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42LD75LGD: No sound and no input/output. Main board fried?

Hi all!

I have an LG 42LD75LGD TV that has recently stopped playing sound and the input/output no longer work (both video and audio). I couldn’t test everything for lack of cables/peripherals. My findings:

The only thing that seems to work is the image from the cable. The sound doesn’t come through.

VIDEO:

the three hdmi ports do not get the picture across to the screen. However, in the input menu, I can see the TV detects which port has a source connected. When I select the connected hdmi port, I can hear a faint buzzing/whistling, but no image comes.

When using the VGA cable, I get some “snow” on the screen.

AUDIO:

I have no sound and I cannot get the sound (of the TV programs) out through e.g. the headphone jack.

So my questions are the following:

  • is it 100% sure that it is the main board that is the cause, or can this be related to something else too?
  • I doubt they still make the main boards, so can it be fixed (I’ve read things about baking the main board in the oven)?
  • Otherwise, is it possible to just use some other board? I actually really don’t care about any of the smart features. I just want to use this TV as a screen (input from HDMI).

Any hints?

Thanks in advance!

Cheers!

Update (10/06/2019)

Hi oldturkey03, and thank you very much for your detailed answer!

The latest part (universal LCD driver board) actually got me really interested, so for the time being I’ll forget about fixing the main board. As I wrote, I’m planning on using that TV set as a monitor only (streaming through chromecast, computer, etc.). So if I can get rid of all the so-called smart functions that take forever to load and that I don’t need, all the better!

I will pop open the TV and look for the LCD panel and power board numbers. And then I’ll try to see where the T-con board is to get its ID too and get back to you.

Update 2 (10/07/2019)

I have re-opened the back panel of the TV, and since I am not sure where to look for the LCD panel number, I took a picture of the whole thing. I don’t suppose the panel number is one of those circled, or could it? I suppose I’ll have to detach the panel from that back metal plate in order to access the panel number…

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Since it was quite late I decided to postpone this next stage, but while removing the main board, something drew my attention. One of the heat sinks does not seem to be in contact with the surface of the component it’s supposed to help cool down. Or at least I’ve never seen heat sinks placed like that, but then again, I have really no notion of electronics, so that’s why it’d be nice if someone could confirm (or infirm) this.

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And just in case the conversion from TV to monitor is unsuccessful, should I try to re-attach this heatsink to its component before I bake the board in the oven as a last attempt at fixing the TV? If so, how should I go about doing it?

Thank you!

Update 3 (10/07/2019)

Don’t worry, it’s not your eyes, it was the picture :-D. I had taken it to have an overview of the whole thing, the idea wasn’t to be able to read those small prints on the labels. But anyway, here are close-ups of those two labels in the center, hopefully one of those is what we’re looking for! I would bet the one that reads LG Display?

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And out of curiosity, I lifted the heat sink, and it was barely touching the component underneath. When I looked at the thermal glue (or double-sided tape or whatever it is), it seemed like it has been attached to the component only from the edge from the very beginning. There’s only a line imprinted in the glue that corresponds to the place where it was in contact with the component. The component itself did not present any apparent damage, but also no trace whatsoever of any thermal glue on it. See the pictures below:

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And I was going to forget the main board number. Here are two pictures, one with all labels on the upper side, and if that doesn’t suffice, there was a number engraved in a piece of metal on the back, if that’s of any help.

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So now, how do I match my display (provided one of those labels did have the display number) to an LCD driver board? And how do I get my hands on one? :-)

And oldturkey03, I cannot overstate how precious your help is! Thank you very, very much!

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If you are ready to convert your smart tv to a monitor all we need is the panel number :-)

由完成的

Yes those are the right labels but my old eyes can't read those :-) You are absolutely right; that heatsink is not where it is supposed to be. This could possibly mean that the IC below it has been fried (if so, you can skip the baking ;-) or is what causes your TV to not work due to thermal shutdown. At this point you should just pull it off and then see what is going on underneath it. Possible that replacing the thermal paste and heat sink may just get your TV back. Zoom in so we can get A. the panel number and B. the board number for the main board.

