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Peavey Windsor - Oscillating at effects loop?!

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  • #1

    Peavey Windsor - Oscillating at effects loop?!

    Hey guys,
    I have a Peavey Windsor here that I've spent quite a few hours trying to debug a high frequency oscillation that varies frequency with the master volume knob.

    At first, I thought the oscillation was in the PI, cause it goes away with the PI driver tube removed. Some other things I noticed:
    - It goes away with C17 lifted
    - It goes away with C7 lifted
    - It stays with power tubes removed.
    - It stays with C16 lifted
    - It stays with "texture" knob disconnected
    - It stays with "presence" and "resonance" disconnected.


    So I basically checked everything in the PI, and tried a few different tubes - with no change. Then I decided to just use a cable to bridge the effects loop - and it went away!

    I gave the jacks and their shorting contacts a clean, but it still happening - and I have no clue why. Has anyone experienced this one before?


    SCHEMATIC: http://bestnetworx.com/uploader/file...0SCHEMATIC.pdf
    Tags: None
  • #2
    Perhaps one of the loop jacks is defective, or the solder connections have broke loose.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      Originally posted by g1 View Post
      Perhaps one of the loop jacks is defective, or the solder connections have broke loose.
      Hey G1, I replaced the jacks and the problem persists.

      The only conclusion I have is that the problem goes away when running through the effects loop because of the additional transistor buffer that's now between the master volume and the PI. So that buffer is stopping the oscillation. What's causing the oscillation though is eluding me big time.

      Comment

      • #4
        We went through this over at the PV board some years ago. I forget how we resolved it, but the loop drive itself is feeding back. Poke a plug in the return jack, does that stop it?
        Education is what you're left with after you have forgotten what you have learned.

        Comment

        • #5
          Originally posted by Enzo View Post
          We went through this over at the PV board some years ago. I forget how we resolved it, but the loop drive itself is feeding back. Poke a plug in the return jack, does that stop it?
          Hey Enzo, just putting a plug in the return jack doesn't seem to stop it.

          I've put power tubes into this thing and it's oscillating like -crazy-, with the master volume controlling the frequency. To be honest, I'm a bit tired of attempting to fix this amp - so I might give up for a while. I may try calling Peavey to see if their techs are familiar with this problem.

          Comment

          • #6
            Might it just be tired power supply caps?
            Try bypassing each in turn with a smaller value good one.
            Low voltage supplies too.
            My band:- http://www.youtube.com/user/RedwingBand

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            • #7
              Originally posted by pdf64 View Post
              Might it just be tired power supply caps?
              Try bypassing each in turn with a smaller value good one.
              Low voltage supplies too.
              Okay, I tried bypassing each capacitor exactly as you said.. including the 24, 12 and 6.3V DC lines and even the bias supply. No luck on any of them!

              However, when I was messing around with the amp this time, I noticed the oscillation was changed by moving the wires going to the primary on the OT (the primary is the side the tubes and B+ are going to right?). I checked the connection and everything, no problems there.. but yeah, the oscillation was affected by moving the OT's wires.

              I measured the OT primary, OL to ground on the taps and CT and 30 and 25 ohms from each tap to CT.

              Comment

              • #8
                Yes, that is the primary - probably blue, brown, and red wires.

                There is nothing wrong with the transformer, those wires have THE hottest signal in the amp, hundreds of volts worth. In a wired amp I would ask you to move the grid wires away from them. But on this amp, all you can do is keep those primary wires as far from any grids as possible. And can you twist them, before plugging them onto the board? A couple wires twisted , like heater wires in some other amp, might help.
                Education is what you're left with after you have forgotten what you have learned.

                Comment

                • #9
                  Originally posted by Enzo View Post
                  We went through this over at the PV board some years ago. I forget how we resolved it, but the loop drive itself is feeding back.
                  Could it have been here rather than at the PV board? Was a similar issue here, involving both hum and squeal, you were wondering about the pointy jack ground spikes (unfortunately not resolved):

                  http://music-electronics-forum.com/t25727/#post221342
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  ​

                  Comment

                  • #10
                    No, not the hum issue. There was some odd circumstance, that if memory serves involved the 1k resistor.
                    Education is what you're left with after you have forgotten what you have learned.

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