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H. H. Scott 420A integrated amp weird buzzing issue

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  • H. H. Scott 420A integrated amp weird buzzing issue

    Hi everyone.

    Im having a weird buzz issue with this, other than that, great vintage amp.

    I'm getting a buzz from phono input, its not the turntable, i tested it on another set up,
    Here is the weird thing..
    When i dont connect anything to aux, tuner & phono inputs turning the input selector i get a sligt buzz from aux and tuner input and a hiss from phono, all getting slightly louder as i turn the volume up. Also i can hear it on headphones too.
    When i connect my TT to phono input all other inputs are 100% buzz free except from phono but no hiss this time just the buzz which gets louder as i turn up the volume and headphones output are buzz free except when i turn selector to phono which results in buzzing.

    Ground loop on the phono stage?

    I'm attaching the schematic if anyone willing to have a look and point me where to look.


  • #2
    On phono, we must not only have the grounded shielded cables, but there must also be a separate ground wire from the TT chassis to the amp chassis. You have that?
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      Hi Enzo,
      Thanks for looking into this.

      TT is a stanton St-150 which is internally grounded.
      It has a switch for ground lifting which only makes things worst.


      • #4
        How can it internally ground without a ground wire? I may be missing something, but that would seem to mean it can short its chassis to its cable ground or not. For what it is worth, my experience is I never had a stereo preamp or receiver that didn't have the separate chassis ground screw on the rear for this purpose, and I never had a TT that lacked a ground wire. Having been in the pro audio game for decades, I have services countless Technics SL1200 for DJs, and they have the separate wire.

        Touching a length of wire from one to the other would answer the question.
        Education is what you're left with after you have forgotten what you have learned.


        • #5
          The amp does have a ground screw for TT on the chassis. I did use a cable to touch TT and amp chassis but the buzz it's still there. I also hooked up my old technics sl bd20 TT which have a ground wire with no luck. Both TT works flawlessly on my other integrated amp. The stanton it's the one in the pic attached


          • #6
            40 year old amp => possibility of corroded or loosened ground points. Unscrew and re-tighten every ground bolt/washer/nut you can find. Even better, remove them and clean up the chassis mating surfaces with something like Caig DeOxit D series in the tiny bottle with the brush in the cap. That stuff's for metal that isn't gold, isn't plated, and doesn't use copper as a base metal. It's really strong. If you have doubts about the metals you're going to apply it to, check out for guidance on which product to use. Also check any internal cables that contain ground wires and are detachable. Un-mate them and clean those as well. I've seen Molex connectors whose pins/sockets were almost white with corrosion. Verify the power cord is to spec all the way back into the amp. If the power plug is non-polarized, try plugging it in the opposite way. If it *is* polarized, verify the socket is wired to spec, including no funny stuff like ground swapped with neutral, or tied together... all that stuff.


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