LG 55LN5700-UH dimming every 2 seconds...

[niswanger]

Hey gang,

I picked up an LG 55LN5700-UH that has an issue and I figured I would have a go at trying to repair it.

I've searched here and I have some ideas but didn't really want to hijack any existing threads. Didn't find any of this exact model.

Attached are the model sticker and software version. And the last youtube video link is a short video of this exact set showing the issue.

The set software is up-to-date and the behavior of the screen dimming every 2.5 seconds and then going back to normal and while the slight dimming occurs faint horizontal lines appear. This is much more noticeable in a white screen and/or brighter areas and it also seems more pronounced on the left side of the panel as viewed from the front.

The symptom occurs with the picture reset, all picture enhancing functions turned off and the backlight turned all the way down. Also it occurs with all HDMI ports and different HDMI cables and devices. I can not test the component video connection.

I also tried this but knew it could be too good to be true: https://www.youtube.com/watch?v=8P6yHTxSDKI

Here's my video of this TV and it's symptoms: https://www.youtube.com/watch?v=Gdcxpnx1Im0

I'm ready to troubleshoot

Thank you,
Roy

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

Guys any good starting point here?

Thanks,
Roy

[budm]

Re: LG 55LN5700-UH dimming every 2 seconds...

How about good clear pictures of the whole back side of the TV so we can see how all the boards are connected together, then good clear pictures of each boards.

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

Thanks budm,

Been busy with real work. Here's a start...a lot different than a Panasonic Plasma I tried fixing. I'm assuming there are more boards under the large metal panel?

Attached are the images.

Thanks,
Roy

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

Looks like posts have edit expiry...too bad it's not more like 12 hours. Anyway, here's some more info. Attached is a close up of the p/s model printed on the board.

Notice the top line says REV 2.0 but the the second line says PCB: EAX64905501 (2.2). Just checking over on shopjimmy.com and all I found is one that says (2.0) http://www.shopjimmy.com/catalogsear...05501%282.0%29

So I'm itching to test and rule out the p/s first if that's possible. Hopefully that's where the issue is at. Recall the symptom in my OP...the panel looks 100% GOOD for 2 seconds, then for 2 seconds the panel dims, exhibiting some horizontal faint lines where mainly the left side of the screen seems to dim more than the right. So I'm thinking the statement from shopjimmy.com "PLEASE NOTE: Models that use this power supply are also HIGHLY susceptible to LED failure. What may seem like a power supply failure may be caused by the failure of one or more LEDs inside the panel (screen)." doesn't fit my panel since the display does work seemingly perfect for 2 seconds at a time.

Thanks,
Roy

[budm]

Re: LG 55LN5700-UH dimming every 2 seconds...

OK, I found good clear pictures of the power supply-LED driver board LG EAY62810801.
There are two string of LEDs, it looks to be one string for the LEFT edge of the panel and one sting for the right edge of the panel. The two strings are individually controlled as you can see there are two PWM driven POWER MOSFETs (Q803, Q805) for each string.
So based on what I see on your youtube, it looks like the left side is not stable.
There are 4 wires on the LED connector P801., 1 pair per each LED string, you can try swapping the pair of wire to see if the problem will follow to the right side of the screen or not.
What do you see if you feed the TV with signal that only has white for the whole screen?
Pictures provided by shopjimmy.
BTW, bad LEDs can flicker bright and dim.

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

budm,

Thank you. I only took a quick look and I couldn't find Q803/805 on the main p/s board. I understand you thinking and found P801 connector. There are 4 pins +/- LED so I guess the best way to swap is to unplug and then use a needle to prob and somehow swap them? Thinking here is that if the behavior follows to the right side then it could be an issue on the main ps board? If the issue remains then it could be the LED controller and/or LEDs?

Also, that board "LG EAY62810801" is not exactly my board, mine says 2.0 on the first line and (2.2) at the end of the second line.

I'll have to wait until tomorrow night or even this weekend to tinker with the led driver connector swapping.

Thanks,
Roy

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

Some quick thoughts here before I head off to the office.

First, I found Q803 and Q805 on the second picture attached in your last post. Is that one board for driving both the right and left sides? Is this board the power supply-LED driver board?

I'm going to have to solder jumpers off the back of the P801 connector and back-probe the jumpers to swap the connections to test this. Too bad the connector configuration wasn't + - - + instead of + - + - Any suggestion on a good all white image? Right now I can see it even with the LG test image which is a football team.

Aside from doing this test I have some further questions:

1. Is it possible to see if the power supply board is not maintaining proper steady DC voltage across each set of LED +/- on P801 to isolate the ps board as a potential culprit? I ask because of the work involved to swap/flip the connector. Basically any other test we can do at the ps board before moving downstream?

2. I would like to get an idea were we'll stop troublshooting because of cost effectiveness. For example, what if we determine if it's LEDs that are bad?

