LG 42LD750 - only Lg logo thengoes black

[domix]

Hello,

i recive this tv LG 42LD750. when turn on sjows only LG logo then goes black, somethimes shows for a second rotating logo whic it's after firs logo.

I open the case and see that someone did replace the capacitors, now are big one 250V 100uF. and other smallers are the same 470uF 63V and 1000uF 35V.

on the pictures on the net i saw that there are the big ones 82uF, and so on.

What do you think what i should to do, to this tv work again

Thaks

[JeffJ]

Re: LG 42LD750 - only Lg logo thengoes black

First step is to post pictures of the inside. high quality ones.

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

After the logo disappears does the backlight remain on?
When you try to change input sources does anything appear on the screen?

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

I will post photos later. There aren any blown caps, just replaced ones.

When logo dissapear goes black and backlight is on only for second then goes completly black without backlight. At the bottom led indicator is still blue. To change input source nothing happend

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

Here is a (pretty incomplete) service manual:http://elektrotanya.com/lg_42ld750-z.../download.html

Found this pic on the net:



It seems power supply and inverter are on the same board.

There is a cable going from the PSU to the control/main board.

I would first measure signals and voltages on this connector. There must be some kind of BL or STB SB or PWM or DIM signal and 5V, 12V or similar.

Take a picture of that connector if a description is printed on the PCB near the connector.

You need to find out if the mainboard commands the backlight off or if the problem is on the PSU board.

Then we go from there.

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

It seems to be P8000 (page 28 of the pdf) . Check INV_ON and PWR_ON and if you have a scope observe also DIM1, DIM2, A.DIM and also measure the voltages 24V, 3.5V, 12V.

[Hamie]

Re: LG 42LD750 - only Lg logo thengoes black

Bet this is a bad main board, this TV is terrible for faulty mainboards.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

Okey, here are the pics. What i must to measure now, to check where is the problem, i have basic skill from this, so i need a litlle help and i think taht together we cuold solve the problem

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

I see that someene, previus user did some solder work here, why i dont know?

[Moreno83]

Re: LG 42LD750 - only Lg logo thengoes black

The mainboard is faulty thats the reason it does not start.
You could try and see if you can find some cold solder joints or bake the mainboard in the oven @ 385 degrees for 9.5 minutes.

Problem is 100% mainboard!

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

So there aren't any other ways to fix it. only to buy new board?

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

if i turn it on and press menu button it show menu but for a second aor two then goes black as before, it it this faulty mainboard, if all functions wokrs but ther a bleck screen and no backlights

and previus user has tv on service and now i see that solder together ERROR and P-DIM look at pic before

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

INV_ON 3,34
PWR_ON 3,16V

A.DIM 1.69V
20V are all 20.7 - 20.8V
3.5V are 3.38V
12V are 12.34

P.DIM 159.8V
P.DIM2 159.9V

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

INV_ON 3,34
PWR_ON 3,16V

A.DIM 1.69V
20V are all 20.7 - 20.8V
3.5V are 3.38V
12V are 12.34

P.DIM 159.8V
P.DIM2 159.9V

We need the voltages while the backlight is ON and while the backlight is OFF.

Do any of these voltages you posted change or are they constant?

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

when turn on and lg logo shows with backlight
Backlight....ON.......OFF
......P.DIM..33.4V...152.2V
......A.DIM..3.15V...1.68V it drops 1 second before backlight went off
INV.on/off..3.33V...3.33V
..P. DIM2...33.4V...159V

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

Pwr_on?

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

To have 160V on this connector seems weird.

Also if you look at the schematic, ERR_OUT (pin24 of the P8000 connector) goes via a 0 (?) Ohm resistor directly to ground? That can't be true if ERR_OUT and P_DIM1 are connected together and you measure 160V and P_DIM1.

I think we need to find out first if this is the right schematic and which of these components are populated. Take a good picture of the top and bottom side of the area where this connector is on the mainboard.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

P.DIM is almost 160 yes. look at pictures

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

Pics of TOP and BOTTOM of the other board near the connector P8000 please.

And you didn't post the voltages for PWR_ON with backlight on / off.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

here they are. i found something on this moss on pic, they were replaced or ?

PWR_ON is 3.16V with and without backlight