LED LG 55LN5710 NO VIDEO


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LED LG 55LN5710 NO VIDEO

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javier678
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Oct 20, 2015, 5:59 PM

Post #1 of 37 (6857 views)
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LED LG 55LN5710 NO VIDEO	Can't Post

hi, i have a Lg 55ln5710 it has audio but no video and when i turn it on the Lg logo appears 1 sec. and disapear

what could it be?

jts1957
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Oct 20, 2015, 6:17 PM

Post #2 of 37 (6855 views)
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Re: [javier678] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Look through the back of the set at the panel (there should be extra mounting holes, vent holes, etc.) and see if backlights are lighting and either staying lit or going out.

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Location: Far, Far Away

javier678
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Oct 20, 2015, 6:21 PM

Post #3 of 37 (6852 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

They light up for a second and then go off

jts1957
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Oct 20, 2015, 6:27 PM

Post #4 of 37 (6850 views)
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Re: [javier678] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

From the front of the set, hold a small flashlight near the panel screen and see if you see any very dim video movement.

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Location: Far, Far Away

javier678
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Oct 20, 2015, 6:32 PM

Post #5 of 37 (6848 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

yes i can see the video with a flashlight

jts1957
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Oct 20, 2015, 6:43 PM

Post #6 of 37 (6845 views)
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Re: [javier678] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Bad LED(s) or bad power supply / LED driver board. SJ has a rebuild kit IF LED bulbs 'PROVE' good. http://www.shopjimmy.com/...board-repair-kit.htm

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Location: Far, Far Away

javier678
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Oct 20, 2015, 6:51 PM

Post #7 of 37 (6843 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

i already changed the power supply board but the same.. could it be the T-con ? or do you think its bad leds?

jts1957
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Oct 20, 2015, 7:03 PM

Post #8 of 37 (6841 views)
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Re: [javier678] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

You HAVE video (and I presume Menu & other OSD with a flashlight), both indicate a working TCON ... so one (or More) LEDs would be suspect. Of course if the PS you obtained was Brand New from the manufacturer - it can be ruled out ... IF the PS you obtained was 'New to You' (fleebay, amazone, even SJ, they are pulled from service 'supposedly' 100% fully tested, pulled from a working set with a broken screen, etc. etc. - until a second is tried it CAN'T be fully ruled out.

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Location: Far, Far Away

javier678
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Oct 20, 2015, 7:16 PM

Post #9 of 37 (6836 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

how can i see if its bad leds? can i open the tv to see them?

jts1957
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Oct 20, 2015, 7:46 PM

Post #10 of 37 (6835 views)
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Re: [javier678] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Some are easy, some are hard. Some use LED 'Strips,' some use an 'array.' Panel part number may help.

All are part of the LCD Panel. Some you can 'peek' into with some judicious disassembly. Careful, TFT is glass and has 'tabs,' both can be easily damaged. IF you decide to look before tossing in trash, you can VERY carefully disassemble panel, plug LEDs back in and look for one that doesn't light. Many times you can see a discoloration in the lens.

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Location: Far, Far Away

Southerner1959
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Nov 1, 2015, 7:20 PM

Post #11 of 37 (6786 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

I have a similar problem with my 47LN570V . . . this set also uses LEDs for the backlight.

Mine also flashes the LG Logo at full brilliance for about 1 second when you turn it on - surely this means the LED array behind the screen must be OK?

(also can see images if using a flashlight, and audio is working OK)

I have replaced the LED psu capacitors . . . and measure 75V across each bank (there are two feeds to the backlight)

But I see on the PCB that the feeds to the Backlight each go through a switching FET . . . is that because the LEDs are pulsed on and off?

I have downloaded a Service Manual which is supposed to be for this model . . . but it is not correct, as there is no LED part on the PSU Board, or test voltages for that section.

There are also no circuit diagrams either - they just talk about replacing the board !!! (I want to find the faulty component)

Hope somebody can advise, as although I'm an electronics engineer I have limited experience of modern TVs.

jts1957
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Nov 1, 2015, 7:44 PM

Post #12 of 37 (6784 views)
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Re: [Southerner1959] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

What happens if 'a' LED fails due to a internal thermally intermittent contact and only lights for 1 second and goes out?

