LG 70LB7100 Led Strip Replacement Question

[gavric]

Hello, I am working on 70LB7100 with led strip issue where led lenses were falling off. I opened screen and glued them back and successfully re-assemble screen back together without breaking it. When I finally assembled tv without breaking glass tv decided to start shutting down random led strings on right side of screen only, sometimes after 15 minutes, sometimes 5 minutes, sometimes 2 minutes. Sometimes whole screen shuts down but sound and faint image is present. Power cycling instantly turns screen on nice and bright and all leds work until it acts up again. Each row of leds is separately powered by its own power input with three wires + - -, 52 VDC, 8 rows of leds, 16 leds in a row, 3 strips in a row. . Increasing brightness to 100% makes screen stay on always. Power supply looks burned up some around outputs to leds as seen in photo from another poster here.

I want to replace these led strips with another simpler brand new set of leds from new vizio 70 inch but one issue is that these lg strips have 3 wires marked + - - and vizio only has 2 wires.

My question is- what does second - wire do and can i do with out it . Can i connect only + and - for leds from vizio that operate on same 52V DC. ?

[budm]

Re: LG 70LB7100 Led Strip Replacement Question

You need to do more research about the two LCD panels to find out what the Voltage and current requirement are to match them properly with the LED driver circuits.
We need to see more pictures of the boards and the LED strips to be able to do more research about them first.
The picture of your board shows that it has 16 LED strings driven by 16 LED driver circuits.
Knowing LG, the LEDs are probably driven to the max so the life span of the LEDs are shortened, the backlights level should be turned down as low as possible.

[gavric]

Re: LG 70LB7100 Led Strip Replacement Question

Thanks for reply,

I took screen apart again today and all leds seem to be working perfectly fine, when they are on. They all seem to be flickering, all at same time. Power does go up/down from 52V to 50V but once screen difusers are in place this can not be seen.

Same issues --where only 8 leds (half string) somewhere on right side of screen only die after several minutes and then random 8 more, then whole screen dies. Left side of screen work nice and bright while right side of leds is shutting down. They are connected from right to left and power goes from right to left.

There is 8 rows of leds with 16 leds in each row (3 sections make up one row, A B C strip) in this tv with 52VDC going into each row. All led bulbs are working nice and bright (with flickering) and i dont know that led strip replacement will fix this, as they are lighting up nice.

I will try to stick a fan behind power supply to see if this helps and prolongs backlight any because as i said only issue that I see is that backlights are flickering little bit because voltage varies from 52-50VDC.

Can someone please explain so i know now and in future why this LG led strip has 3 wires on it where all others i seen have 2. i am looking at 47inch, 50inch, 55inch led strips, all two wire connectors. At this 3 wire plug when powered from power supply I am measuring 54V at + mark, 24V at middle - and ground at outside - mark. Meter ground is on tv's chassis. What does 24V do? See pictures of plug connector.

Thank You

[budm]

Re: LG 70LB7100 Led Strip Replacement Question

They should not flickering at all.
How are the strips are connected to each other? It may be that when the two strips are connected together via the 3-pin connector it will form 1 long LED string and put all the LEDs in series.

[gavric]

Re: LG 70LB7100 Led Strip Replacement Question

They should not flickering at all.
How are the strips are connected to each other? It may be that when the two strips are connected together via the 3-pin connector it will form 1 long LED string and put all the LEDs in series.

They plug directly into each other, there are A B C strips. 8 power cables into 8 rows. power goes into A, then A continues into B, B into C. But when right 8 leds die it shuts off right in middle of B strip and left 8 stays on. They all flicker and I suspect that this might be issue with power supply. I will try to cool it off with a fan and see if it stays on longer.

Any tips on what to check on this power supply? And please any explanation on 3 wire plug, Thank You in advance

[dskall]

Re: LG 70LB7100 Led Strip Replacement Question

I would say bad connections between strips

[budm]

Re: LG 70LB7100 Led Strip Replacement Question

What is the LCD PANEL model number?
The 50~52VDC feeding the Anode of 16 sets of LEDs is OK.
"Same issues --where only 8 leds (half string) somewhere on right side of screen only die after several minutes and then random 8 more" The key point is that you do have Voltage to run all the LEDs but some of them go out after several minutes so it is telling me that the problem can be faulty LED that goes open circuit, the connection goes open circuit, the MOSFET that is connected to the Cathode return of the LED set may stop conducting. As long as you have the 52V feeding all the LED set, none of them should stop from coming on, it is the individual set of LED or the connection, or the MOSFET that can cause the set of LED to stop working. All the L+ are electrically connected to each other and to the 52V power supply.
You can look at it this way: you have 16 sets of Christmas lights that are fed by the same 120V and then you have 16 switches (In this TV you can see 16 SMD Switched MOSFETs on the bottom side of the board, they are PWM controlled to maintain regulated constant current flowing through the LEDs) for each set of the Christmas lights.

