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Thread: LG 55LB6520-TB - Is it Going to be Worth it???

  1. #41
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    gnd & gnd are ok.
    12v is 12vdc and NC was 24vdc <<-- odd, but m'kay, multi model PCB
    12v & 12v were both 12vdc
    PIDM2 and PIDM1 are both 2.4vdc <<-- LED array 1 & 2 brightness controls (lowers/raises LED driver output voltage)
    Pwr on was 3.4vdc <<-- service voltage, pull to ground to turn on (active_low)
    DRV-on was 3.5vdc <<-- LED voltage inverter control, not sure if active low/high but easy to divine

    *when on & backlight flickering, check voltage/state of DRV-on, PIDM2 and PIDM1 just to be sure logic board is sane =)

    LED array circuit is in two parts, with one controller IC ... C801/802 are the bulk caps for primary array, C805 is the 2ndary array bulk...these are switched by Q803/Q806 respectively

    These controller chips monitor LED current draw as part of function ; screenprint on the board of LOW/HIGH output HV circuit, points back to this;



    *if I were sure the logic board wasn't being silly, I'd measure the LED arrays for current draw

    My guess is you saw the low LED array going wacky -- this is C805 you'd suspect ...33uf/200vdc caps are cheap as chips, but you'd wanna measure/test that LED string first before even replacing that cap

    *if the LED array tested good, and C805 measured ok (removed from circuit), I'd start looking at the controller IC under that slathering of goop with hairy eyeballs

    edit: as I don't know what IC it is, I'm not sure of what fault detection response it has
    Last edited by wotnot; 26-12-20 at 01:23 PM.

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  • #42
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    Ok.
    Thanks for the follow up Wotnot:
    On the white LED connector that Loopy asked me to check;

    White LED + 260VDC
    Black LED - 260VDC

    Blue (Loopy), LED + 110VDC
    Black LED - 90VDC

    All fluctuated a bit, but BLUE and BLK were all over the place...!


    DRV was 3.5VDC (PWR on is Blue i take it?), as in the wires on the lower are on the right of the PCB:
    C801, C802 and C805 are all ok, dead on 33-36uF.

    C801 and C802 were at 260VDC

    C805 was at 110VDC

    What I'm assuming is the LED Driver IC is IC801 (big IC close to the area of the White LED connector) - I removed the gunk and there's ZERO info on the chip.


    Obviously under voltage for Blue LED + and BLACK LED - as C805.


    I cannot find Q803 or Q806 - I'm worried it's a Doh! moment...

    Wotnot - where can I measure those voltages you have above - 145v LOW and 218V High...?

  • #43
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    I cannot find Q803 or Q806 - I'm worried it's a Doh! moment



    where can I measure those voltages you have above - 145v LOW and 218V High...

    You already have =) These are the two LED array outputs ..... and they're screwy 'as measured' by youself .....you shouldn't see over 218v on that array, and nor should you see under 145v on the other.

    Is C219 on spec? (my guess is it's good)


    White LED + 260VDC
    Black LED - 260VDC
    I probably don't expect to see that ~ I think I should be seeing 260v minus the the voltage drop across 'x' LED elements (no idea how many there are, but more than 10 at a guess, so at least 20~30volt drop)

    Makes me wonder if that strip's shorted or something (the other array that has voltage all over the place, may be a red herring -- ie; it's all over the place because the high array is shorted? And because of that short, the driver IC is winking out)

    *test LED strips as mentioned before

    Do a search on this string 'how to test tv led backlight strips'

    Choose a methodology and see what you find =)

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  • #44
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    Ok.
    Tks for the follow up wotnot

    As I mentioned there's a PSU board being sold for $40 as 'New: Never Used: An item in excellent, new condition with no wear'.

    I've spoken to the seller and he had a "Flashing picture - going from black to picture"...
    He bought the board but it didn't fix it and said it was an LED strip failure.
    He had pickup only but is prepared to post it to me.

    Obviously, his fault is similar to mine. So I'm going to have to confirm that the LED Strips in mine are ok - as Wotnot is saying.

    What I don't understand is that the 'flashing is the LED strips coming on'. But then they are going off. So the actual 'Lighting' of them is working?

    If there is a 'short' does that mean that I will get a picture - then it shorts out - then unshorts (as picture comes back in), then repeats...?

    And would you believe this guy has an LG TV.... ha ha..!


    This YT video is (as the above), testing to see if they come on.



    Should I have a go at disassembling the tv down to where I can see these LED Strips or should I still be looking for the PSU issue?

