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Asus power adapter strange problem after repair

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    #1

    Asus power adapter strange problem after repair

    Well i have an asus laptop charger (19v 4,74a)for repair and it seems it is going to beat me

    Charger was dead...One of the diodes in diode bridge rectifier failed and killed the charger...I have replaced:

    -diode bridge
    -input filter inductor before diode bridge
    -blown big primary filter cap(was 100uf 450v ,i put 100uf 400v as didn`t have other at the moment)
    -blown fuse

    Now when i turn the adapter on it gives 19,4v for a second and than turns off...voltage just drop at the output...but...
    If i connect load(for test purposes put two 12v fans in series ) and then turn adapter on,it works well and it is steady at 19,4v...As soon as i disconnect the load charger turns off...reconnecting the load won`t trigger the charger to turn on again...
    I spent a lot of time here and have tested all componnents in feedback circuit and it seems to be ok...
    I`ve noticed that when there is no load, and no output respectivly ,voltage on a primary filter cap is oscillating from 506v to 515v which seems to be a little higher than it should be i think...
    Any toughts ?

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    #2
    Re: Asus power adapter strange problem after repair

    Probably a triggering switch Ic going faulty . It's the one in the middle of the second picture . Maybe something of TEA series . It could be also the mounted components around it . Soldering might fix the problem if it was only a case of bad joints due to heating ..

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      #3
      Re: Asus power adapter strange problem after repair

      I thought also it was the pwm but ...At the moment i posted the topic i found the culprit...
      Bad C3 block filter capacitor after diode bridge...
      I measured it before but i must have done something wrong...it was not shorted but it had 30nf capacitance ...bad filtering caused higher voltage on big filter cap and higher voltage on the input of pwm which probably caused pwm shutdown as protection measure...

      Comment

        #4
        Re: Asus power adapter strange problem after repair

        Originally posted by Djosla View Post
        I thought also it was the pwm but ...At the moment i posted the topic i found the culprit...
        Bad C3 block filter capacitor after diode bridge...
        I measured it before but i must have done something wrong...it was not shorted but it had 30nf capacitance ...bad filtering caused higher voltage on big filter cap and higher voltage on the input of pwm which probably caused pwm shutdown as protection measure...

        Nice catch

        Comment

          #5
          Re: Asus power adapter strange problem after repair

          Originally posted by Djosla View Post
          I thought also it was the pwm but ...At the moment i posted the topic i found the culprit...
          Bad C3 block filter capacitor after diode bridge...
          I measured it before but i must have done something wrong...it was not shorted but it had 30nf capacitance ...bad filtering caused higher voltage on big filter cap and higher voltage on the input of pwm which probably caused pwm shutdown as protection measure...
          Originally posted by jiroy View Post
          Nice catch
          With no load the peak voltage was probably high enough to trip a high line protection circuit. With a load that voltage was also loaded down.
          PeteS in CA

          Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
          ****************************
          To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
          ****************************

          Comment

            #6
            Re: Asus power adapter strange problem after repair

            Originally posted by PeteS in CA View Post
            With no load the peak voltage was probably high enough to trip a high line protection circuit. With a load that voltage was also loaded down.
            exactly...

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