No commercial links

I will delete all comments, which contain links to commercial web sites, and report them as SPAM. This blog is about sharing, not advertising.

2015-07-11

LG 42LV4500 - Not booting - relais clicking - cold solder joint on mainboard

This LG TV started its boot process until the standby LED lit up white for a second and then the power supply main relais switched off and back on again into standby. The seller said that the defect appeared suddenly.

The device's production year is 2011. I did not suspect any faulty caps in it. From what I have seen in LG TVs so far, this is not a likely fault, even more so after a little more than three years.

Nothing special under cover (no plastic, nice one). The usual. The cover is made of steel, I guess this is to give the very flat TV more stability.



Ok. First test all the voltages. 3.5V standby, 12V, 24V, all there. The backlight voltage was only 36V and is supposed to be 64V. However, the DRV-ON line was low all the time, so the backlight was never activated. Nevertheless, I checked all diodes and FETs for shorts and the electrolytics for ESR. No findings.

I used two 10k pull-up resistors to let the power supply activate all voltages. The main board was disconnected. One R between STDBY and PWR-ON, another one between STDBY and DRV-ON. And lo and behold, the backlight turned on.


The power supply was flawless. It had to be the main board. So, next, I checked the voltages at all the swiching converter's output inductors. Nothing extraordinary. But what's that? An electrolytic capacitor with no voltage at its plus leg?

This is the schematic from the service manual:



It ought to have 24V! Something had to be wrong with the inductor between the 24V line from the pwr supply and the cap.


When I put the probe of my multimeter on the solder joint (red arrow) sparks where flying. Wow. This joint was severely broken. I reflowed it and tadaaa, Frau Johansson looking good again:


I think the problem here is the glue, which is supposed to secure the white socket. It covers one side of the inductor and so this side is more rigid than the other. The mechanical stress of expanding through heat and shrinking while cooling down affects mostly the side without the glue. This will eventually break the solder. If the inductor could expand in both directions, this would probably not be a problem.

One last easy job was left. The power button on the remote did not work. Whiping the black contact surface and the rubber button with cleaner fluid fixed it.


This is a good TV. Nothing extraordinary, but it has a very solid picture with pleasant colors, and an acceptable sound. It sports two expert modes for fine adjustments of white balance and colors. And it looks good. LG manages to build middle class TVs, which look nice even though everything is plastic.

The service manual as always is missing the power supply and TCON, even though the panel is made by LG. No PCB layout graphics, just the schematics of the main board. The boards are not very complex, though.

The manual has a useful repair guide section with repair workflows. For an experienced TV repairer, they contain not much new.
Another section showed TCON and panel fault photographs. They looked very familiar to me. I have seen them in a PDF from prehertech.com, called LCD display failures and possible causes. Now who stole from whom here?


18 comments:

  1. You are a fucking freak! Well done :-)

    ReplyDelete
  2. Thanks a lot! I really appreciate your experiments. Et voilà, it resurrected our blacked out LG.
    Awesome!

    ReplyDelete
  3. Thank you very much! I got LG 42LW4500 for repair with the same symptoms. I re-soldered L407 and it started working!

    ReplyDelete
  4. Hi,
    thank you very much for the detailled instruction. On our flatscreen, which is also produced in 2011, occured the same mistake after 4,5 years.
    Now its fixed ;-)

    ReplyDelete
  5. I got excited when I saw this but then released my board is a different rev. and doesn't have a L407. I do have the same 36v on the backlight though..

    ReplyDelete
    Replies
    1. That means your motherboard does not activate the backlight. You will have to measure your way through all power supply points on the power supply board and main board. Maybe the topology is a little different, but the problem is the same. Who knows.

      Delete
  6. Thanks for these lines. I did it without measuring and it worked very well!

    ReplyDelete
  7. Thanks again to the community! Worked at first run!
    Danke Hermann!

    ReplyDelete
    Replies
    1. Btw.:
      I have a LG 42LW5400-ZA (November 2011)

      Delete
  8. Hi,
    I will test this.
    If it works, you are the best.

    ReplyDelete
  9. Many many thanks to you, I had the same problem and could not find (on the power board) and now, after I read it, my TV still work again.

    ReplyDelete
  10. Hard to believe, yet my LG 32LW4500 (made in Poland in June 2011) failed some 3.5 years later due to the exact same joint crack of L407! By that time it didn't cross my mind to google the net for a fix... so I got a new tv (still in use today), and I stored the LG for better times. Now, some 4 years after the failure, I was able to fix it thanks to your post here! Many thanks!!!

    P.S. In my case the glue seemed uniformly distributed along the entire side of L407, so perhaps the respective joint was insufficient flowed by the manufacturing machine.

    ReplyDelete
  11. Hi, greatings from Norway. stumbled over this article when searching for ideas for mine lg lcd. And you never guess... a little solder job on L407 and is's alive!

    Many thanks for this article!!

    ReplyDelete
  12. Hi, I have a 47LW650T and found your blog by chance, I read all your information and though many years old now, can anybody help please
    I have joined so many sites and blogs trying ti fix it, but to no avail.
    No picture no sound, no power to USB or Optical.
    The PCB has all 3.5,12,24v voltages when on and 3.5V when on standby.
    I jumpered the PSU as suggested here and my backlight turns on.
    From research I realised that my TCON is not getting 12v from mainboard on pins 48-51. I tried to trace the 12v voltage but its too complex for me to figure out where exactly. There was a mention of Q505,Q506 transitors and Q507 mosfet, but Im not sure what to do next. Thx

    ReplyDelete
    Replies
    1. I don't have any experience with this specific model. However, when the TCON does not get 12V although the power supply is fully switched on, the main board does not switch the 12V through. The reason for this can be a dead FET switch or a not fully booting software. If you are not experienced with measuring on component level on the mainboard and reading schematics, I suggest you replace the main board.

      Delete
    2. Thanks for replying. I'm not fully experienced in electronics but have been doing the basics for years, but the tracing and components are so minute, they're so hard to follow. I have followed the 12+ trace that passes through the transistors and mosfet, after which I'm lost as to where it goes next..I thought to the pins for connector but there must be somthing on the topside of board.
      Unfortunately in the UK, it's hard to find replacement main boards but OUS boards are easy. It seems to be quite a common problem that spares have been all brought up. I rather not throw away the TV..electrical waste too

      Delete