SMPS low voltage

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  • Sojke
    Senior Member
    • Apr 2017
    • 131
    • Serbia
    #1

    SMPS low voltage

    I was repairing an SMPS that had failed. I managed to get it up and runing but t he main voltage rail is 1/3 less than what it is stated at around ~24.3V.

    I have marked the components I have replaced just in case with red dot, those with red and green have failed. All SMD components measure well.

    The amplifier also failed in this device, it is powered by the supply that now I have at low voltage. I have replaced amplifier hybrids and the same thing happens either way. With amp it is 22V with out it is 24V.

    I suspect LM3850 but I can not test it.


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    • R_J
      Badcaps Legend
      • Jun 2012
      • 9535
      • Canada
      #2
      Re: SMPS low voltage

      What is your line a/c voltage? 230vac? I suspect the pfc boost circuit is not working.
      What is the PFC_VCC voltage? It should be around 16~18v. You could have a bad electrolytic on the pfc_vcc line, CE521, CE520, CE513.
      Also check the source resistors on the pfc mosfet's
      Last edited by R_J; 05-18-2020, 11:12 AM.

        Comment

        • Sojke
          Senior Member
          • Apr 2017
          • 131
          • Serbia
          #3
          Re: SMPS low voltage

          Spot on, VCC was 10.7V.
          Now there is +32V , but no sound from amplifier, all outputs are at 16V (both + and - side on all channels). Without TAS5538 outputs are at 350mV (Like in working one I had). I exchanged it 3 times with new ones from China and same thing every time.....

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