Lg55uj620v no backlight

[Edgas10]

Hi there ,first post be gentle on me😁
Have a tv to try to fix , its an lg55uj620v
Has no backlight,image is there and sound.
So bought a led tester and both lines measure the same
57.4v
There are 44 leds.
I have all the correct voltages on the connector .
But im only getting around 18v on the socket where the leds go on both sides.
Measure resistance on the connector i get 32 kohms.
I went to the icdriver i have 13 volts on vcc
I get the 3. Something whem power on but i have no voltage geting out.(ic is a BD94121F).
Hopping for some help thanks.
Im not an expert ,and forgive if i have some mistakes ,im portuguese.😁

[handsworth]

Re: Lg55uj620v no backlight

1 or more bad leds. Replace all

[Edgas10]

Re: Lg55uj620v no backlight

1 or more bad leds. Replace all

Thanks for the aswer ,but they all lit with backligt tester
Measure then individualy and they all light the same brightness .
Am i missing something here?🤔

[Edgas10]

Re: Lg55uj620v no backlight

Heres some pics

[Diah]

Re: Lg55uj620v no backlight

@handsworth give you the correct answer..
LG use LED units built in in reserve way protection zener diode. tester cant figure out if they are open or short. you need with yours DMM diode mode test each in reserve way to lit on dim and other way to read diode round 0.650

[handsworth]

Re: Lg55uj620v no backlight

@handsworth give you the correct answer..
LG use LED units built in in reserve way protection zener diode. tester cant figure out if they are open or short. you need with yours DMM diode mode test each in reserve way to lit on dim and other way to read diode round 0.650

Had the same experience with 50UH5530. All leds light with tester but tv does not come on when powered up. Replaced all leds and tv works fine.

[Storyteller12]

Re: Lg55uj620v no backlight

Plug tv in with the screen and diffuser out and power it up. you will see them light up except the one that’s bad.

Replace all of them though as mentioned by everyone else

[Edgas10]

Re: Lg55uj620v no backlight

Plug tv in with the screen and diffuser out and power it up. you will see them light up except the one that's bad.

Replace all of them though as mentioned by everyone else

(A side note the tv did a half second backlight flash when turned on )

Actually i did that test ,the first rime and open the tv and i plug the tv with the diffusrr and screen not connected and they lit up.
But im no expert and i tought they were good so closed the tv reconnected everything and tested but no backlight again.
So i tested the strips again with the tester but i connected both positives from ech string and no more light... On one of the strings
Opened again and one of the leds was bad.
Ive ordered new strips,so fingers crossed

[Edgas10]

Re: Lg55uj620v no backlight

@handsworth give you the correct answer..
LG use LED units built in in reserve way protection zener diode. tester cant figure out if they are open or short. you need with yours DMM diode mode test each in reserve way to lit on dim and other way to read diode round 0.650

What is reserve way?

[Diah]

Re: Lg55uj620v no backlight

What is reserve way?

Reverse way.. see LED with ZD diagram used by LG LED TV

[Edgas10]

Re: Lg55uj620v no backlight

Reverse way.. see LED with ZD diagram used by LG LED TV

Ah ok wlready did it i have 2.4v in foward and nothing on reverse

[Donny2derby]

Re: Lg55uj620v no backlight

Reverse way.. see LED with ZD diagram used by LG LED TV

What’s the purpose of having zener diode in parallel with LED?

Wired this way would require each diode to have its own driver no??

Ahhh. Edit! Reverse polarity protection! Of course!

What does a zener diode do extra that a normal LED can’t though seeing as they both serve to protect from reverse polarity?

[Edgas10]

Re: Lg55uj620v no backlight

Thanks to all ,problem solved .
Leds change and tv works fine.