[budm]

Re: LG 70LB7100 Led Strip Replacement Question

3-PIN LED explanation:
"There are 8 rows of LED's with 16 LED's in each row (3 sections make up one row, A B C strip)"
You have 16 LED's per one row of LED. So you have two sets of LED's, each set has 8 LED's.
Then on the connector you have 1 pin for the (+) which is the L+ pin that is connected to the 52V power supply, that (+) pin is the Common ANODE two sets of LED. The two (-) pins are the Cathode return lines, one per set of LED, the (-) is then connected to the VC_x.
So if you count the L+ wires you will see that you will have 8 of them, the each row of the LED will have two VC_x wires, so the total of VC_x wires are 16 wires and the total wires to the panel are 24 wires. BTW, the Vf of the LED in this case is 6V instead of 3V because it has two LED's in one body, so 6V x 8 LED's = 48V so + 4V Vdrops on the MOSFET will need about 52V power supply.
Bottom line, each row of LED has two control circuits for two set of LED's in that row.
That is why you have 3 pin on the LED strip instead of just two. Your TV can do what they call local dimming zone control of the LEDs for the 3-D and dynamic contrast.
https://reviews.lcdtvbuyingguide.com...-70lb7100.html
That is why you have 16 sets of LEDs.

[gavric]

Re: LG 70LB7100 Led Strip Replacement Question

What is the LCD PANEL model number?
The 50~52VDC feeding the Anode of 16 sets of LEDs is OK.
"Same issues --where only 8 leds (half string) somewhere on right side of screen only die after several minutes and then random 8 more" The key point is that you do have Voltage to run all the LEDs but some of them go out after several minutes so it is telling me that the problem can be faulty LED that goes open circuit, the connection goes open circuit, the MOSFET that is connected to the Cathode return of the LED set may stop conducting. As long as you have the 52V feeding all the LED set, none of them should stop from coming on, it is the individual set of LED or the connection, or the MOSFET that can cause the set of LED to stop working. All the L+ are electrically connected to each other and to the 52V power supply.
You can look at it this way: you have 16 sets of Christmas lights that are fed by the same 120V and then you have 16 switches (In this TV you can see 16 SMD Switched MOSFETs on the bottom side of the board, they are PWM controlled to maintain regulated constant current flowing through the LEDs) for each set of the Christmas lights.

Understood about 3 wires plug,global dimming and all, thanks.

This tv uses Sharp screen HC700CUF-VHHD1-11XX . I really do not care for local dimming feature working or not. I would like to know if led strip is at fault or power supply i causing blackout after a while but i can not figure it out because random strip goes out at random times on right half of screen and then screen blacks out and no voltages are present to leds. Not sure what caused what to shut down, if led tripped power supply or other way around??

I would think that voltage to led strip would all be separate (seeing all those MOSFET's) but then i realized from backside picture of PS that all positive leads to led strips +LED are connected together on PS and its actually one 52VDC. why they complicate it with 16 different VC lines, anyway way to make it one line also? what would happen if i connected together VC1+VC2+VC3+...+VC16 and basically made it one big negative. and make it like on other led tvs where there is only two wires going into screen.

How would i check if all those MOSFETs are good, compare them to each other? while working and when off? would like to check and see if i can find what causes backlight flickering.
Anyway to measure resistance of whole row of strips when working and when off and compare it another row?

[budm]

Re: LG 70LB7100 Led Strip Replacement Question

The fault did not trip the power supply otherwise you will not have the 52V for the other LED set to be on, it will have over current protection that will shutdown thw whole power supply. I would swap the strip that quit with another position to see if the problem will follow..
"I would think that voltage to led strip would all be separate (seeing all those MOSFET's) but then i realized from backside picture of PS that all positive leads to led strips +LED are connected together on PS and its actually one 52VDC." I already explain how it is configured, the MOSFET is on he return side of the LEDs. You cannot directly connect all the return together and expect it to work, each string has its own MOSFET to regulated the current and since not all LED's are equal so some of them may not light up at all due to the Vf.
Swapping the LED strip will be the easiest since there is no soldering involve.
They do 16 string for zone dimming, you buy cheaper TV will use one big string connected in series like Christmas lights one go out the string will go out.

