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PowerBook M4896 Adapter

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    PowerBook M4896 Adapter

    Hello,

    I've inherited a few PowerBook 3400C laptops, two of which are working and one potentially repairable, though may be used as spare parts. All have power supplies but only one power supply actually works. All of the adapters appeared in good shape, so I disassembled one of them in hopes of figuring out what was wrong. However, I haven't had any success thus far and I'm unsure of the circuit design so I don't want to go poking at it too much and end up on the AC side.

    I have tried different input AC cables but no matter the input, there is no output. I am attaching photos of the internals and hoping for some suggestions on where and what to test.

    Thank you for your time.
    Attached Files

    #2
    Re: PowerBook M4896 Adapter

    Going by a couple of date codes I'm going to guess it was built in May, 1997. That means you could have electrolytic caps that have dried out, going high ESR. Checking to see if the fuse is open is probably the first step.

    The manufacturer was Astec, a very good company. The topology is flyback, probably discontinuous. The PWM IC is a 3843, located on the bottom of the PCB. One critical component that can cause it to fail to turn on is an electrolytic capacitor, often around 47uF. This is typically charged after turn-on through a high value resistor from the DC voltage across the big electrolytic capacitor. If that small capacitor is high ESR or the high value resistor opens up, the 3843 won't get the Vcc it needs to start up. I would look at the small electrolytic cap between the big electrolytic and the transformer and the small electrolytic cap between the board edge and the white film capacitor. The high value charging resistor might be one of the two green resistor between the white film cap and the bridge rectifier.

    Given the age of this adapter I'd check to see if the output capacitors are still OK.
    Last edited by PeteS in CA; 11-26-2020, 08:36 PM.
    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.
    ****************************

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      #3
      Re: PowerBook M4896 Adapter

      Hello,

      It turned out that the small electrolytic capacitor between board edge and white film capacitor had leaked, though it was impossible to see until removing it from the board. Along the way I replaced output capacitors and the capacitor between the transformer and big cap. The latter capacitor was bulging on the bottom slightly.

      For future reference, because the values were difficult to see until I removed them from the board, the electrolytic capacitor between the transformer and big capacitor is 120uF 16V from Nichicon and the small capacitor at board edge is 4.7uF 35V. The three larger electrolytic capacitors near the output are 220uF 35V

      Thanks for the help.

      Steve

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        #4
        Re: PowerBook M4896 Adapter

        Thanks for the feedback! The flyback topology is pretty stressful for output capacitors, and at over 23 years old replacing them was probably a prudent move, assuming you used good parts.
        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


          #5
          Re: PowerBook M4896 Adapter

          Hopefully good parts. Mouser-sourced for everything except the 4.7uF 35V for which I had to salvage a close-enough used part until I have enough need to pay shipping for another order from Mouser or Digikey... which reminds me...

          Steve

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            #6
            Re: PowerBook M4896 Adapter

            Happened across this thread while browsing for information on recapping a Powerbook 3400c. While I was particularly interested in finding out whether it was worthwhile to proactively recap the logic board, I do in fact have a dead M4896 power supply. Several years ago I bought an M7332 "yo-yo" style adapter as a replacement, but it is neat to know I could refurbish the original power supply. Thank you for posting this info!

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              #7
              Re: PowerBook M4896 Adapter

              Steve, do you have any tips for opening these power supplies without damaging the case? I do not see any screws, so I assume it is all plastic latches.

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                #8
                Re: PowerBook M4896 Adapter

                psu's are usually welded shut,
                a vice can be used to squeeze the cases till the joins crack

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                  #9
                  Re: PowerBook M4896 Adapter

                  Originally posted by marciot View Post
                  Steve, do you have any tips for opening these power supplies without damaging the case? I do not see any screws, so I assume it is all plastic latches.
                  From what I recall, the Apple adapters are usually glued. I've tried variations of heat gun and pry tools to pry them apart with the least damage. Snap ring pliers work good too, once you have enough space to fit them in.

                  I could be misremembering, but I thought some of the Apple adapters had a screw underneath a label. But I've done enough tear downs on power adapters that I could be completely mixing that up with another brand.

                  Steve

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                    #10
                    Re: PowerBook M4896 Adapter

                    To anyone looking to this thread in the future, there is a single screw under the label, right about where the "M4896" text is located.

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