LG 42LV5500 only backlight on

[tibimakai]

I have done, which I should have not done, purchased it from somebody, who already tried to repair it before me.
He replaced the PS and mainboard already. Only thing left, is the T-Con board, but he gave up. At least, that is what he said.
Basically, the TV turns on with backlight, which turns on and them dims.
I have checked the fuse at the T-Con and it's good.
Do you think the I could check something on it, or could I have other issues at well, other then the T-on board?
I have checked on shopjimmy, to be sure that is the right mainboad, and it is.
This is my board(picture is from shopjimmy for reference):
http://www.shopjimmy.com/lg-6871l-2554a-t-con-board.htm
Any help would be appreciated.

[tibimakai]

Re: LG 42LV5500 only backlight on

So, no ideas?

[tom66]

Re: LG 42LV5500 only backlight on

12V both ends of T-con fuse?

[tibimakai]

Re: LG 42LV5500 only backlight on

Yes, that was the first thing, that I have suspected and I was almost 100% sure that is the problem with this TV.
He said, that they had a power surge, when the TV went out.

[tom66]

Re: LG 42LV5500 only backlight on

So 12V is good?
Next see if you can locate test points AVDD, VGH, VGL and gamma on T-con. Check these...

AVDD 14 to 18V
VGH 20 to 30V
VGL -5V to -10V
gamma as with AS15 chips

[tibimakai]

Re: LG 42LV5500 only backlight on

There are multiple VGHs voltages, but I'm measuring 29.95V, -5.03V, -4.99V, 9.33V, 7.14V on those test points at CN4.
The Gamma pin out it's weird too, I can't understand which way have to be measured.
I can't find the AVDD test point and all other test points are weirdly placed, I'm not sure which one is which.

[tibimakai]

Re: LG 42LV5500 only backlight on

I will take a picture, because the shopjimmy picture is bad.

[tibimakai]

Re: LG 42LV5500 only backlight on

OK, here are some pictures, though it's still hard to see the markings. Even when I look at at them up close, it's hard to see them.
If needed, I will take some microscope pictures too, just let me know.
Thank you for trying to help.

[tom66]

Re: LG 42LV5500 only backlight on

AVDD check voltage across C237 top right corner of this photo https://www.badcaps.net/forum/attach...8&d=1398637689

about 14~18V

VGH: R271 near bottom left
VGL: C252 middle left

gamma voltages GMA1 thru GMA18 far right

Vcore: C219 (probably 1.2V, 1.5V, 1.8V)
VCC: C212 (probably 3.3V)

do all with and without panel hooked up

[tibimakai]

Re: LG 42LV5500 only backlight on

AVDD= 16.71V
VGH= 29.83V
VGL= 5.029V
Gamma voltages:
I'm still not sure about there order, but I will start from lower right test point, the going up, then down and so on in zig zag pattern.
1= 0.001V
2= 14.37V
3= 13.62V
4= 12.55V
5= 10.58V
6= 0.001V(?!)
7= 0.001V(?!)
8= 6.226V
9= 4.226V
10= 3.176V
11= 2.454V
12= 0.001V
13= 0.001V
14= 1.656V
All the measurements, are the same with panel connected or disconnected.

Vcore= 1.006V
Vcc= 3.312V

12V at the fuse is 11.92V
Gamma voltages don't look good.
Gamma IC could cause this issue?
Which one it would be the gamma IC?
Thank you Tom for the great help.
I would also need help with a Sony KDL55EX720, it seems that I'm not getting much help with that TV. It's a 3D 55" and I would like to see it on. You know how it is.

[tom66]

Re: LG 42LV5500 only backlight on

Gamma IC is probably U3. to confirm try googling part #s. looks like it might be a Maxim chip and they make a lot of gamma references.
Gamma voltages might cause your issue, but it is strange, I would only expect a distorted picture.
I don't know if you can replace U3 easily - it's a QFN (leads are not exposed) - I struggle with those at the best of times...

[tibimakai]

Re: LG 42LV5500 only backlight on

I have never tried one of these before.
I have to check the IC numbers and see if I can actually find it somewhere.
If I will, then I will give it a try.

[tibimakai]

Re: LG 42LV5500 only backlight on

This is U3:
http://www.ti.com/product/tps65194

[tibimakai]

Re: LG 42LV5500 only backlight on

I guess, this one is the gamma IC (U6 the one on the far right):
https://cdn.badcaps-static.com/pdfs/...746ad8e194.pdf

[tibimakai]

Re: LG 42LV5500 only backlight on

OK this is weird.
I have replaced the IC U6 and now I'm getting in every point previously measured where they were voltages, now I'm getting in every point 8.31V - 8.34V.
Where they were missing, it's still missing. The last point 1.65V, it's the same.
I have ordered another board, but I'm curious what do you guys have to say about this development?
Still no change on the screen.

[tibimakai]

Re: LG 42LV5500 only backlight on

The replacement T-Con board came in, replaced it and no change. I have purchased it from retechtv.com.
Gamma voltages seem the same, I haven't compared yet with the old measurements yet.
Any more ideas, what else could it be?
I have hooked up a media player to it and the sound works, though the volume it was at maximum and I couldn't lower it from the buttons. I don't have a remote.

[audiofreak19]

Re: LG 42LV5500 only backlight on

If the gamma voltages are the same and not in that descending or acending order then that t con might be bad as well.

[tibimakai]

Re: LG 42LV5500 only backlight on

They are descending, but in the middle there are two 0V pins on both of the T-cons(old and new). I'm not sure, that is how it suppose to be, or it's bad?
But this shouldn't still show an image because of the gamma voltage is off?
I'm not getting anything, only the backlight lights up, dims, lights up again and then dims.
There anything else that could cause this symptom, no video signal? Maybe the mainboard?
The previous owner replaced it, he purchased it from Amazon, I'm guessing that should be a good board, no?

[tom66]

Re: LG 42LV5500 only backlight on

Do you still have the Rigol scope? Here's where you can use it to see if you have LVDS signal going into the T-con. If the LVDS is missing the main board is bad. If it is present then the T-con or panel is bad. You have tried with one panel ribbon at a time as well?

[tibimakai]

Re: LG 42LV5500 only backlight on

I have the Rigol.
I have tried removing the ribbons, with both T-cons and no change.
The big heat sink, on the main it's pretty warm.