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Thread: Fender Mustang II amp no output, looking for schematic

  1. #1
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    Fender Mustang II amp no output, looking for schematic

    Hi All fellow repair techs,

    I'm working on a Mustang II amp with no output, first I thought it would just changing the TDA7294 IC, which I did, still no sound.
    After looking up the IC's specs and getting some voltage info, I found the mute and standby voltages were missing.
    Using two 9v batteries I replaced the missing power to the IC, and the amp came on.
    Without a schematic this looks like a real tough repair to find the problem, does anyone have a schematic of this amp?

    Regards
    Dean C

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    If you do not have a schematic on hand, make one.

    Follow the mute pin back & draw it.

    Marshall uses a transistor & a zener diode/ capacitor to slower ramp up the voltage (anti turn on thump).

  3. #3
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    Hi jazz p Bass

    Thanks for the reply, IF it was a Marshall I could do that, the problem is its all surface mount components and has a large micro processor board mounted on top of it.
    The mute voltage is not coming from the switching power supply directly cause I've checked that out already.

    Regards,
    Dean

  4. #4
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    I haven't been inside one, but check the input jack it could trigger the mute as well as the headphone jack, etc.

  5. #5
    Senior Member Enzo's Avatar
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    So call Fender and ask for the schematic.
    Education is what you're left with after you have forgotten what you have learned.

  6. #6
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    Thanks for the idea Bill,

    I checked the jacks and they are good.

    Regards
    Dean

  7. #7
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    Thanks Enzo,

    But I am not an authorized service repair center for Fender.

    Regards
    Dean

  8. #8
    Senior Member Enzo's Avatar
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    And you don't always have to be. Call them, the worst that could happen is they say "no."


    Never think up reasons not to check something.
    Education is what you're left with after you have forgotten what you have learned.

  9. #9
    Supporting Member Jazz P Bass's Avatar
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    I would check the resistance of the mute pin to ground.

    If it's open, then something in the mute circuit is blown.

    At that point, find a 5 volt source & hook it up to the pin.

  10. #10
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    Hi Dehn, I have the same problem, how can I fix it? Thanks is my first time in this forum

  11. #11
    Senior Member vintagekiki's Avatar
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  12. #12
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    thanks vintagekiki
    But I already have the schematic of the amplifier, my problem is that I have no voltage in the mute or stand by, I have no sound at the output, I see that comes from the transistor Q100 2N4403 ,
    That's why I ask if that's the problem, sorry my English im Spanish

  13. #13
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    Hi Petard,

    After begging to Fender at least four times, someone did send the schematics.
    It was a bad 15 volt regulator IC either U107 or U108 I don't remember it was a couple years ago.

    regards
    Dehn

  14. #14
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    Ok Dehn thank you so much,i will watch IC ,i hope that's the problem, this IC is on the motherboard?

  15. #15
    Senior Member vintagekiki's Avatar
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    Mustang main board schematics page 6
    U107/ U108 positition on schematics A (left) 5 (down)
    The Dude likes this.

  16. #16
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    Thanks to all and find the problem of my fender mustang, resistance R160 1W completely burned, U170 also damaged, will soon sound again ...
    Without your help it would have been impossible greetings from Spain

  17. #17
    Senior Member vintagekiki's Avatar
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    If R160 (22R/ 1W) burned check (replace) C142, C181

    Before any intervention first dismantle U108 (7915) and with an ohmmeter check for possible short circuit between -15V supply and the ground.

    If U108 (7915) burned check (replace) U108 (7915), D122, capacitors in -15V supply C145 ... C155, C183 (0.1uF) C144 (10uF / 16V)

  18. #18
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    Thanks for your help vintagekiki I am looking at the schematic and I see everything I have to replace ..in my city there is nothing I have to buy outside, I think the quality of this amplifier is bad they all burned

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