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Thread: LG TV wipeout

  1. #241
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    The MICOM chip gets data Vsync from the TV board and also monitors the backlight circuit the errors should not show up if everything is ok.
    Did you measure the backlight voltage output? (should be 50-60V)

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  • #242
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    Quote Originally Posted by Skepticist View Post
    The MICOM chip gets data Vsync from the TV board and also monitors the backlight circuit the errors should not show up if everything is ok.
    Did you measure the backlight voltage output? (should be 50-60V)
    No I didn't. I thought I might just see if the main board will start it up for me.
    Got busy watching Michael Mosely. Going to flatten that BC337 to the board and fit it to the TV in a minute. Of course I will be utilizing the load lamp too.
    Last edited by loopyloo; 17-08-17 at 10:55 PM.

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    Bummer, not a good result.
    When mains is applied all seems fine but the moment I hit power on the remote the load lamp lights up brightly for a good second then goes out, so the TV must be dropping back to standby after it sees a problem. I also tried it with the backlights unplugged from the power board and got he same result.
    I'd say there's a major short in the backlights circuit. Time to start checking FETs again.
    Last edited by loopyloo; 18-08-17 at 12:04 AM.

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    With the TV board connected the demand might be creating too much voltage drop across the series lamp.
    Try checking the voltages with a multimeter over the time the lamp glows and whether the lamp going out is the result of the TV board switching the power off via POWER_ON input to control Q501.
    The nominal voltages for the outputs are printed on the PCB in the pic you provided IE 3.5V, 12V, 24V, 58.5V and the rated currents for the major 3 outputs indicates a possible loading near 100W with the backlights being the most signficant so yes, the series light is a major impedance. Unplugging the backlights is likely to produce an error even if the voltages are better.

    So you might have to try without the series lamp to be sure - if the TV board is switching POWER_ON off that is.
    If the power supply is all back to normal the overcurrent sensing should handle any abnormal loading in the TV board safely by nobbling the output.
    Last edited by Skepticist; 18-08-17 at 12:47 AM.

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    Quote Originally Posted by Skepticist View Post
    With the TV board connected the demand might be creating too much voltage drop across the series lamp.
    Try checking the voltages with a multimeter over the time the lamp glows and whether the lamp going out is the result of the TV board switching the power off via POWER_ON input to control Q501.
    The nominal voltages for the outputs are printed on the PCB in the pic you provided IE 3.5V, 12V, 24V, 58.5V and the rated currents for the major 3 outputs indicates a possible loading near 100W with the backlights being the most signficant so yes, the series light is a major impedance. Unplugging the backlights is likely to produce an error even if the voltages are better.

    So you might have to try without the series lamp to be sure - if the TV board is switching POWER_ON off that is.
    If the power supply is all back to normal the overcurrent sensing should handle any abnormal loading in the TV board safely by nobbling the output.
    The load lamp lit brightly with the backlamps unplugged too, that's why I thought maybe another short on the power board. I hope not.
    Yep, I can pick up the 58V at the test point top right of the power board, so I'll check that.
    Pretty sure the main board is turning it off just by seeing the reaction so I'll have a look at that pin too.
    It didn't occur to me that the load lamp could be why it's not running but you're right, it's fair bit of extra load for the board to sense and it prob doesn't like it.
    I just went quickly over some of the major components on the secondary side and I couldn't find any shorts. Maybe things are ok after all. ... soon find out in the morning.
    Last edited by loopyloo; 18-08-17 at 01:44 AM.

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    The power-on is switching it off so that part is working.
    When measuring the backlight voltage both of my meters come up to 36v then it shuts off back to stby. This is either because my meters are not quick enough (which is strange since the meters are different and they both get to the same reading) or I assume it's because that's as far as the power supply gets before deciding to cut off. I did these measurements with the backlight plugs disconnected. Can't imagine there are any shorts if it gets that far, just sensing overload.
    Are we ready to try it without the load lamp ? (getting excited )

    PS Just tried it with the backlights plugged in and 36v is again where it stops.
    Last edited by loopyloo; 18-08-17 at 10:59 AM.

  • #247
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    What was that saying? sometimes you just gotta roll the dice
    The power supply should be able to take care of itself non-destructively now IMHO

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    Ok well a good result, at least partially.

