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Yamaha DSP A-990

Discussion in 'Electronic Repair' started by steve, Nov 6, 2006.

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  1. steve

    steve Guest

    I'm hoping that there may be somebody with a 5 channel amplifier Yamaha
    DSP A-990. I like my old machine, 6 years old, a fantastic sound and
    video processor. Happened to short the outputs and blew most of the 14
    power darlington transistors. I have repaired with newer equivalents
    and all e-c-b voltages are OK, but 5 seconds after start, there is a
    safety relay that switches power off. I can't figure out why and what
    the sensors react on. Complicated electronics. Now I just wonder if
    someone happen to have such a machine and if there is also someone who
    have schematics. Yamaha don't not respond positively on my request, say
    they have no service on such a obsolete amplifier.

    Thanks!
     
  2. propman

    propman Guest

  3. Usually looking for a DC offset to protect the speakers. Maybe also
    oscillation. A scope should tell all.
     
  4. Not finding this model number, at least not on the Yamaha's US tech site.

    When the outputs blew, they probably took out one or more resistors,
    sometimes a 330 ohm or so which is between the emitters of the two driver
    transistors, or a series resistor going to the base of an output transistor,
    usually about 4.7 ohms, an emitter resistor, usually .22 ohms at 5 watts
    dual package, or, and this seems most likely to me, a resistor supplying B+
    or - to the collector of a driver transistor. Usually in the one to ten ohm
    range.

    Mark Z.
     
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