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Harman Kardon amp died - PS caps?

Discussion in 'Electronic Repair' started by Scott Kelley, Jun 15, 2005.

  1. Scott Kelley

    Scott Kelley Guest

    I recently bought a Harman Kardon HK870 amp from a guy. It worked fine for
    a minute or two at moderate power level, then blew fuses & quit. Replaced
    the two fuses on one channel of the power supply (right off the transformer,
    before the bridge). The new fuses went out immediately, heard lots of 60 hz
    from the speakers for about half a second before they blew.

    It's possible this amp had not been used for awhile. I'm thinking about the
    capacitors having been sitting without use for awhile . . . would they tend
    to short out due to lack of use??

    Scott Kelley
  2. Terry Bull

    Terry Bull Guest

    Its unlikely its duff caps as they would be short from the moment of turn on
    if faulty, so nasty noises would be there from the start.

    It sounds like one channel has ' let go' ie the main 2 output transistors
    blown... or perhaps you could be lucky & just the main rectifier pack has a
    short diode.
    The load your speakers present may have been a factor if they're lowish
    impedance, but also its worth checking to see if any of the big output trs
    have been replaced recently ( shiny solder on pins~) indicating a recent
    (incorrect) repair.
    I suppose some component value changes could have upset the amps drive
    bias/offset thus causing it to blow up.

    Quite a few factors really, but it would have to have been left stood for
    quite a few years (5+) for time to be a contributary factor

  3. Scott Kelley

    Scott Kelley Guest

    Thanks for the feedback. Looks like I have one blown output transistor.

    Having a tough time finding a replacement - Mouser & DigiKey Nothing - MCM
    does nave an NTE replacement. The manual calls for replacing with similarly
    'selected' part, either 2SC755 "O" or "R". The parts in the amp are marked
    "O" - the NTE parts are not so marked. Any other sources?

    Anybody have a 2SB755-O lying around?

    No previous repairs. HK did send me the manual, so I will be able to set it
    up properly.

    Scott Kelley
  4. DaveM

    DaveM Guest

    Are you sure about the 2SC755??? My NTE software crosses that number to
    NTE123A which is a small signal transistor in a TO-18 case. Definitely not
    an output transistor.

    Dave M
    MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
  5. lubbie

    lubbie Guest

    According to my manual there is a 2sb817 & 2sb863 that has the same
    specs as the 2sb755.. Confirm package and pinout though..

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