    The lights are on but nobody's home.
    The screen lights up fine and even eco works because if I press that the screen backlights change from bright to not so bright and vise versa BUT, nothing is displayed on the screen at all.
    Must have sustained some damage to the main board. I know it works to some degree, responds to the remote and controls the front led (which doesn't flash any errors by the way), controls the power board and the eco, just no images or menu or even startup clock etc

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    So near but still so far...
    I wonder if there's a schematic online for the TV board...
    Did all the voltages come up to spec? (12V and 24V mainly)

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    Quote Originally Posted by Skepticist View Post
    So near but still so far...
    I wonder if there's a schematic online for the TV board...
    Did all the voltages come up to spec? (12V and 24V mainly)
    Yep voltages are good.
    I've got the schematics for the main board too. Might start by looking over it for fuses.
    Should be this one :
    Last edited by loopyloo; 18-08-17 at 01:04 PM.

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    That'll help but it could turn into a lifetime project

    There's enough data there and the schematic to go probing the power circuits on the main board

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    Quote Originally Posted by Skepticist View Post
    That'll help but it could turn into a lifetime project
    Ha ha couldn't possibly be as bad as that power board could it ?

    I just wanted to say at this point in time, thanks so much for all your help getting that power board running.
    I couldn't have done it without your help and I really appreciate it cos I learned a lot throughout the process.
    You can pull out now if you like. I might just plod along for a while at least to see what I can find out.
    Last edited by loopyloo; 18-08-17 at 01:58 PM.

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    Hey it's been fun. Such a little elusive fault causing so much trouble and maybe the TV board is the same

    Could be as simple as forcing a firmware update, factory reset or similar

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    Quote Originally Posted by Skepticist View Post
    Hey it's been fun. Such a little elusive fault causing so much trouble and maybe the TV board is the same

    Could be as simple as forcing a firmware update, factory reset or similar
    Yeah that's something I should try but I'll have to figure out to force an update.
    I put in a USB flash drive to see what reaction I'd get and it must be working cos it light up and did some random flashing.
    Thought I'd better try an antenna to see if there's any sound so I moved the TV into another room for it. There is no sound so the tuner might be out too but I noticed a couple of things.
    Firstly it wasn't the eco button I was pressing where the screen brightness changed , it was the input button (I've got some different remotes). Second thing, I was looking from almost perpendicular in this room and saw faint vertical bands on the screen that I didn't see before, similar to if the t-con was faulty. Might try to get a photo of it.
    Last edited by loopyloo; 18-08-17 at 02:45 PM.

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    The lines are not real contrasty so have to view it straight on. You can also see very faint lines on the right hand side.
    Very hard to get a pic.

    One other thing, the processor can't be totally stuffed because it gets warm.


    Last edited by loopyloo; 18-08-17 at 04:17 PM.

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    We have some more specialised TV guys here who'd be better at providing clues as to what's going on there. I'd say there's signs of life, just not as we know it

    I have seen similar symptoms in monitors with static damaged LCD drive circuits (the circuit mounted on the LCD film itself)
    Last edited by Skepticist; 18-08-17 at 04:36 PM.

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    Yep, that would be good.
    Would someone else like to hazard a guess on this one ?

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    Pretty sure I got an update to go through.
    USB drive flashed for quite a while then the TV turned off then on.
    Unfortunately the screen is still the same.

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    The troubleshooting flow chart suggests 'replace main board' but we could have worked that out without the flow chart
    Sounds like the CPU is working OK and from what we've seen the HV damage appears limited to the primary side of the power supply.

    What would be nice to know is the state of the TV at the time of the fault, particularly whether it was connected to any external earths at the time because the natural condition of the set is that it has a floating earth unless an external device is providing the true ground connection (a large lightning discharge to ground can raise the ground potential considerably). You could check all the voltages shown on the MID_POWER page but, in the end, the main board was never intended to be repaired it seems.
    They have a history of BGA failures as well so you could end up replacing every board only to find the LCD itself is damaged.

    Who was it that was making a coffee table out of his old plasma?

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    Quote Originally Posted by Skepticist View Post
    The troubleshooting flow chart suggests 'replace main board' but we could have worked that out without the flow chart
    Sounds like the CPU is working OK and from what we've seen the HV damage appears limited to the primary side of the power supply.

    What would be nice to know is the state of the TV at the time of the fault, particularly whether it was connected to any external earths at the time because the natural condition of the set is that it has a floating earth unless an external device is providing the true ground connection (a large lightning discharge to ground can raise the ground potential considerably). You could check all the voltages shown on the MID_POWER page but, in the end, the main board was never intended to be repaired it seems.
    They have a history of BGA failures as well so you could end up replacing every board only to find the LCD itself is damaged.

    Who was it that was making a coffee table out of his old plasma?
    It's an unfortunate thought but I did reason that it could be the LCD. Just one thing might foil that, there's no sound, of course at this stage I don't know if it's lost its channels.
    I've been going over the board tracking power by continuity. Tomorrow i will take some measurements.
    Hope I don't have trouble there, I've already found a couple of discrepancies in the diagram.

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