3. Now to get to the board (whatever it's called) that the cable from P801 goes to I have to remove they entire metal cowling. Any tips for removing it? I see it's held on with very small screws and of course I will have to disconnect two ribbon cables at the bottom and a few more around the panel.

Thanks,
Roy

[budm]

Re: LG 55LN5700-UH dimming every 2 seconds...

'Thinking here is that if the behavior follows to the right side then it could be an issue on the main ps board? If the issue remains then it could be the LED controller and/or LEDs?' That is correct. Make sure to label wire before swapping.
' I couldn't find Q803/805 on the main p/s board' They are SMD and located on the bottom side of the board close to the LED connector per shopjimmy pictures.

'Also, that board "LG EAY62810801" is not exactly my board, mine says 2.0 on the first line and (2.2) at the end of the second line.' Then you need to provide good clear pictures like shopjimmy.

'Is this board the power supply-LED driver board?' Yes, it is power supply/LED driver board combo.

'Is it possible to see if the power supply board is not maintaining proper steady DC voltage across each set of LED +/- on P801 to isolate the ps board as a potential culprit?.' There is ONE LED controller/ Voltage Boost converter SMD IC and POWER MOSFET that produce the boosted Voltage that feeds both LED+, then the two power MOSFET for the LED- return for each string, these two MOSFETs are driven by the IC so if one of the MOSFET or the drive signal is bad then it affect only one LED string. So what you need to find out in details is to get the P/N of the IC.

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

'Also, that board "LG EAY62810801" is not exactly my board, mine says 2.0 on the first line and (2.2) at the end of the second line.' Then you need to provide good clear pictures like shopjimmy.

Check post #5, I have a macro shot of the board model. So that yellow/orange'ish board smack dab in the middle is the power supply-LED driver board correct?

I did find those MOSFETs you mentioned in the second image you posted in your second to last post. I don't believe I have revealed that board yet since there's a metal cowling covering it.

So no way to prob or test voltage at P801 for any fluctuations etc? I should just go ahead and make the jumpers ("and label them") and test that way.

Thank you,
Roy

[budm]

Re: LG 55LN5700-UH dimming every 2 seconds...

You can probe the DCV on the LED - pin of each string.
The LED+ are tied together and connect the Output of the Boost converter.

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

On P801 I see LED+ LED- LED+ LED- Set my multimeter to DC volts on range of .1 resolution. Then probe + lead on LED+ and - lead on LED - (two left ones) and then do the say on the LED+ LED- on the two right ones. Just take note of voltage over 5 seconds for each? What voltage range and fluctuation would I expect to see?

Thanks,
Roy

[budm]

Re: LG 55LN5700-UH dimming every 2 seconds...

Black probe on the chassis, red probe on the connector pin, you will be looking for fluctuation in sync with what you see on the screen. The LED + should be very steady.

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

Black prob on chassis GND and red probe on LED+, measure for 10 seconds? Record and stop? Or do I do the same for the other (second) LED+ (thinking this doesn't matter from your statement about LED+ tied together).

I know I'm being picky...but I'm just wanting to be 100% sure I know exactly what you are asking to be done. I'm in no hurry

Thanks,
Roy

[budm]

Re: LG 55LN5700-UH dimming every 2 seconds...

The LED+ of both pins are electrically tied together. I would look at the LED - pins one at at time and see which one is flickering in sync with the flickering of the screen what you are seeing on the screen.

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

Measured P801 both LED - (negative) leads to Chassis GND for 10 seconds each and read constant 35VDC...once it went to 34.9VDC.

[budm]

Re: LG 55LN5700-UH dimming every 2 seconds...

Measured P801 both LED - (negative) leads to Chassis GND for 10 seconds each and read constant 35VDC...once it went to 34.9VDC.

So during the 10 seconds period you do see flickers on the screen but the Voltage did not change much?

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

Screen behavior is always the same as I describe and posted video in my OP. Remember the screen dims slightly (mostly on the left side of the screen) about every 2 seconds for a period of about 1 sec. So during any voltage test the condition exists. I figured 10 seconds was enough...probably went through at least 4 dimming cycles.

Thanks,
Roy

[budm]

Re: LG 55LN5700-UH dimming every 2 seconds...

I still wonder if it s the Video flicker not the backlights flicker since you also indicated some horizontal lines on the screen. But one way to find if it is the LED or not is to swap the LED wires to eliminate the LEDs problem, otherwise it may be screen problem since T-CON will not cause horizontal lines.

[niswanger]

Re: LG 55LN5700-UH dimming every 2 seconds...

Okay I observed really close again...largely the left side dims and when that occurs there are horizontal lines about 1/2" apart or less and they too are more pronounced starting at the far left edge and gradually disappear around 2/3rds of the way to the right.

I'm not creating pig-tail jumpers to swap the LED +/- on P801...I may not have time to complete this test tonight.

Thanks,
Roy