LG 47LN578V - Brief Backlight flash on power up.

[tezzanator]

Hello everyone!

I'm trying to repair an LG smart TV, 47LN578V and have so far come to conclusion that the PSU has a fault but would like a second opinion.

Symptoms
Powering TV on causes the screen to come on briefly with LG logo then the backlight goes out, the smart functions of the set continue to function and the TV can be controlled both via the remote and over the android application.
Shining a light into the screen allowed me to view the android application pairing key.

Measurements
Main, Large, Cap: 394V

Main connector to main board (P201):
Row 1/Row2
------------------
GND/GND
11.8V/24.8V
11.8V/11.8V
GND/GND
24.9V/24.8V
GND/GND
3.52V/1.12V (PDIM2)
3.51V/1.12V (PDIM1)
3.4V/3.22V

Backlight LED Output:
0V, brief spike that my DMM cant pick up quick enough beyond a flash of a number >24V then returns to 0V

Backlight Resistance Between pins:
Nothing found

Backlight Resistance to ground:
Nothing found

Notes
The central transformer on the PSU gets quite warm, unsure whether this is normal.

At this point should I just replace the Power Supply? My thinking is that if there was an open circuit on the back lighting strips then the BL would not come on at all, and measuring the pins on the BL connector found no obvious shorts between pins or to ground.

Are there further troubleshooting steps I can take to narrow the problem down further?

I hope I've provided all the required information, Any help is much appreciated

[budm]

Re: LG 47LN578V - Brief Backlight flash on power up.

I need good clear pictures of the backside of the power supply/LED driver board. More likely to be bad LEDs problem.

[tezzanator]

Re: LG 47LN578V - Brief Backlight flash on power up.

Hi Budm,
Thank you for taking the time to look at my thread
I hope these photos prove adequate.

[budm]

Re: LG 47LN578V - Brief Backlight flash on power up.

OK, this TV uses two LED driver circuits one per string.
If one of the string goes open circuit, it will cause the circuit to go into protection mode.
You need to check the DCV on the + leg of CAPS 1 (any + of the two caps 1 legs) and Ground when you first turn on the TV, then turn the TV off and then check the + leg of the caps 2 when the TV is turn on.
I expect one of the reading will show a lot higher DCV than the other.
The two caps in CAPS 1 are connected in parallel and they are for the boosted DC filter caps.
Same for CAPS 2.
Right now I believe only one string of the two LEDs lights up.

[JeffJ]

Re: LG 47LN578V - Brief Backlight flash on power up.

PSU and the innards are almost identical to mine and two others who recently posted with the same issues. This seems to be a spreading issue for these production runs if this turns out to be LEDs as well. Very much a pain to source, they are going for a mint on the used market as well.

[tezzanator]

Re: LG 47LN578V - Brief Backlight flash on power up.

Hi Budm,

Cheers for the diagram

I have done the voltage measurements as requested and these are my findings:
Cap set 1: 0V, Quickly rises to 160V then slowly discharges after backlight has turned off
Cap set 2: 0V, Quickly rises to 96V then slowly discharges after backlight has turned off

I'm guessing this points to a backlight strip fault? Intuition tells me that the strip powered by the circuit containing "Cap set 2" has a short or is at the very least taking excess current?

JeffJ,
It certainly is looking like a common issue, luckily I Bought this TV in it's current state as a project and don't have much money tied up in it!

I am lucky in that I have a potential source for a replacement backlight and if that falls through I may attempt a chip-level repair with LEDs sourced from a similar model's backlight. If that is possible, I haven't seen them in person yet. I do have SMD soldering experience (I'm not new to electronics, just TVs )

Is it possible they are just driving the LEDs with far too much current?

[budm]

Re: LG 47LN578V - Brief Backlight flash on power up.

"Cap set 1: 0V, Quickly rises to 160V then slowly discharges after backlight has turned off" You have open circuit, no load so the Voltage went up real high and so it goes into shutdown.
Open LED (bad LED), connections, etc. You need to open up the LCD panel assembly but good luck finding new LED or LED assemblies, one is already bad so you can expect the CH2 to go out later also.
LED backlights TV are very unreliable!

[tezzanator]

Re: LG 47LN578V - Brief Backlight flash on power up.

Thought I may as well tear into it, in for a penny in for a pound =)

I set up my DSLR with an 8 second exposure time and switched the TV on so I could easily see if any LEDs weren't coming on, the result is quite interesting...

On two of LED strips there's one LED out... I'm assuming they are in series so surely these must me shorted? And then there's three entire rows that aren't lighting up. However one of the rows isn't in adjacent.

At the very least it's a cool photo!

[budm]

Re: LG 47LN578V - Brief Backlight flash on power up.

