LG 42LD750 - only Lg logo thengoes black

[budm]

Re: LG 42LD750 - only Lg logo thengoes black

P.DIM is almost 160 yes. look at pictures

The reading is mili-Volt (mV), Not Volt (V).
It is 0.158V

"I open the case and see that someone did replace the capacitors, now are big one 250V 100uF" The power supply has PFC Voltage booster, so 250VDC cap for the main filter is not going to work.
I see there are two main filter caps laying on their sides, one on them is rated at 100uF 450VDC. Are they connected in parallel? I think someone put in the wrong cap (250VDC).

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

sorry my bad you have right

i dont know which one must be

if i meassure it shows me 198.7uF

here it is

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

A_DIM and P_DIM seem to be PWM and/Clock signals, you'd need an oscilloscope to check them. Since the values you posted are actually "mV" it changes very little.

All voltages and ON/OFF commands are present, it looks more like a problem on the PSU board.

[budm]

Re: LG 42LD750 - only Lg logo thengoes black

sorry my bad you have right

i dont know which one must be

if i meassure it shows me 198.7uF

here it is

OK, the main filter caps are connected in parallel, so who ever worked on this before you did not know what they are doing, get the correct filter cap installed first before the damages to the PFC Voltage booster occurs, the boosted Voltage can be as high as 400VDC that is why the 450VDC rating caps are used. Do not power up the TV until the correct caps are installed.
Who knows what else was done to the board or wrong parts are replaced.

The PWM dimming signal is 200~300Hz with variable pulse width, 10% pulse width = min brightness, 90% pulse width = max brightness.
The signal level is 5V, so at 50% setting the DC Voltage will be 2.5VDC, or you can just set the meter to AC to see the activities.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

Which caps must i pit in. In pictures of the board on net i see that is 82uF voltages i dont see, and i dont know where to check. I want to replace all caps, which ones was replaced.

[budm]

Re: LG 42LD750 - only Lg logo thengoes black

Post 1:"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."

https://www.badcaps.net/forum/attach...2&d=1429542048
Isn't that the picture of your board?

"Which caps must i pit in. In pictures of the board on net i see that is 82uF voltages i dont see, and i dont know where to check. I want to replace all caps, which ones was replaced." I do not understand.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

It was wrong key pressed. Now are the big ones 450V 100uF, on pictures on ebay and etc, i see that is the big one 82uF. This caps that are now on plate was replaced by previus user. So you said it is wrong caps. Must i replace it? But i dont know where to check which one must be, like on new board.
All pictures are of my board.

[budm]

Re: LG 42LD750 - only Lg logo thengoes black

OK, This picture here:
https://www.badcaps.net/forum/attach...2&d=1429542048

Is this the picture of YOUR BOARD? One of the cap has 100uF 450V printed on it, the other one we cannot see the printing.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

this is what's now on board, yes all pictures are from my board

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

this is what's now on board, yes all pictures are from my board

The original board seems to have had 82uF/450V for both of these big caps.

No problem using 100uF as long as both have a rating of 450V you should be good to go.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

So than, what else must i to check to see where is the problem, can anyone help me

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

First I would open the connection between P_DIM and ERR_OUT.
Then measure again P_DIM while backlight is on and while it's off.

I would also measure the secondary winding resistance of the two big black transformers. If you don't know where to measure take a picture from the bottom of this area.

[budm]

Re: LG 42LD750 - only Lg logo thengoes black

The A_DIM is the analog 0~5VDC dimming control signal, 5V = MAX brightness, <1V = min brightness.
The pw_dimming does not look like it is being used.
Are you getting >360VDC between the two legs of the main filter caps when TV is on?

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

This is the bottom side, tomorow mornig i will measure what you said and sepparate err in dim cable.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

P_DIM and ERR_OUT was only taped together, not soldered.
so previus results are correct

i measured 386V between the two legs of the main filter caps when TV is on

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

Any suggestion what to do? I realy want to fix it

[vinceroger69]

Re: LG 42LD750 - only Lg logo thengoes black

as paul says post 32 we need to measure the inverter transformers next the two black oblong transformers is there any numbers printed on them if you dont know the pin layout of them as paul says post a picture of the pin area of the transfomers on the bottom side of the board.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

hello, i was away from home these days, now i'm back,

where have i to look for these nubers can any shows me

https://www.badcaps.net/forum/attach...2&d=1429682664
https://www.badcaps.net/forum/attach...3&d=1429725867

[paulstef]

Re: LG 42LD750 - only Lg logo thengoes black

What numbers?

Did you measure resistance of the transformers. Have a look at the attached picture. You need to find the secondaries. My guess is where the red lines are.

[domix]

Re: LG 42LD750 - only Lg logo thengoes black

i measured it there you mark it, one has 113,1 ohm and other 112,9ohm