"if i connected together VC1+VC2+VC3+...+VC16 and basically made it one big negative. and make it like on other led tvs where there is only two wires going into screen" That will be parallel connection, not series connection, and you will also need much higher Voltage to drive series connection since the Vf of each LED will be added together. 128 LEDs x 6V = 768V + Vdrops for the MOSFETs.

"Anyway to measure resistance of whole row of strips when working and when off and compare it another row?" You can measure the Voltage on VC_1~16 to see what you get, but you can already see which strip of the LED stop working so why not just swap the strips?

[budm]

Re: LG 70LB7100 Led Strip Replacement Question

BTW:
"then screen blacks out and no voltages are present to leds." Are you saying that the 52V goes down to 0V? I thought only certain strip stops lighting up.

[gavric]

Re: LG 70LB7100 Led Strip Replacement Question

BTW:
"then screen blacks out and no voltages are present to leds." Are you saying that the 52V goes down to 0V? I thought only certain strip stops lighting up.

Yes, only half of led strip dies dies only 8 leds out of 16 in whole row.but my issue with swapping places of strips is that its random 8 leds, never same strip.

Let me explain.
Now its very top one, then middle one, then whole screen is black and 0 volts anywhere but sound works.

After instant power cycling screen in nice and bright again and then again maybe very bottom strip might die , then middle one, then whole screen is black and 0 volts. random strip dies at random times-all on right side causing screen to blackout and voltage to go to 0VDC.


With connecting all return lines together (like cheap set)-as you said BUT i was thinking to connect everything to be same 54VDC power input into every strip row but this way + to +to +..... - to - to -to-.... not + - + - + -

If I would make all strips connected in parallel(all +with+ as it is currently on PS and all - with -(not currently) ) I am not sure how you say i would need 700+ VDC because they are not in series. Each row would run on 54VDC, this way even when one row goes out others continue to work. Same way as now just bypass that 16 mosfet circuit complicated circuit and make it one big return circuit.

Why dont they make led strips in parallel
problem with current led strip design where they are in series is one dies they all die. If they were in all in parallel and running on 3.3V they would work all independently of each other, one dies,others continue to work. only thing needed would be little beefier power supply and it would solve world's greatest problem.
Not sure how would dimming and control of backlight work but i assume voltage control from 3.3V to lower voltage would dim backlights ?

[dskall]

Re: LG 70LB7100 Led Strip Replacement Question

If it were me I would concentrate on fixing it not trying to redesign it.

[budm]

Re: LG 70LB7100 Led Strip Replacement Question

Yes, only half of led strip dies dies only 8 leds out of 16 in whole row.but my issue with swapping places of strips is that its random 8 leds, never same strip.

Let me explain.
Now its very top one, then middle one, then whole screen is black and 0 volts anywhere but sound works.

After instant power cycling screen in nice and bright again and then again maybe very bottom strip might die , then middle one, then whole screen is black and 0 volts. random strip dies at random times-all on right side causing screen to blackout and voltage to go to 0VDC.


With connecting all return lines together (like cheap set)-as you said BUT i was thinking to connect everything to be same 54VDC power input into every strip row but this way + to +to +..... - to - to -to-.... not + - + - + -

If I would make all strips connected in parallel(all +with+ as it is currently on PS and all - with -(not currently) ) I am not sure how you say i would need 700+ VDC because they are not in series. Each row would run on 54VDC, this way even when one row goes out others continue to work. Same way as now just bypass that 16 mosfet circuit complicated circuit and make it one big return circuit.

Why dont they make led strips in parallel
problem with current led strip design where they are in series is one dies they all die. If they were in all in parallel and running on 3.3V they would work all independently of each other, one dies,others continue to work. only thing needed would be little beefier power supply and it would solve world's greatest problem.
Not sure how would dimming and control of backlight work but i assume voltage control from 3.3V to lower voltage would dim backlights ?

