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After rplaceing caps transformer starts to fizz

Discussion in 'Troubleshooting and Repair' started by sizzle, Mar 26, 2013.

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  1. sizzle

    sizzle

    19
    0
    Apr 19, 2012
    After rplaceing caps transformer starts to fizz or sizzle or i don't know how to call this sound in english.
    pic of transformer:
     

    Attached Files:

  2. bullfrog

    bullfrog

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    Mar 26, 2013
    This appears to be an invertor board for an LCD panel - monitor/TV. If you supply the make and model of the item that your trying to fix , I/we might be able to help.

    I could suggest a few places for replacement invertor panels, but i've not read the forums rules yet to determine if I'm allowed to post that sort of thing.

    Bullfrog
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Did you have the CCFL tubes connected?
     
  4. sizzle

    sizzle

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    Apr 19, 2012
    No difference if CCFL is connected.

    LG - W1942S
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Do the caps have the correct voltage ratings? A CCFL inverter generates high voltages. If the capacitors are not rated for these voltages, there can be sparks (even inside the capacitors) generating that fizzing sound.
     
  6. sizzle

    sizzle

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    Apr 19, 2012
    i tought so too but tacked the fizzing down to this transformer using microphone and headhones. Replaced 3 caps to same reading caps (35V 470uF).
    Now i turned the display ower and checked if baclights even come on and find oud thy don't.
    before dissassebling it came on for a second. Showd LG logo on screen and then turned off.
    Took it apart and found bubbling caps. changed those 3 and now fizzing and no backlight at all.
     
    Last edited: Mar 26, 2013
  7. bullfrog

    bullfrog

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    Mar 26, 2013
    Service Manual located here - http://www.hobbielektronika.hu/forum/getfile.php?id=161534

    Page 24 is diagram for CCFL invertor. Page 23 is PSU.

    I believe you've replaced C301, C206, C203. Which are all associated with the 22V rail.

    I'm assuming you've checked your soldering and looked for stray solder blobs that may have dropped off your iron while soldering the caps, and I also assume you've fitted the caps the correct way around.

    So firstly try measuring the 22V rail and see if your getting 22Volts. Probably one end of R202 which appears to be a 2Watt 24ohm resistor with respect to Ground. Remember this circuit uses a high frequency oscilator and with the help of the transformer generates high voltages.

    If you can hear a sound then the oscillator is running at too low a frequency, there can be several reasons for that, amongst them are low input voltage or excessive loading on it's output.

    Failing the above you can get replacements from the likes of http://www.lcdinverter.co.uk/ although at the moment I'm not seeing one specifically for your model.

    Bullfrog
     
  8. sizzle

    sizzle

    19
    0
    Apr 19, 2012
    22V shows 25,8 when display powered off, 28,7 when power on.
    Don't have oscillato. Measured with my MAS830L between GND pin and 22V pin from where connects the videoboard to powerboard. Also measured between GND pin and 35V caps + legs same there.
    5V is exactly 5V. Transistor doesn't fizzle when power poard is screwd off from metal caseing.
     
  9. bullfrog

    bullfrog

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    Mar 26, 2013
    OK

    Not sure how much more help I can be at this stage. U301 (FAN7314) is the oscilator , I assume you are referring to having no oscilloscope to view the output, if you did I'd suggest looking at the outputs on pins 18 and 19 of that chip.
    Of course I'm also assuming that so far the unit you have matches the service manual I found, it's not unknown for there to be more than one varient of a specific model.

    Are you able to post overall pictures of the top and bottom of the board. Perhaps it's possible to spot something that way. It's a bit of a long shot.

    Bullfrog
     
  10. bullfrog

    bullfrog

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    Mar 26, 2013
    I've tracked down the data sheet for the FAN7314

    Slightly stumbling about in the dark here. The frequency of oscilation appears to be controlled by the following components.

    C320 - 220pF 50V
    R312 - 27Kohms
    C324 - 10pf 50V
    R304 - 68Kohms
    C309 - 0.1uF 50v

    OLP on the diagram stands for Open Lamp Protection, so if the lamps are connected the oscilator is supposed to shutdown. From your previous post this doesn't appear to be the case, so perhaps this part of the circuit is malfunctioning. This is controlled by the OLP capacitor attached to pin 1 of U301, which I believe therefore is C328 1uF 25v.

