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Yamaha DSP Amp has hum and low sound on one channel

Discussion in 'Electronic Repair' started by ElectroNovice, Jan 6, 2006.

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  1. I have a Yamaha amplifier that's about 5 years old. The right channel
    is normal, but the left channel has a low hum and significantly reduced
    sound volume/quality.

    I've tried the following:

    Switched speakers- Not a speaker problem.
    Switched from A to B speaker output- no change.
    Tried multiple audio sources and input jacks- Not a source or input
    jack problem.
    Tried headphones- Same results as speakers. Hum and low volume, low
    quality sound.
    Tried changing DSP program modes- hum stays the same in bad channel,
    sound changes as it should in working channel.
    Tried adjusting all knobs including balance- no effect other than as
    Tried all switches off/on- also no effect other than as expected.

    The hum exists even if no audio source is connected to the unit.
    The hum does not increase or decrease when I move the volume knob.
    The tone of the hum changes when I switch into the mode that lets you
    change the volume of five speaker output channels (front, rear, center)
    and when I switch from one channel to another.
    Adjusting the volume of channels otherwise has no effect.

    I am a newbie to electronic repair but can read and follow directions
    and am willing to use this amp to learn on. I don't have any test
    equipment but would consider buying or borrowing it.

    Any help would be appreciated!

  2. JVC Dude

    JVC Dude Guest

    Giving the model number would be useful.

    I would suspect probably an edge connector problem or dry joint on a
    board.(after the volume control so follow the leads from that to the tone
    board or main board).
    Its unlikely to be the amp channel itself or the amp protection would have
    detected a problem and cut out.

  3. Other than a bad connecting cable or two, your chances of fixing this
    yourself are probably pretty remote. Yamaha's are nice pieces, but even many
    experienced tech get their comeuppance on these. Yamaha's are rather
    complicated and difficult even for the professional.

    Mark Z.
  4. Bobscar

    Bobscar Guest

    Sounds like a leaky capacitor in the left channel of the amplifer. The
    capacitor may be a bypass filter component that would cause signal
    attenuaition by either 1/ dragging the supply voltage down if in power
    supply circuit, or 2/ causing loading of previous stage of amp by
    bypassing/shunting the signal to ground if found connected across input
    of each amplifer stage esp. the final power stage.
  5. Thanks for the tips. Sorry it took so long to reply...

    The model is a Yamaha RX V795.

    I will poke around a little (carefully) and see if I can find anything
    obvious. Thanks...
  6. I have some used Metcal gear including the Net Power Meter (NPM-50) and
    the MX-500P. When I connect the NPM in line with either of two
    handpieces, it shows power levels between 5 and 7 watts. I have not
    seen it go higher or lower.

    Question: is the power supply output low or is the meter faulty ?

    While it is possible to solder with a handpiece, I am not overwhelmed
    with the ability to deliver soldering heat. I would guess it higher
    than 10W but not 40W. This suggests that the NPM-50 is in error.

    The MX-500 power supply draws 40-50 watts from the AC line (using an AC
    wattmeter between the power suply and the wall socket). The power
    consumption does not seem to be related to what the tip is doing. I.e.
    AC input power does not obviously go down when the iron is idling and
    up when it is heating the work. This suggests that the MX-500 is not up
    to snuff.

    If the MX-500 could only put out, say 7W, then the iron would be
    getting some power but it would fit with most all the symptoms. What
    can I measure to know what's up ? Are there some common points of
    failure on either the MX-500 or the NPM-50 that I should look at ?


  7. Somewhere in the US Patent index their is a circuit for the Metcal iron.

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