I am talking about series connection, when you parallel LED since LED forward VOLTAGE will never be the same so the one with lower Vf will light up and draw more current than the one with higher Vf, you do not directly connect LEDs in parallel amd expect them to conduct evenly.
I still do not understand about no Voltage to the LED, all the LED strings are fed by the same power supply source, 52v, that is why we need to know if you are losing the 52V at the power source or you are losing the 52V at the LED strip that is not lighting up, that is a big difference. as I indicate, all the LED+ are ALL tied to the same 52V power supply, if you lost the 52 Volt at one of the strip only then it cannot be the power supply fault but the connection fault.
LED is current device and the brightness is controlled by the current, the power supply is set up as constant current so the LED get fed by constant current and it is controlled by the MOSFET. So if you think about it how would you insure even current draw for each string that is connected in parallel. Right now you have 16 string and if each string is drawing about 300mA, so the total current draw from 52V power supply is 0.30A x 16 = 4.8A that is why the power supply for the LEDs is so beefy in your board. If connected in series you will still dissipate the same amount of power. Right now the LEDs are kind of connected in parallel but each string has its own controller (MOSFETs), normally you will have current limiter resistor per each string and then you parallel them up.

"Why dont they make led strips in parallel
problem with current led strip design where they are in series is one dies they all die. If they were in all in parallel and running on 3.3V they would work all independently of each other, one dies,others continue to work. only thing needed would be little beefier power supply and it would solve world's greatest problem." It is not that simple, and the problem we are having the LED failure is due to the LEDs are being driven at the max current specification and with poor heat management; the circuit is also need to maintain constant current to to temperate change, etc. That is also why I always tell them to turn down the backlights to as low as possible. I have my LG running at only 25% setting and it is still pretty bright.
I think you have bad solder joints on those SMD connectors.

"Same way as now just bypass that 16 mosfet circuit complicated circuit and make it one big return circuit. " And again, it is not that simple. How would you regulate the current? if it is that simple the TV makers would have done that and save lots of money. You have to understand what the requirement are for the circuits and the TV to function, if you do not need 3D, zone control, smart TV, etc. then buy the lower end model that does not provide those features you do not need. When was the last time you use 3-D?

[budm]

Re: LG 70LB7100 Led Strip Replacement Question

http://www.edn.com/design/led/442453...el-LED-strings
http://www.ledsmagazine.com/articles...-magazine.html

[gavric]

Re: LG 70LB7100 Led Strip Replacement Question

I still do not understand about no Voltage to the LED, all the LED strings are fed by the same power supply source, 52v, that is why we need to know if you are losing the 52V at the power source or you are losing the 52V at the LED strip that is not lighting up, that is a big difference. as I indicate, all the LED+ are ALL tied to the same 52V power supply, if you lost the 52 Volt at one of the strip only then it cannot be the power supply fault but the connection fault.

Tanks for quick reply budm,

I was not able to catch half of led row die and measure voltage on it when screen was taken apart , only seen it where tv was all together and where glass was on.

I got this tv for $50 with issue of several bright white spots on screen and it seemed to be really simple fix where led lenses fell off and had to be glued back in place. I got it from school janitor and i took tv apart, glued lenses back in place, put tv together and after 10 minutes led string started shutting down.I had tv on while it was taken apart on table and it never shut down quick. after i would say 15 minutes it would shut down completely-click off-no signal shut down. i did notice some flicker to leds .

its is strange how "basically" one big led string row with 16 leds on it- exactly one half of it dies after few minutes- side that gets power to it first??? same as if it is getting only half of power to it and only able to power on 8 leds. so power goes thru 8 non working leds and goes into working 8 leds- this rules out bad connection ??? then after first second random one on same side of screen dies in same manner-other far half continues to works, then whole screen dies, same as if power supply is weakening and losing half of power to some outputs to led rows and finally it shuts down.

Power supply looks browned and my next move would be to take hair drier to power supply to see if it speeds up shut down and to look for cracked solder and to retouch solder on power supply. Not sure if heating up board will do anything since screen comes on nice and bright right after instant power cycle.

How do i test rule out weak MOSFET while they are on ps ? Thank You

[vinceroger69]

Re: LG 70LB7100 Led Strip Replacement Question

i wonder why the led lens fell off in the first place has someone replaced any leds? i wonder if there at fault

[dskall]

Re: LG 70LB7100 Led Strip Replacement Question

Saw a video of lg repair with white spots all over screen. When they took it apart almost all of the lens had fallen off.
https://www.youtube.com/watch?v=Nvwk7mpyx9Q&t=598s

[diif]

Re: LG 70LB7100 Led Strip Replacement Question

i wonder why the led lens fell off in the first place has someone replaced any leds? i wonder if there at fault

Heat causing the glue to fail ?

[vinceroger69]

Re: LG 70LB7100 Led Strip Replacement Question

Heat causing the glue to fail ?

Yes that makes sence