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Harman/Kardon AVR5500 (AVR520)

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    Harman/Kardon AVR5500 (AVR520)

    Hello, I'm sad

    When I turn on my AVR 5500 it shuts off immediately. According to the previous owner it only turned off occasionally. For me it never worked. So I opened it and saw the big 10000uF caps where bulging so much it lifted of the board and separated the glue. So I order all new Nichicon electrolytic caps for the front and the surround amp boards. Also replace te caps on the small board that have the small transformer. After I replaced them I have the same issue.

    One thing I noticed when disassembled I had 50V on one of the 10000uF cap but the second was 0V. Both 8200uF caps on the surround amp board had 50V as well. So I thought the bulging cap was so bad it was shorting.

    Now the same replaced cap has 0V again so something must be faulty there in the circuit. I can't see anything else that's blown. The current limiting resitors that goes to all of the lm7805/7905 and lm7815 that I seen people replace is measuring fine.

    Did a ohm reading on all of the transistors connected on the heatsinks. Don't know if that's wrong way or if you can check them like that. All of them seemed to pass thru to give readings on the ohm/diode setting.

    I'm a novice but trying to learn something as I go arround measuring and reading the schematic. But I feel that I need some help on what to measure or look for to fix this.

    Thank you for you time!

    #2
    Re: Harman/Kardon AVR5500 (AVR520)

    i would think there is a bridge rectifier for the main rails .. check for the +- voltages there .

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      #3
      Re: Harman/Kardon AVR5500 (AVR520)

      Thank you for your respond!

      Was thinking to check that too. But was also thinking how could the other cap be charged up if the recitifier is bad? The C481(10000uF) capacitor is connected to the + on the rectifier. The C481 and C482(10000uF) is connected together with the ground and the otherside of the C482 is connected to - on the rectifier.

      Could it be something further down the line that is grounding the C481 cap so it's getting discharged?

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        #4
        Re: Harman/Kardon AVR5500 (AVR520)

        Measured the C481 cap when turning on the amp. The cap charged up to 50V but when protect turned on it slowly discharged withing 30 sec. The C481 cap stays at 50V. So I guess it's something shorting?

        The plus side on the C481 cap is feeding the transistors and VCC out to 2 different terminals. The negative side is common to both caps so my guess it must be something down the line on the + side?

        How to you check transistors in a good way?

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