Series circuit, so when one component failed, the whole series circuit will be dead. Just like Christmas lights.

[tezzanator]

Re: LG 47LN578V - Brief Backlight flash on power up.

"Cap set 1: 0V, Quickly rises to 160V then slowly discharges after backlight has turned off" You have open circuit, no load so the Voltage went up real high and so it goes into shutdown.
Open LED (bad LED), connections, etc. You need to open up the LCD panel assembly but good luck finding new LED or LED assemblies, one is already bad so you can expect the CH2 to go out later also.
LED backlights TV are very unreliable!

Timing!
Ah, That makes sense.

Thank you very much for your help. I shall see if I this potential backlight comes good, if not I think it may be destined for the scrap heap.

[newtothis]

Re: LG 47LN578V - Brief Backlight flash on power up.

Ok there are so many LED strips failing these days... So what can we do about repairs?... We all know it is hard to obtain the strips and the individual LEDs... But what is so special about the LEDs in these sets? Surely there is something we can use to substitute one?

[budm]

Re: LG 47LN578V - Brief Backlight flash on power up.

If we can find the spec sheet of the LED then the replacement can be found. I think if you want to make them last longer then run the backlights at lower level, heat kills semiconductors.

[newtothis]

Re: LG 47LN578V - Brief Backlight flash on power up.

If we can find the spec sheet of the LED then the replacement can be found. I think if you want to make them last longer then run the backlights at lower level, heat kills semiconductors.

Can we not just figure the voltage needed for said LED and find one of equal brightness? Or is it not so simple?

[budm]

Re: LG 47LN578V - Brief Backlight flash on power up.

The Vf of the LED will be about the same, but we need t know the current handling (important), viewing angle, the foot print (important if they are used in tight spacing), the color temperature, the lights output (important), etc. Otherwise will try and error.

[newtothis]

Re: LG 47LN578V - Brief Backlight flash on power up.

Right so once I get my bench power supply I am going to get allsorts of SMD LEDS and blow the buggers up figuring what they can handle, colour etc etc

I have successfully transplanted LED's from one strip to another when having a few identical shorted strips so hopefully after some figuring out we can start fixing these bloody things

[budm]

Re: LG 47LN578V - Brief Backlight flash on power up.

It will be nice to get a bunch of LED samples to experiment with.

[newtothis]

Re: LG 47LN578V - Brief Backlight flash on power up.

It will be nice to get a bunch of LED samples to experiment with.

Even if the footprints are different convertors can be made, I have the equipment for BGA work here so shouldn't be too much of a hassle to do it.. I will start hunting around for wrecked LED sets from different manufacturers etc and see what I can find out

[JeffJ]

Re: LG 47LN578V - Brief Backlight flash on power up.

If we can find the spec sheet of the LED then the replacement can be found. I think if you want to make them last longer then run the backlights at lower level, heat kills semiconductors.

This is an interesting comment hat made me have a eureka moment. Th failed LED, (i only had a single led bad, thank god) was over a part of the panel chassis that was reassessed so there fore the back of the strip was not affixed to the chassis there and was not able to allow the heat to sap off to the chassis.

My tv i fixed failed after 13 months

If this comes to be a large scale issue i can either see a class action or a recall to avoid one.

Even if the footprints are different convertors can be made, I have the equipment for BGA work here so shouldn't be too much of a hassle to do it.. I will start hunting around for wrecked LED sets from different manufacturers etc and see what I can find out

For what its worth every failed led strip i have seen was made by LG and are found in a wide assortment of tv's these days. yay LG...... /Sarcasm

[budm]

Re: LG 47LN578V - Brief Backlight flash on power up.

This edge lit screen uses in Philips has big heatsink for the LEDs assembly but I will never find out how long they will last because it has broken screen. They are REALLY BRIGHT!
http://s807.photobucket.com/user/bud...?sort=3&page=1

[newtothis]

Re: LG 47LN578V - Brief Backlight flash on power up.

This is an interesting comment hat made me have a eureka moment. Th failed LED, (i only had a single led bad, thank god) was over a part of the panel chassis that was reassessed so there fore the back of the strip was not affixed to the chassis there and was not able to allow the heat to sap off to the chassis.

My tv i fixed failed after 13 months

If this comes to be a large scale issue i can either see a class action or a recall to avoid one.



For what its worth every failed led strip i have seen was made by LG and are found in a wide assortment of tv's these days. yay LG...... /Sarcasm

Most LED strips are outsourced

The LEDs are manufactured and even if not readily available they are available somehow... It is a case of lots of emails and talks with the manufacturers until a supply chain is achieved. There are as far as I know no repair services for bad strips... I think it is about time there was one!