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Marantz PM 57

Discussion in 'Electronic Repair' started by Jack, Jun 6, 2007.

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  1. Jack

    Jack Guest

    My IA drops out every once in awhile. I hear what sounds like a relay
    clicking when this happens. I have the SM and it looks like it might
    be the "Speaker Relay which is controlled be an IC and some components
    that is called the "Speaker Protection Circuit." Has anyone had any
    experience with this and how this SPC works ?

    Thank You
    Jack
     
  2. Arfa Daily

    Arfa Daily Guest

    If it's a simple protect circuit, it only monitors for DC offset at the
    outputs. There is usually a resistor or diode network from each output, to
    some collective point that the chip monitors for average zero voltage. When
    the unit is initially powered, this chip also provides the turn-on delay,
    via a simple R-C timing network on one of its pins, to allow for the various
    supply rails to establish and stabilize, before the speakers are connected
    by the relay. This serves the dual purpose of preventing false triggers of
    the protect circuit whilst the rails are settling, and also of preventing
    major speaker thumps if the main rails don't come up exactly together.

    When they get a bit more elaborate, these protect circuits can also check
    for output stage overloads and total shorts, and the most sophiticated
    protect circuits also monitor AC supplies to the various rectifier circuits,
    as well as the DC outputs from them.

    If you hear a relay click, and the unit appears to otherwise stay powered,
    then almost certainly, it is the speaker relay you are hearing. The first
    thing is to listen whether the relay ever comes *straight* back on. If it
    does, then it could be something as simple as a bad joint on one of its coil
    pins - it's fairly common. If there is always a delay, then it's probably
    that the chip is sensing a real problem. It will always put the delay back
    in after an 'event'. The fact that the problem is intermittent, and there
    seems to be otherwise no signs of trouble with the performance or audio,
    might well suggest a simple bad joint, so the next move would be to run the
    amp with the lid off, and have a bit of a tap around with the butt end of a
    screwdriver. If this makes the problem occur, you can refine the location of
    the area, by switching to something lighter like a plastic Biro pen for the
    tapping.

    Other possible causes, depending on the sophistication of that particular
    protect circuit, are wire whiskers at the output terminals, and an
    intermittently defective speaker. It is unlikely (although possible of
    course) that there is any genuine component fault in the output stages being
    monitored. Most outputs, discrete or hybrid module, either work or don't.
    One slight rider on that though. If the amp uses an STK series hybrid, these
    can get intermittent connections where the pins connect to the ceramic
    substrate. This gives all the appearances of a bad joint that you just can't
    find, no matter how hard you look.

    Finally I have, in the past, had the timing cap mentioned above, go faulty
    or low value or poor ESR, which has resulted in intermittent erroneous
    operation of the protect circuit on various makes and models. Usually, there
    are other symptoms as well, such as the relay being reluctant or slow to
    kick in at initial power up. Hope this helps.

    Arfa
     
  3. Jack

    Jack Guest


    Hello Arfa,

    Many thanks for your reply. I will be removing the cover and will
    monitor some place with a
    DT scope. Sometimes you can be overwhelmed buy such circuits - they
    appear to be
    exotic but in further view they are simple. Will post my results in a
    few days.

    Thanks Again, Jack
     
  4. **It will most likely be the driver IC. STK3102, as I recall, for that
    model. They very commonly develop intermittant faults. The protection
    circuit rarely causes problems. When it does, it is almost always a timing,
    or decoupling cap. The protection ICs are quite reliable.
     
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