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

[Jermo133]

Hi,
I have a LG 55LF5700_UA LED tv

The standby light is on, I can see the picture with the flashlight test, and there is no back light. I test the standby power at the power board and there is no power. The 2 Led+ test points are only showing 30.3 V.

I can not find schismatics for this board but is it possible that there is a separate ground for the standby voltage? Thanks

Power Supply Board ((X)PLTVEW401XAT9) Voltages:

1. DIM: 2.33
2. O/F: 3.34
3.PS: 3.36
ON
4.NC 0.00
5. GND
6. GND
7. GND
8. GND
9. 12v_A: 11.96
10. 12V_A: 11.96
11. 12V: 11.95
12. 12v 11.95
13. 12v 11.95
14. 12v 11.95
15. 5v 0.00
16. 5v 0.00

LED Voltage

+Led 1: 33.3V
+Led 2: 33.3V

[budm]

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

We need to see the pictures of the backside of the TV showing all the boards and good clear straight shot of the whole power supply board so we can see what we are dealing with.

[Jermo133]

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

Sorry I forgot to add these. Thanks again for the help.

[Le Fils du Boulanger]

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

Open your TV and try to access the LEDs. Attention, very deliquate operation because it will remove the screen. As soon as it is done, remove the light diffusers from the LEDs. Put your multimeter in the diode test position and check if each LED is lit. This will allow you to detect LED bares to be changed. You can bridge faulty LED terminals, the ultimate solution that will give you dimly lit areas on the screen.

[nomoresonys]

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

Maybe try unhook cable from powerboard to mainboard and see if your standby voltage comes back.

[budm]

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

Hi,
I have a LG 55LF5700_UA LED tv

The standby light is on, I can see the picture with the flashlight test, and there is no back light. I test the standby power at the power board and there is no power. The 2 Led+ test points are only showing 30.3 V.

I can not find schismatics for this board but is it possible that there is a separate ground for the standby voltage? Thanks

Power Supply Board ((X)PLTVEW401XAT9) Voltages:

1. DIM: 2.33
2. O/F: 3.34
3.PS: 3.36
ON
4.NC 0.00
5. GND
6. GND
7. GND
8. GND
9. 12v_A: 11.96
10. 12V_A: 11.96
11. 12V: 11.95
12. 12v 11.95
13. 12v 11.95
14. 12v 11.95
15. 5v 0.00
16. 5v 0.00

LED Voltage

+Led 1: 33.3V
+Led 2: 33.3V

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

[Unspun01]

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

You can bridge faulty LED terminals, the ultimate solution that will give you dimly lit areas on the screen.

Honestly, no offense but this is poor advice. Do NOT simply bridge a faulty LED.

According to ohm's law, if voltage remains constant, and the resistance of the LED string is reduced by bridging faulty open LEDs (or even by a shorted LED) then the current through the LED string increases.

Bridging a faulty LED with a wire or solder blob does not "use" any electricity, and does not create an associated voltage drop like an LED. Therefore you will essentially force more current through the other good LEDs in the same series path and result in over-driving the other LEDs until they fail as well.

This is called a "cascading" failure, where one LED fails, which places extra current on other adjacent LEDs, forcing them to fail as well.

If you absolutely have to, remove the bad LED and replace it with a suitable diode or something similar.

Truly the best solution is to replace the LED and diffuser lens cover. This is difficult for most people without proper tools since the LEDs are delicate and don't like direct contact heat from a solder iron.

Another solution I like because it can be done with basic tools is called "stacking". This is where you cut a good LED from a LED strip complete with its attached lens cover, and glue/attach it in place of the original LED that was removed. + and - wires are soldered to appropriate points on the LED strip to restore the circuit. There are no special tools required, and no excess heat which can damage the existing backlight strip or new LED. Very easy for anyone to do. The only concern is observing correct polarity of the LED.

[nomoresonys]

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

Ah ok, was wondering why everything but the 5 volt was there so 15 n 16 won't be connected.

[Le Fils du Boulanger]

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

Merci pour ces infos Unspun01. Nullement offensé.

[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.

[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!

[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

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

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

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

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

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

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

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

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?