    I've no idea if you can check any of those.

    Beware though the OLP signal appears to be derived from the supply to the lamps which is in the region of 650-700Volts.

    Bullfrog

    Bullfrog
     
  11. sizzle

    sizzle

    19
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    Apr 19, 2012
    http://imgur.com/a/IdW5I hi-res images.

    I'll start over by checking cap legs connectivity tomorrow. 3times soldering has left it's mark. no to be 100% i'll solder long leg caps so i can put probes on both sides on board on them.
     
  12. davenn

    davenn Moderator

    13,833
    1,950
    Sep 5, 2009
    those caps WONT be 35V rating, that I can garantee
    3500V ie 3KV or so is more likely see the markings on the PCB about danger hi voltage ?? :)
    I would doubt 470uF as well they are more likely to be in the 100's to 1000's of pF

    Dave
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    In the first image the caps are labeled C419 and C402 (I can't decipher the name of the third capacitor). Are these the capacitors you replaced?

    The service manual bullfrog linked says: C402=C419=15pF/6kV
    Considering the "caution, high voltage" marking on the PCB (image in first post), this is much more logical than 35V 470uF.
     
  14. bullfrog

    bullfrog

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    Mar 26, 2013
    If the OP has been replacing components on the secondary side of the transformer then yes replacing them with 35V caps would be wrong. And is most likely to have caused his replacements to explode.

    The high voltage section of the board is the area contained within the dashed lines on the left hand side of picture 3 where it says caution high voltage.

    C402 is over on the left hand side of the picture within this marked area by the CCFL connector, this looks like the original fitted part (picture 3) , this doesn't appear to have been changed. There is no sign of the OP soldering that component on the reverse of the board

    His soldering appears to be over by the mounting hole top right of first picture - cap C203 which is on the primary side of the circuit the 22volt side or if you believe what is on the description of the CCFL invertor - Page 10 section 2 of the manual then it's actually 18Volts that generates the required 700VRms. However there is so much muck and gunge over in that area it's hard to tell what's going on. Some Isopropyl alcohol or a proprietary PCB cleaner would improve matters both from being able to see what's happening and also to ensure that there isn't something adversely affecting the circuit.

    Picture 7 (second page of pictures) indicates that he has replaced C301 which is 470uF 35V and C207/C297?? . According to the diagram in the service manual C207 is NC top right where it shows 22V/1.1Amp with arrow pointing upwards - I assume NC means not connected.

    Actually I see C206 isn't fitted - over by mounting hole , but J38 is fitted and L201 isn't fitted so electrically position C207 and C206 are in parallel anyway, so a 470uF 35Volt cap in these positions C301, C207, C301 would be correct.

    Perhaps the OP in addition to cleaning the board , should confirm which caps he's changed as in their circuit referance number.

    Bullfrog
     
  15. bullfrog

    bullfrog

    11
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    Mar 26, 2013
    Hi

    Sizzle can you check your soldering on C207/297, picture 7 close up of top centre of board. I believe your solder may be bridging to an adjacent track. It's hard to tell.

    Bullfrog
     
  16. sizzle

    sizzle

    19
    0
    Apr 19, 2012
    Copper has come loose there c207. But still connected as all other caps i soldered there.
    Heres a pic of old(brown) and new caps i replaced.
    http://i.imgur.com/5YTkPXo.jpg
    can there be a problem ?

    Replaced C207, C301 and C203
    In C206 and C204 were no caps.
     
    Last edited: Mar 29, 2013
  17. sizzle

    sizzle

    19
    0
    Apr 19, 2012
    UPDATE: found that D201 is shorted.
    SB5200
    SSG 07496

    UPDATE: FALSE "alarm", D201 is working fine
     
    Last edited: Mar 29, 2013
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    This is typically caused by VERY bad soldering technique.

    It requires far too much heat applied for far too long, possibly aggravated by mechanical damage caused by pulling or pushing on the joint or "digging" with the soldering iron.

    The solder should melt within a second or so of placing the iron on the joint and you should not have to leave it there for more than a couple of seconds longer.
     
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