    I'm assuming they are ok (in brightness), as i can see a someone small section of screen at the LG Logo that flashes up.

    I'll try and do some measuring on those two Q803 and Q806 in moment

    Cheers,
    GT250.


    EDIT - TESTED Q803 & Q806
    Q803 is measuring 96vdc to 105vdc with slight fluctuations.

    Q806 is around 16vdc to 21vdc and all over the place, going crazy....

    I measured the solder pads on the back of the IC's.. Is that the correct place to have tested?
    When I tested the middle leg, it gave similar results. legs on the outside of both didn't register a thing.

    NB: On the small video i posted of the flashing, this was at the top of the screen. There are TWO types of flashes.
    1) Super bright and 2) Slight bright.
    On the lower part of the panel (on/off switch and speakers), there is NO SUPER BRIGHT, only SLIGHT bright.

    Question is: If the LED strips have gone (and it's going to take a long while to get to them - lot's of little tiny screws), is it going to be worth to get this $40+P&P $20 board, to find out if it's the PSU board after all...?

    Oh well, all fun and games...
    Last edited by GT250; 27-12-20 at 05:43 PM.

  • #45
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    What I don't understand is that the 'flashing is the LED strips coming on'. But then they are going off. So the actual 'Lighting' of them is working?

    If there is a 'short' does that mean that I will get a picture - then it shorts out - then unshorts (as picture comes back in), then repeats...?
    You're not quite getting the picture here (pun =) There are 2 LED array circuits - you don't yet know if both arrays are working ; lets hypothesize the LOW array is working, and the HIGH array is cactus. When you switch on, the backlight driver applies power - the working array lights up, the dead (perhaps shorted) array doesn't light up, but because it's buggered it's still drawing current. If that is so, that strip might be drawing too much current (especially as things warm up), and the driver IC resets. This cycle repeats ad infinitum ; you see backlight going on/off.

    No...nothing will 'unshort', the backlight array(s) are switching on/off per above. If per chance you could get a bright light/torch and shine it in the back of the display, if the mainboard is good you should still see an image on the screen.

    edit: this'll perhaps help you understand things

    Last edited by wotnot; 28-12-20 at 11:38 AM.

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  • #46
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    Update:

    I have spoken to the guy in Camden (NSW), and it looks like he had the same issue as me.
    He bought a new PSU board and it turned out (in his case), to be the LED strips...

    Would have been nice to have had a 'new' PSU Board to rule it out - but it does like now it's the LED strips.

    Thanks to Wotnot and others who 'hinted' that it might be the LED strips.

    I live alone at the moment (Partner is in the UK as of 10mths due to Covid). Yet if she can hit a cricket ball, play tennis, or fruit pick (Seasonal worker - Tasmania), or be a politician, she would be allowed in!

    I'm ok (have the time), to take the TV to the level of replacing the LED's.
    I'll work out how to get to them and for basic testing, do the Three 9vlt battery thing to start with
    I don't have any SMD equipment, just a fine solder point and a heat (paint stripper), gun.

    So all good fun.


    Cheers.
    GT250.

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  • #47
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    Hi all,

    I have decided not to continue with the repair, as I'm very confident it's the LED strips at fault.
    I don't have any SMD gear at all and looking at all the SMD IC's and Hot Air Solder guns - which I don't have, It's just one of those times where I can't justify going any further.

    So I'd like to say a HUGE thanks for all the input I have received and after looking at many You Tube videos, it looks like a large amount of LG LED Tv's have had this problem.
    Shows the LG quality!

    At least it wasn't the caps and more than likely wasn't the PSU board, all leading to the LED Strips at fault - which meant dismantling the screen(s), of it's many layers...
    I have a small table to work on and I envisaged needing a large flat bed clean dust free area to lay it all out - and I just don't have it.

    So once again everyone thank you so much for being a guiding hand in my attempt to save this TV.


    Cheers,
    GT250

  • #48
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    You do need a clean area and table tops to work on the things ~ it's not difficult but fickle work and careful hands needed at all times.

    Just so you don't think you --need-- to go the SMD LED replacement route (which is the cheapest way to do it) -->>

    A note about that -- it's always best to get a visual of the LG part number on the strips before ordering replacements like the above

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    I've done them before. I just used a soldering iron. Got the spare leds from another broken LG and it worked fine.
    It's not so much the quality of LG but some people set the brightness way up. That will destroy them leds eventually in any TV.
    You will need a large flat area to work on it and somewhere to lay out the parts. Maybe put a wanted ad on your local facebook page for an LG with a broken screen (free).
    Last edited by loopyloo; 30-12-20 at 02:26 PM.

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