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Magnetone Help Needed

Discussion in 'Amp Tech Center' started by String Tree, Dec 1, 2020.

  1. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    image.jpeg This is a late 60's Audio Guild Amp but it was built by Magnetone under License.
    It has two input channels and no Master Volume.
    Channels 1 and 2 have separate 1/4" inputs and a third input that can be switched between those two channels.

    Channel 1 works fine.
    Channel 2 has the faintest of sound from it, even at full volume.

    Front panel
     
  2. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    View from behind Channel 2 Volume and Tone Pot.

    image.jpeg
     
  3. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    View from behind the InoutJacks and Switch.

    Switch Issue?
    I can hear changes in sound, but just barely, when Volume 2 is turned up.
    I looks to me like that signal has to go through that switch.

    Tubes are new, channel 1 is loud and clear.
    Any help would be appreciated, thanks in Advance!
    -ST

    image.jpeg
     
  4. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    Couldn't help it.
    This caught me off guard and cracked me up.

    image.jpeg
     
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  5. BigDaddy23

    BigDaddy23 Tele-Holic

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    Have a good look at the connections on the jacks and the switch. One of those jack mounted resistors could be the problem (they look to be 2.2meg) or the actual grid stoppers, if any, on the board. Some shots of the rest of the board would be handy - follow those x2 green wires coming off the switch.

    Do you have a schematic you are using?
     
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  6. Jon Snell

    Jon Snell Tele-Holic

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    Solid state? Looks like a PCB in there!
    Valve?
    Model name/number?
     
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  7. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    I don't have a Schematic, can't find one.
    Audio Guild Corp was owned by a guy named Bonham. He worked for Lowrey Organs and then Magnetone. He invented a style of Vibrato that is closer to an Analog Chorus.
    He struck out on his own for a very short time.

    Audio Guild Corporation.
    Bonham 120.
    That is all I can find on it.

    Tube Pre amps, Tube Power.
    Kind of an odd bird.

    Two 12ax7's, One 6GHS (?), Two 7591's.
     
  8. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    More pics coming.
     
  9. elpico

    elpico Tele-Afflicted

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    Put a signal in and measure AC voltage along channel 2.
     
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  10. Dan R

    Dan R Poster Extraordinaire

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    "Manufactured by beautiful girls in Milton, Wis. under controlled atmosphere conditions." That's worth the price of admission right there.
     
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  11. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    Tube Types

    image.jpeg

    Fisheye view of everything.
    As you can see, they tucked things in like nobody's business.

    image.jpeg

    The Green wires from input jacks one and two go to the first 12ax7.
    image.jpeg image.jpeg

    Thanks again.
    I only have a small amount of time I can give it on the evenings.
     
  12. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    What kind of signal?
    Test tone generator at -20 db?

    Measure between which two points?
     
  13. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    I posted some more Pics for you.
     
  14. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    I posted some more pics.
    -ST
     
  15. BigDaddy23

    BigDaddy23 Tele-Holic

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    Try measuring the resistors on the jacks. They look to be 2.2meg. Maybe one of those is open?

    I would then look at tracing the signal through from the jack tip through the circuit on the 2nd channel if you have the capability to see where it drops off.
     
  16. Jon Snell

    Jon Snell Tele-Holic

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    Check the anode load resistors.
    The grid leak resistors; 2.2M, won't affect the lack of sound, just the quality.
     
  17. elpico

    elpico Tele-Afflicted

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    A steady one with a frequency in the audio band. 1kHz around 100mV is traditional but anything will work. Clip the ground lead to the chassis and use the other to measure the ac voltage along channel two's signal path.
     
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