Sinotec LCD TV Model ST-42KC42F "CCFL lamps not switching on"

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  • angelos
    Badcaps Veteran
    • Dec 2012
    • 568
    • South Africa
    #1

    Sinotec LCD TV Model ST-42KC42F "CCFL lamps not switching on"

    Hi Guys,
    It's been a while that i have been active on the forum, sorry for that. My primary job has been keeping me quite busy.
    The above set starts up with sound working just fine but the CCFL lamps (Inverter) runs for one second and switches off.
    My 24V supply is steady and the ON/OFF pin to the inverter is at +5V.
    The only thing i see is the PWM signal pin is at +5V without any square wave being emitted. Is this normal? as far as i can remember there needs to be a square wave pulse there. Am i right?

    I will now measure with my oscilloscope probe on each lamp output and compare the waveforms and send you feedback on the next thread.
    I have added photos of the board layout.
    Attached Files
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    • angelos
      Badcaps Veteran
      • Dec 2012
      • 568
      • South Africa
      #2
      Re: Sinotec LCD TV Model ST-42KC42F "CCFL lamps not switching on"

      I measured with my scope on the inverters outputs of the 10 lamps and got the exact same waveform. The peak voltage is 700V at 50Khz.
      I also added a picture of the inverter for you to see.
      Attached Files

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        • lotas
          Badcaps Legend
          • Jan 2016
          • 4555
          • Russia
          #3
          Re: Sinotec LCD TV Model ST-42KC42F "CCFL lamps not switching on"

          Check these capacitors for capacitance and esr, also measure the resistance of the high-voltage windings for the resistance on the inverter of the transformers (the resistance should be the same for all).
          Attached Files

            Comment

            • angelos
              Badcaps Veteran
              • Dec 2012
              • 568
              • South Africa
              #4
              Re: Sinotec LCD TV Model ST-42KC42F "CCFL lamps not switching on"

              Hi Lotas, all the caps check out fine. I still have to check the ballast transformers but I wanted to know if you have ever scoped the PWM input to the inverter. Mine is a permanent 5 volt. Not pulsed. Without a frequency present I wanted to know if maybe that's why the OZ964 shuts down. I even bypassed the OZ964 IC using a resistor and led diode like I saw done online but the inverter still shut down.

                Comment

                • lotas
                  Badcaps Legend
                  • Jan 2016
                  • 4555
                  • Russia
                  #5
                  Re: Sinotec LCD TV Model ST-42KC42F "CCFL lamps not switching on"

                  A common problem is poor soldering of this inverter, smd capacitors lose their resistance or the resistors in the piping of this OZxxx are open, you need to carefully check all the components and solder them.
                  Attached Files

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