由完成的

@oldturkey03 I have updated my original post with pictures of the labels. The numbers you mentioned I need are the following: LCD panel: LC420WUH (SC)(M1), Main board: 42LD75LGD. There might be some complementing numbers for the board (revision or other), which can be found from the picture. I'm not sure why I needed the main board number if I'm going to ditch it and replace it with a universal LCD driver board. I've had a quick look on ebay, but most universal boards don't state any panel number. Am I mixing things up here? Thanks!

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Pinf C let’s see if we can get all of your questions answered:

  • is it 100% sure that it is the main board that is the cause, or can this be related to something else too?

No. Since we don't physical have access to your TV and cannot measure see your boards we can give you guidance and ideas but anything on here is never 100%. Yes it most certainly sound slike main board issue since that is where video and audio are being processed. This “I couldn’t test everything for lack of cables/peripherals” would require more information from you.

  • I doubt they still make the main boards, so can it be fixed (I’ve read things about baking the main board in the oven)?

Depends. You would need to have the schematics as well as tespoints etc. for that board to determine which part has failed. You may also need test equipment like an oscilloscope since you would be looking for signal lines etc. To get more information on that you needed to at least tell us what main board this is. i.e., PCB number or added multiple good images of it with your question. 在已经存在的问题里加入图片

  • Otherwise, is it possible to just use some other board? I actually really don’t care about any of the smart features. I just want to use this TV as a screen (input from HDMI).

Okay so most likely not since you would need something to match your power board and your T-con board etc. Here is something to consider and not sure if you are up for it. Depending on your panel there may be something that could be done. You will need the real panel number. This LG 42LD75LGD is just not anything of value for and circuit boards since most of the companies have different varieties of boards with multiple revisions etc. so you need to get those numbers. Consider using a universal LCD driver board. I have only done a conversion on two TV’s where all I could use was the display and had to discard the rest but at least I had some decent size monitors. Can’t call it a TV since I did not have the ability to select channels etc. but great for streaming from laptop and things like Chromecast.

Update (10/09/2019)

Pinf C here is the datasheet for your panel for now you want to browse through it but hang on to it for future reference. We’ll need it LC420WUH-SCM1-LG.pdf

More to follow

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Alright, so here’s the answer to the title’s question: it was indeed the mainboard that was faulty. I glued back the heatsink onto the chip, and then I baked the whole thing in the oven for 10 min. at 200°C (392°F), I let it cool down, then re-mounted it and voilà! All input ports work and so does the sound. I haven’t tested the sound output, but I guess it, too, works now. [Edit: works now, too]

However, this is not over, since I am really interested in using a universal board to turn that “smart” TV into a dumb monitor, but for the time being, since it works as it is, there’s no need for me to do that just yet. Also I’m not sure baking the board in the oven will offer me a long-term solution. It might fail again soon and I’ll be back at square 1. Of course, I can then bake it again and see what happens :-D.

While looking on the net for a universal LCD board, I was puzzled to see that all those I looked at have only one connector for the display, while my main board has two, connecting to the two ribbons coming from what I believe is the T-con board (or boards, because there seems to be two, one for the left and one for the right) on top of the set. So are there some universal boards with two connectors, or how does this work? Well, since I don’t have any knowledge in electronics, I might be completely wrong, so if someone can push me in the right direction and tell me what I’m looking at, I’d really appreciate!

Cheers!

Pinf

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For some of those you keep your T-con board but lose the power board and main board and in many case the IR board as well. For others you connect the replacement board directly to the panel via the LVDS cable. It's all depending on panel and board. https://youtu.be/11E68FprebU for example

由完成的

@oldturkey03 Thank you very much for this, and I only now just noticed you had sent the panel datasheet. Thank you for that as well. I was really puzzled at first when trying to figure out what universal board I should use. All the boards I looked at only had one LVDS output, and my panel requires two FFC connectors. In the video you shared the guy uses another kind of board (some universal T-con?) between the universal board and the panel, but I'm guessing that in-between board is not universal and I should make sure the one I purchase matches my panel specs. While universal LCD boards seem to be everywhere, I wasn't that succesful in my hunt for that second board. If anyone has a clue about what they're called, or keywords to use in my searches, I'd be really grateful. :-)

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