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Location: Far, Far Away

Southerner1959
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Nov 1, 2015, 7:56 PM

Post #13 of 37 (6781 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

I appreciate that if they are all in series, if one fails you will get nothing (just like Xmas tree lights!)

But is this what often happens with them? That a faulty LED works momentarily every time you switch on, but it then goes open circuit?

I'm just surprised that would keep happening every time, rather than it just failing completely.

Southerner1959
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Nov 1, 2015, 8:00 PM

Post #14 of 37 (6780 views)
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Re: [Southerner1959] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

ALSO . . . I notice there are TWO feeds to the LED backlight . . .

Is that because one does half the screen, the other does the other half?

If so, surely BOTH halves wouldn't happen to have an LED fail at the same time?

jts1957
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Nov 1, 2015, 8:22 PM

Post #15 of 37 (6776 views)
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Re: [Southerner1959] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Manufacturers run LEDs at default settings of 100% duty Cycle (best brightness output=best picture ... also most current draw & heat. Current/Heat has detrimental effect on parts and therefore shortens lifespan). They are making money selling sets. The fact that you're running the engine with your foot on the firewall most of the time doesn't matter to them. As long as it lasts the warranty period.

Circuit was a simple EXAMPLE. Voltage shown a suggestion. You may add extra R2s and bulb strings to top of C2 as required. Also TVs will have feedback circuits monitoring and kill drive to bulbs as Mfg won't want customer watching a sub par lit screen.

BTW - There should be an EDIT button at the top right of each of your posts.

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(This post was edited by jts1957 on Nov 1, 2015, 8:23 PM)

Southerner1959
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Nov 1, 2015, 8:42 PM

Post #16 of 37 (6771 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Appreciate your help, by the way!

OK . . . so the FETs in series with each of the two feeds to the Backlight are just to adjust the brightness, I presume.

And having done some online searching, I now gather that there are typically just two strips of LED for backlighting, one either side of the screen.

But as I say . . . surely an LED diode wouldn't fail on BOTH of these strips simultaneously?

However . . I have just tried measuring the resistance (using an analogue meter) of each of the LED strips . . . and both measure completely open circuit - presume you wouldn't see that if they are OK? (although still can't understand how you see the LG Logo at full brilliance for a second when you switch on!

(This post was edited by Southerner1959 on Nov 1, 2015, 8:44 PM)

jts1957
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Nov 1, 2015, 8:54 PM

Post #17 of 37 (6766 views)
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Re: [Southerner1959] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Not usually.
One to three series strings wired in parallel with source, current being supplied by FETS which are driven by a driver IC that monitors bulb strings current and shuts down drive (to ALL strings) if fluctuations appear. Arrays are similar.
Wire 12 - 16 1N4004s in series and measure across whole string.

Try measuring Each LED and/or put two AA / C or D batteries in series, and observing polarity, fire Each bulb for a few seconds.

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Location: Far, Far Away

(This post was edited by jts1957 on Nov 2, 2015, 10:11 PM)

Southerner1959
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Nov 1, 2015, 9:35 PM

Post #18 of 37 (6764 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

I haven't tried taking the screen apart to get at the LED strings . . .

BUT . . . I have repeatedly been turning the set on and off . . . and every time you get a full picture - it's not for a second, more like a quarter of a second . . . but that makes me think it COULDN'T be the LEDs failing. Surely it wouldn't be so repeatable?

So . . . I hooked up wires from one of the pair of LED feeds . . . and as expected, there is no voltage there, except when you first turn on, and the meter kicks up momentarily (hard to guess what the voltage is, but I would guess it might be about 24V)

So I guess it is the control circuitry going wrong, that feeds a signal to the switching power FET? The output of each FET goes directly to the LED Backlight feeds (there are 2 completely separate PSUs for each LED string, ie rectifier diodes, smoothing caps and a Switching FET.)

