistance of the LED strips, since LED's are diodes and they are in series (I would believe) One direction would have extremely high resistance, the other direction extremely low. With knowing this LED CH1 has 50k ohms in one direction, and 1m ohms in the other, and LED CH2 has 30k ohms in one direction, and 50-60k ohms in the other. This would mean that LED CH2 probably has a short somewhere (I would imagine). Now is this 55" PSU smart enough to detect this and shut that channel down to keep operations going, or what.
Can I place a diode+resistor in place of the shorted LED strip to trick the 47" PSU into thinking it's operational?

I am not looking to resell or sell these TV's they are just for my personal use so any hacks to get them working are welcome.

[newtothis]

Re: LG 47LN5400 TV Backlight problem

https://www.badcaps.net/forum/showth...t=diodes+5+led

[abduct]

Re: LG 47LN5400 TV Backlight problem

Thanks for all the help guys.

It was easier than I thought to strip the TV down to the LED backlighting and I used my DSLR camera with extended exposure to figure out which LEDS were broken. I had 2 LEDS shorted closed, and 1 LED shorted open. The open led caused 1 entire LED channel to fail causing the PSU to shutdown.

Instead of buying new parts I simply scratched the solder mask off the PCB and soldered in a piece of wire to complete the circuit bypassing the broken LED's and using epoxy to secure the connection/insulate it. With the diffuser in place you can hardly tell there are broken LEDs anyways unless you are watching something with allot of white or black, then again it just looks like normal light bleed anyways and doesn't bother anyone.

TV works like new, and I am going to apply similar methods to the TV's that have the same problem.

Thanks again.

Edit:: As a final question, is there an easier way to tell which LED is causing an open in the circuit? I had to pop open a handful of the diffuser lenses before I found one. None of the LED's looked burnt or damaged. Is it just luck of the draw I guess if you start popping them open at random?

[hendricks026]

Re: LG 47LN5400 TV Backlight problem

Hey Abduct,

I've got two of these 47s with the same issue. Im not sure if you took off every diffuser but since I had one good LED string I took apart the bad string and substituted one strip of the good string for one from the bad. (I peeled the LED strips off the back shell and pulled them apart into the individual strips that each had 5 LEDs) With the good string complete with 1 strip from the bad string I would turn the TV on and see if the LEDs flashed, if they did I'd consider the strip from the bad string to be okay and then I would move on to the next one.
This allowed me to narrow it down to one of 5 LEDs instead of 1 of 30.

I was going to try and replace the LED for a new LED but am somewhat confused when I take readings of the LED. I tried using the 'diode test' setting on my DMM and tried the leads both ways (since I dont know which side of the LED is positive and which is negative). Measuring them one way the DMM reads .780 volts, measuring with the opposite polarity the DMM reads Open but the LED will light up. Is it safe to say that the forward voltage on one of these LEDs is .780 volts or is there something going on that I dont get?

[budm]

Re: LG 47LN5400 TV Backlight problem

"Measuring them one way the DMM reads .780 volts, measuring with the opposite polarity the DMM reads Open but the LED will light up." The LED has built-in protection diode connected in anti-parallel..

[JBP]

Re: LG 47LN5400 TV Backlight problem

I have a 47ln5700 LG that I am having backlight or board problems with . Ruled out bad caps on power board and output voltage seems to be 24v dc going into main board ok . Symptoms were screen went blank ( black ) turned it on and off would show LG logo then right back off . All LED lights seem to be working correctly when first turned on then right back off. T board seems to be working correctly . Have tried flashlight test , but nothing seems to be showing .Don't know whether to replace power board , which seems to be outputting correct voltage or replace CPU ( main board ) ?

[budm]

Re: LG 47LN5400 TV Backlight problem

Please up load good clear pictures of the power supply board (top and bottom sides) and the Voltage reading at the pins LED backlights connector (see post 4).

[sspoonmann]

Re: LG 47LN5400 TV Backlight problem

use a diode to take the voltage drop instead of just wire. that extra voltage needs to go somewhere other wise more voltage on remaining diodes will make them burn out faster.

[JBP]

Re: LG 47LN5400 TV Backlight problem

From left to right pin voltage is and this is based on 200 DC setting on multimeter 78.5 ,21.6,78.5,0

[budm]

Re: LG 47LN5400 TV Backlight problem

From left to right pin voltage is and this is based on 200 DC setting on multimeter 78.5 ,21.6,78.5,0

Put the meter on the connector, then hit the power switch to turn the TV ON, you should then the see the DCV Voltage jumps up high and then settle down to around 70VDC or so when the protection circuit kicks in.
It is more likely that you have bad LEDs.

[JBP]

Re: LG 47LN5400 TV Backlight problem

Yep spikes to about 130 on 1 and 3 then goes back down to 78 . The second pin goes to about 70 DVC then back down to 21 . Can't send pics yet keeps timing out.

[budm]

Re: LG 47LN5400 TV Backlight problem

You have open circuit in the LED strings.

[JBP]

Re: LG 47LN5400 TV Backlight problem

Ok thanks will check those next.