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H/K receiver repair: looking for help with diagnosis

Discussion in 'Electronic Repair' started by sdmorr, Jul 3, 2007.

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  1. sdmorr

    sdmorr Guest

    I was given a free Harman/Kardon AVR 40 receiver, with a caveat: only the
    center channel works. Also, the unit puts itself into standby after a short
    time. (The first time you turn it on, it'll last for maybe two minutes;
    after that, it generally goes into standby after less than a second. Cycling
    the power repeats this cycle.)

    It's not under warranty, so I cracked the thing open. Didn't see any obvious
    blown fuses or leaking caps, but that's all I know how to look for. What
    other components might be the culprit? I'm interested to learn how to check
    the various components and figure out where something's gone wrong.

    Is there an easy way to determine whether this is, for example, a toasted
    power transistor vs. some other component? (As a physics student, I have
    access to all sorts of equipment--I just don't know what to look for.)

    Thanks,

    Scott Morrison
     
  2. Dave

    Dave Guest

    Grab the service manual at:

    http://manuals.harman.com/HK/Service Manual/AVR40 sm.pdf

    There's not much that's easy about these amps, sorry. If you don't have a
    bit of diagnostic skill and background in electronics theory, you're likely
    in for a tough time. You may want to go pay $20-30 for a shop to diagnose
    it for you. Depending on what they come back with, it may be quite
    inexpensive to repair.

    As far as the amp shutting down, there are two protection circuits in this
    amp which will shut it down completely, one monitors current through
    Q215/Q216 and powers down via Q217/Q218/Q219 if current is excessive. You
    may wish to check R227 which determines this cutoff current.

    There is also a thermal protection circuit with two posistors, one in the
    transformer winding and one on the heatsink. Either of these going
    high-temp (or being faulty) would shut down the amp.

    This amp looks like a bastard to diagnose. I count about 30 IC's and
    several hundred passive components.

    You ARE using 8-ohm speakers, and only one pair, right? When you say only
    the center channel works, do you mean that neither the front L/R nor the
    rear L/R outputs function?

    Dave
     
  3. .. You may want to go pay $20-30 for
    Man. I charge 55.00 for diagnosis on surround receivers, and I'm about to
    raise it to 75.00 , at least on the flagship models.

    Mark Z.
     
  4. Sounds like maybe 2 separate problems.

    The center channel only problem relates to the DSP board in my mind anyway.
    The problem of going back to standby reminds me of a couple HK's I've seen
    which had a signal sized diode open up on the display pc board. The diode
    was supposed to drop .6 volts in series with the 5 volt line to the micro.
    When it opened up, the micro lost power. One of the involved units was
    intermittent like yours.

    Mark Z.
     
  5. Dave

    Dave Guest

    And you probably still lose money on at least half... people just choke at
    spending upwards of $50 for "nothing". They feel better about spending $500
    for a new unit than $200 to repair an "old" (1 or 2 year-old) unit.
     
  6. Yeah, people aren't even bringing in that many regular sized surround
    receivers anymore. It's pretty amazing the features and relative quality you
    can get in a new "mid-fi" HT receiver in the 250.00 to 350.00 range.
    Yamaha and Onkyo, and other middle range models all seem to be adequate for
    any but the real tweakoids out there.
    And when they do come in, it's often the result of a lightning strike which
    may be simple or a real can of worms and often no way to tell for sure until
    you are way too deep into it...

    I'm the audio guy in my shop but more and more I'm working on TV's.

    Mark Z.
     
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