What I find odd is that the DC voltage going to each FET is actually 75V . . . surprised it is that high.

I managed to see the TV's backlight settings (using a flashlight on the screen) and it was set at 100% . . . tried turning it up and down a bit, but obviously made no difference)

EDIT: Just had an idea . . . think I will try unplugging the Backlight lead from the PSU Board and feed some DC to it . . . presumably the screen will then light up? But what sort of voltage will it be looking for? Any idea?

(This post was edited by Southerner1959 on Nov 1, 2015, 9:44 PM)

Southerner1959
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Nov 1, 2015, 10:22 PM

Post #19 of 37 (6755 views)
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Re: [Southerner1959] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Well I just tried feeding 18V to one of the two LED feeds . . . but nothing.

However . . . based on what you said, I presume that inside the screen there is a constant-current driver IC feeding each diode string.

If so, that would explain why I could measure no resistance across each. And perhaps it takes the full 75V with a big voltage drop to work (if so, that would explain why I see nothing using 18V)

If all the above is true, I would imagine the Switching FET just reduces the backlight brilliance by pulsing the feed on and off (like a normal domestic light dimmer switch), rather than reducing the voltage.

If that's all true, then presumably I could just by-pass the FET and feed the full 75V straight to each LED Feed? I know I could no longer use the menu to reduce the Back-Lighting, but most sets look best at 100% anyway!

I would appreciate your (or anyone else's) comments . . . as I am just using my electronic knowledge to make assumptions here . . . I don't know what kind of circuitry is used in practice in these TVs.

jts1957
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Nov 1, 2015, 11:04 PM

Post #20 of 37 (6750 views)
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Re: [Southerner1959] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Panel number? PS/LED Driver board part number? Apparently on other side of big pond. I can't see that far.

Panel usually only has LEDs. Sometimes if two parts of a series LED string (a left and a right) are joined via a plug-in connector, the connector contacts can be suspect.

75V divided by 3V per diode equals approximately 25 diodes per series string (not allowing for any dropping/surge resistors).

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Southerner1959
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Nov 1, 2015, 11:38 PM

Post #21 of 37 (6748 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Unlike earlier LG TVs I have seen, this one only has 2 boards ! And both are remarkably small.

The PSU Board, which drives the Backlights directly is EAX64905501

And the Main Board, with everything else is EBT62345937

So is 25 LEDs each side typical? (on a 47 inch set)

jts1957
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Nov 1, 2015, 11:58 PM

Post #22 of 37 (6742 views)
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Re: [Southerner1959] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

http://www.tv-forums.com/...=EAX64905501;#160692

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Southerner1959
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Nov 2, 2015, 12:11 AM

Post #23 of 37 (6740 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Yes, I read that thread before posting . . .

But firstly, I don't think the LEDs could be the fault, as surely they would not KEEP working and failing repeatedly.

And secondly, they are not getting any supply from the board, only momentarily when you switch it on - so the fault must be in the control circuitry. (which is why i wondered if I could just by-pass it)

jts1957
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Nov 2, 2015, 12:38 AM

Post #24 of 37 (6738 views)
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Re: [Southerner1959] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

P201
3.5V standby/alltime
PWR-ON 12 & 24 - on when high
DRV-ON for LEDs - on when high

"800" area on rear of PS/LED Driver.

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Southerner1959
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Nov 2, 2015, 8:47 PM

Post #25 of 37 (6720 views)
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Re: [jts1957] LED LG 55LN5710 NO VIDEO [In reply to]	Can't Post

Not quite sure what you mean by that last post !

The connector from this PSU Board to the Main Board only has 18 Pins, not 24.

However, I have found a table for this connector in the Service Manual, listing the various voltages that should be on each pin . . . but is says Pin 1 is PWR - ON, and Pin 2 DRV - ON . . . plus Pin 4 is PDIM1 and Pin 6 PDIM2

So what should I be seeing on those pins? (and what is DRV and PDIM?)

(This post was edited by Southerner1959 on Nov 2, 2015, 9:07 PM)

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