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Marshall AVT50 Guitar amp repair

Discussion in 'Repairing Electronics' started by Gillieshill, Sep 20, 2010.

  1. Gillieshill

    Gillieshill New Member

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    HI,

    I have a Marshall AVT50 which has stopped working. The speaker has been blown but there are also other problems. I can get output from it by plugging a CD player into the CD in jack and unsing headphones in the headphone socket. Both of the clean and drive channel LED's light up but I only get a slight hum from the drive channel through the headphones. The main fuse is OK.

    Clearly I need a new speaker, but there is obviously another issue. The guys at Marshall that I've spoken to (parts dept) suggested that the power stage of the amp could be blown. Does anyone have exoprience of this amp/problem?

    Richard.
     
  2. krytenrobot

    krytenrobot New Member

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    Last edited: Sep 20, 2010
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It almost certainly won't be class A, and 100% certainly not if it's transistor or IC.
     
  4. dave

    Dave New Member

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  5. rfs100

    rfs100 New Member

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    Man...Tghis thing has Gotta be Tubes!
     
  6. rfs100

    rfs100 New Member

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    Sory Nigel, didnt see your post before responding...kinda redundant what I said
     
  7. Gillieshill

    Gillieshill New Member

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    Marshall AVT50

    Guys,

    Tha amp is a hybrid. It has 1 tube in the pre amp and a solid state power amp section. Other advice that I've had suggests that it's the power amp which has gone and taken the speaker with it. Any views or comments on this?
     
  8. ben7

    ben7 Member

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    Maybe just a bad filter capacitor causing it to hum, or maybe a stressed out tube that has gone out of spec.

    I don't have much expertise on tube amps though.
     
  9. Torben

    Torben Well-Known Member

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  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    According to that circuit, the power amp is a TDA7293 IC - presumably that is blown, and took the speaker with it?.

    However, it's easy to tell - just stick a voltmeter across the speaker output, there should be no DC on it (or certainly only a few 100mV). If there's no DC, it's probably just the speaker that's died - try hanging any old speaker on just to try it (get one out of an old TV or stereo).
     
  11. flounder

    flounder New Member

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    I have the same head.I'm havin problems with it playing out of the cabinet. It turns on fine. If I plug headphones into the headphone jack it sounds perfect but nothing comes through the speaker output. I test the speaker cabinet with another amp an it worked fine. So given the evidence from troubleshooting, 1. It plays fine though the headphone jack. 2. The cabinet plays with other heads but not the Marshall. Therefore no signal or power is coming through the speaker jacks. Can anyone help?
     
  12. gearhead63

    gearhead63 New Member

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    I know this is an old thread, but here's something to be added.
    I have an AVT50H that wouldn't power a cab. The TDA7293 was bad. The effects send worked fine, no headphone output.
    I replaced it, checked components on the power board, put it back together and found that it would intermittently power a cab.
    The headphone jack worked fine. I cleaned the contact surfaces on the
    headphone jack and slightly tensioned (bent) the contacts with a jack installed in it and now it works great!
    Remember, the headphone jack breaks the circuit going to the speaker
    outputs and if it has bad connections/broken solder joints then no power will go to the speaker.

    So if you can use headphones on these amps, then the TDA7293 is ok and the problem is in the
    headphone jack or the circuit from the jack to the speaker output.
    NOTE: IF REPLACING THE TDA7293, BE ABSOLUTELY SURE YOU DISCHARGE ALL CAPACITORS IN THE AMPLIFIER AND ON THE POWER BOARD BEFORE REPLACING OR THE CHIP WILL BE TOAST BEFORE YOU EVEN TURN THE AMP ON!
    You can find the proper schematic at www.schematicsunlimited.com
     
  13. badass leadguitarist

    badass leadguitarist New Member

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    I have the ATV 50H kid of the same problem but I tracked mine down to a resister but the color bands are unreadable it is the only resister that different in size , does anybody have a copy of the schematics that list all the resisters / parts ?
     

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