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Marshall AVT 20 Hum

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  • #1

    Marshall AVT 20 Hum

    I am not experienced with these types of amps, or this symptom -

    At power-on, there is an immediate, fairly loud hum (240Hz would be my rough estimate of the frequency) that is the same volume regardless of any panel settings.

    Power transformers is putting out approx. 13.5 VAC on each leg.

    Tube tests ok.

    Speaker good

    The board says AVT 2 - 60 - 00, looks to be from 2003

    Should I be replacing filter caps or.......?

    Any ideas appreciated.

    RWood
    Tags: None
  • #2
    You should be finding out what is causing the noise, not guessing what parts to change.

    So is there DC on the speaker? Is there excess ripple on any of the power rails.

    But before any of that, is either end of the reverb open? Briefly touch the tip of each reverb cord to chassis. Does that stop the noise?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Fair point. Here's what I got:

      Grounding the hot pins on the reverb made no difference to the hum.
      Speaker has 13vdc across it.
      Not sure about the ripple on the power rails - would that be measured with a volt meter or a scope?

      Thanks for posting your schematic over at DIY Audio.

      RWood

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      • #4
        OK, DC on speaker means output stage is blown. With that happening, you can't see any ripple anyway.

        This is a simple amp, that 5-legged TDA2050 on the heat sink IS the whole power amp. I'd be replacing it. Oh it runs off the +/-20v rails or whatever voltage they are, and one of them could be screwed, but I doubt it. I'm betting my lunch money on the 2050.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Thanks Enzo. The part is cheap enough, and accessible without removing the board - woohoo.

          I appreciate the guidance.

          Comment

          • #6
            Oops....my bad - I thought the IC was in a socket. That would have made life too easy!

            My next question - in this circuit are C71 and R53 the Zobel network and would I be wise to replace at least the cap while I am in there?

            RWood

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            • #7
              If the resistor is not burnt out, in my experience, the cap will be fine. VERY seldom I have to replace those caps.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                A new TDA2050 was exactly what it needed; amps works fine now. Thanks, Enzo, for making it easy for me to fix.

                RWood

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