Connect with us

Attempting audio amp (Pioneer A-9) repair - any tips?

Discussion in 'Electronic Repair' started by andrew queisser, Nov 23, 2005.

Scroll to continue with content
  1. My trusty Pioneer A-9 has a problem on the right channel. If I listen on the
    headphones the sound is great, on speaker the sound is fuzzy but only on the
    right channel. The headphone ouput actually comes from the speaker output,
    not the preamp, so it seems that fundamentally the signal is getting through
    but under load it gets fuzzy. I can hear the distortion in the headphones
    when I click the speakers on but when I click the speakers off the sound is
    fine.

    The power supplies feed both channels in parallel so I doubt it's a power
    supply problem.

    I have a scope and a little audio probe gizmo that lets me listen to audio
    signals with an earphone. I can generate signals from my PC and feed them
    into the amp, if that helps. I also have the schematics so I can trace the
    signal but I'm not quite sure how to go about it.

    Any tips?

    Thanks,
    Andrew Queisser
     
  2. jakdedert

    jakdedert Guest

    One tip might be to reverse your speaker connections left to right and
    see if the problem remains in the same channel.

    It might not be a problem with your amp....

    jak
     
  3. Maybe it's a problem with the speaker?
    Have you tried switching the speakers?
    Carefully checked the speaker wires?
    Maybe the right speaker is bad.
    Maybe there is a frayed wire. maybe even a short at one of the ends.
    A frayed end just barely touching the other wire could be causing the
    problem, and if it is allowed to continue like that you'll blow the channel.
     
  4. Guest

    I did switch the speakers and the problem stayed on the right side.

    thanks,
    andrew
     
  5. default

    default Guest

    Try feeding a 400 HZ square wave into it and see what comes out
    (compare channels)

    If the leading or falling edges look funny describe the bad one.

    I'm not familiar with the A-9 but some discrete amps used to use fuses
    on the speaker terminals so if the amp shorts it won't set fire to the
    speaker cones. The fuse holders or fuses would corrode and cause
    distortion when the current was higher (speakers versus phones).
     
  6. Asimov

    Asimov Guest

    "andrew queisser" bravely wrote to "All" (23 Nov 05 12:28:03)
    --- on the heady topic of "Attempting audio amp (Pioneer A-9) repair - any
    tips?"

    aq> From: "andrew queisser" <andrew.queisser at hp com>
    aq> Xref: core-easynews sci.electronics.repair:349443

    aq> My trusty Pioneer A-9 has a problem on the right channel. If I listen
    aq> on the headphones the sound is great, on speaker the sound is fuzzy
    aq> but only on the right channel. The headphone ouput actually comes from
    aq> the speaker output, not the preamp, so it seems that fundamentally the
    aq> signal is getting through but under load it gets fuzzy. I can hear the
    aq> distortion in the headphones when I click the speakers on but when I
    aq> click the speakers off the sound is fine.

    aq> The power supplies feed both channels in parallel so I doubt it's a
    aq> power supply problem.

    aq> I have a scope and a little audio probe gizmo that lets me listen to
    aq> audio signals with an earphone. I can generate signals from my PC and
    aq> feed them into the amp, if that helps. I also have the schematics so I
    aq> can trace the signal but I'm not quite sure how to go about it.

    aq> Any tips?

    aq> Thanks,
    aq> Andrew Queisser


    Connect the scope to bad channel and inject a 400Hz triangle wave test
    signal, increasing the amplitude until the buzz is obvious. If the
    triangle wave is mishaped at the zero crossings then it is crossover
    distortion. This is most likely due to a defect of idle bias
    circuitry. If instead the triangle wave is clipped then positive
    clipping means a defective NPN output, and negative clipping a
    defective PNP output. Perhaps intead one of the drivers or supplies.
    A triangle wave is a good test signal because it consists of straight
    lines and it also stresses the output devices less than a sinewave or
    squarewave.

    A*s*i*m*o*v

    .... A stereo system is the altar to the god of music.
     
  7. I've seen mainly controls and switches need cleaning on these, including the
    speaker switches, and speaker relay.

    Mark Z.
     
  8. Chuck

    Chuck Guest

     
  9. jakdedert

    jakdedert Guest

    My thoughts exactly....

    jak
     
  10. rpbc

    rpbc Guest

    Yes, the outputs, one or the other on the affected side is open in such a
    way that allows the driver to power the output. The driver will pass
    enough current to operate the headphones, but not the speakers... which
    quickly clip the output of the driver. Check the outputs with a vom and
    you'll find one of them open. Replace in pairs, adjust bias, and offset
    voltage measured at speaker outputs. Check associated resisters for
    opens. Power up with variac, watch current rise at low voltage to ensure
    everything is good before applying full line voltage.
     
  11. Guest

    Yup, I've been unable to use the balance and tone controls due to dirt
    or corrosion on this amp for years. I can only run it in
    "straight-through" mode where it works fine.

    And after cleaning the connector that goes to the power amps everything
    works fine again.

    Thanks!
    Andrew
     
  12. Guest

    Thanks to everyone who replied. My amp is working again but the
    solution was a bit anticlimactic. I took everything apart and when I
    hooked up the scope the signals looked ok and the sound over the
    speakers was clean again.

    I think the problem was in the connector between the main amplifier
    board and the board with the power amp for the right channel. After
    cleaning the connector everything started working again.

    Andrew
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-