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Vox AC15H1TV Noise Scream Issues

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  • Vox AC15H1TV Noise Scream Issues

    This amp has an odd issue. Owner said it is cutting in and out and also signal strength drops. Issues on both channels.

    I plugged a 1K signal into channel one. Played fine for about 20 seconds, then noticed the signal strength drop a bit, then a thumping noise started, followed by a screech noise. I removed the input signal (pulled plug out) and lowered volume channel one to zero.

    Screech continued, I touched EF86 tube and it got louder, like a scream.

    All tubes test good (I know that does not mean some may be bad).

    Any initial thoughts?

    Never worked on this type of amp and with these pins on the board. Are the pins filled with solder and the leads drop into the hole of the pin? How do you solder these?

    Thank you, Mark

    vox_ac15h1tv_ac15h1tvl_ver.1_sm.pdf

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  • #2
    Sounds like the EF86 tube is microphonic. The pins are known as turrets, common way to build a amp. Thumping sounds like bad filter caps.

    Comment


    • #3
      I think you got it on the filter caps. I removed all preamp tubes and still have thumping/crackling after warmup. If I chopstick the 22uf main filter caps I can create the crackling sounds.

      Did not find anyone selling cap kits for this amp.

      I have 22uF/500V F&T that are a tad large but I can place them in at an angle that I would like to use. I also have Ruby 10uF I can use in the preamp filters.

      And I need to order a EF86 tube.

      Any other thoughts?

      Thank you

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      Comment


      • #4
        Did you check the solder joints on the caps that crackle when you chopstick them?
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #5
          Solder connections on the main caps were good. I replaced them with the 2 F&T 22uF. Owner wanted new filter caps anyway. Problem has not gone away.

          I should have mentioned before that taping any component on the PS board creates the cracking noise. This includes the board itself, the wires going to the tube sockets, the tube pins and tapping the tubes (2 EL84 and 1 EL81).

          All solder connections look good. Could the EL81 rectifier cause this? I would think a bad EL84 would only be isolated to its own tube connections and not the entire board being noisy.

          The board ground has good continuity to the rest of the chassis.

          Thank you

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          Comment


          • #6
            Try holding the tubes one at a time with a rag while tapping on the board. Something must be very sensitive for everything to trigger the issue like that.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #7
              Occasionally you can get what looks like a good looking joint on a turret but there's not a good connection to the component lead. Sometimes another look under magnification may reveal something suspect.

              Comment

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