LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

[handsworth]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

In order to test the leds in this tv with the tester, Remove all the connecting wires from the strips. There is a small U shaped clip on the ends of two of them. Put one of them on the end of one complete strip. Not sure if its the A or B strip since I cant remember now. Test that strip now. It will light up the entire strip. Repeat process on other 4.

[neilc6]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

LF and LB series have high rates of failure in the LED strips. The heavier lenses also get loose over time and can fall off leaving bright spots. In this model, the backlights will flash when an error is detected but if a LED fails open circuit, they will not flash. Proper repair is to replace them with new strips. Lots of sellers on ebay and Aliexpress. https://techhometravel.com/lg-ln-ser...ed-backlights/

[Rso30]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Budm,
I am having the same issue, the only difference is that my voltage does not go down to 33v. It does for a literal moment, then it stays at 52v. I get around5.8v on the vled returns. The other difference is on c8017 and the one next to it only read 52 on the + and nothing on the -. Finally, I get nothing on c9815 + or - and the same for the one next to it.

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Okay, I tested and I see lights with one test on the +LED1 and one of the -LED1, nothing on the rest of the test. I check the +LED1 a third time and now the lights don't come on at all. So I am assuming this means that I need new LED's?

Thanks again!

[budm]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Remove the LED connector from the power supply board then keep red probe on +VLED 1, black probe on -VLED1 one at at time.
Do the same for the +VLED2 and the -VLED2.
4 tests total.

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Budm, the LED tester came in today. What would be the best way to go about testing the LED's? Thanks!

[budm]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Here is the picture of all the boards, and The model numbers I see. The rating on the C8110 cap is 100v

Thanks!

Thanks, at this just wait for the LED tester so we can find out what is going on with the LED strips.

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Here is the picture of all the boards, and The model numbers I see. The rating on the C8110 cap is 100v

Thanks!

[budm]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

BTW, can we see the pictures of the whole back side of the the TV showing all the boards?
Did you find the LCD panel sticker model number?
It is kind of sad that these new TV's only last a few years.
BTW, what is the Voltage rating as printed on cap C8110?

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Thanks again budm, I really appreciate the help. The tv is only a few years old and I would hate to throw it out. I ordered the tester and it should be here in a couple days. I'll let you know what I find.

[budm]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

388v

OK, that is good, the PFC Voltage booster circuit is working.
I am still not sure why you are see 0V on the -VLED return of all the LED strings.
Having the LED tester in this case will help to tell us if you have open circuit in all 4 LED string or not. We do not want to open up the LCD panel unless we really have to.
Looking at the LED strips and the way the LED connector is wired up, the panel has 72 LED's and there 4 LED strings, two strings are connected parallel so total of two pairs of LED strings, that means it has 18 LED's per string, so if they are 3V LED at 200 ~300mA, then the power supply output needs to be about 18 LED's x 3 = 54V + about 20V vdrops for the constant current control element, so output should be about 75V.

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

what voltage do you have between the two legs of cap c9815 (68uf 450vdc) when tv is turned on?

388v

[budm]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

What Voltage do you have between the two legs of cap C9815 (68uf 450VDC) when TV is turned on?

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Yes sir, I left the probe on and turned on the tv. I get no voltage at all on the
-VLED1 or -VLED2

[budm]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

budm,

I get the same reading on the +VLED1 and +VLED2 52.5 when I power it on and drops 33.3v

I get no voltage when I test -VLED1 or -VLED2

Where do I find the LCD panel model number?

I was looking at the same LED strips in your link and I am pretty sure they are the correct ones. I am also looking at a SID LED Lamp and TV Backlight Tester. I hope to start working on more tv's instead of everyone in the family just going out buying new ones all the time.

Thanks!

You did leave the red probe on the -VLED1, or -VLED2 before turning on the TV, correct? It is kind of unusual to have all 4 LED strings with open circuit on all four.

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

budm,

I get the same reading on the +VLED1 and +VLED2 52.5 when I power it on and drops 33.3v

I get no voltage when I test -VLED1 or -VLED2

Where do I find the LCD panel model number?

I was looking at the same LED strips in your link and I am pretty sure they are the correct ones. I am also looking at a SID LED Lamp and TV Backlight Tester. I hope to start working on more tv's instead of everyone in the family just going out buying new ones all the time.

Thanks!

[budm]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

OK so the Voltage did get boosted up to 52V (seems kind of low) but then it goes into protection mode and drops down to 33V.
You have 4 LED strings so we need to fine out what Voltage you have on the Cathode return pin of each LED string when you first turn on the TV.
1) Leave the TV off, meter set to DCV mode, keep black probe on the chassis.
2) Put red probe on any one of the -VLED1 pin, then turn on the TV while watching the meter to see how high the Voltage goes up to then wait 10 seconds to see how low it goes down to.
3) Repeat step 2 but with red probe on -VLED2.

BTW, what is he LCD panel model number? The LCD panel spec may provide some info about the LED's.

I am not sure if your LCD uses these LED strips or not: https://www.shopjimmy.com/lg-insigni...5dr420na16.htm

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

1) The power supply PCBA for your TV does not have 5V standby section stuffed with parts that is why there is no 5V (as you can see from the board layout https://www.shopjimmy.com/lg-pltvew4...ply-board.htm#), it use 12V as standby.
2) All the control signals are normal.
3) Put the Volt meter (set to 200vdc scale) probes on the two legs of capacitor C8107 then turn the TV on while watching the meter to see how high the Voltage jumps up to and then wait 10 seconds to see how low it goes down too.
4) To probe the Even number pins, you can use skinny probe to probe from the wire entrance side of the connector.
5) Taking the panel apart is the last thing you want to do after all the testings are done.
Pictures provided by shopjimmy: LG PLTVEW401XAT9 Power Supply Board

budm,

When I test C8107, power on Voltage is 52.5, then it drops to 33.3

Thanks for your help!

[Jermo133]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Thanks for all of your replies, I will work on it tonight when I get home and report back.

[Le Fils du Boulanger]

Re: LG 55LF5700-UA.CUSDLH - Black Screen, can see image with flashlight test

Merci pour ces infos Unspun01. Nullement offensé.