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Peavey Classic 30 signal bleed at zero volume

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  • #31
    Originally posted by nickb View Post

    That tells us it's not coming from the U1A stage so that is progress. But you have an anomaly. The signal is still bigger ("nothing changed") on the U1A end of R20 than the V1 and that is with the resistor floated at the U1A end? You have to start lifting more (associated!) things to eliminate them. Then what is left, however improbable, is the culprit.
    At this point I don’t know if you are encouraging me or screwing with me, lol! This project has my head all messed up.

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    • #32
      I'm not screwing with you at all.

      What I was trying point out was that you said nothing changed. Earlier, the V1 end of R20 had 7mV while U1 end had 20mV. Is that still true with the U1 end of R20 lifted? I'd find that hard to believe unless there were some kind of HF oscillation present or the ground changed. So what really happened? I don't understand why if the signal was bigger at one end and it's now lifted that you still have exactly the same problem.

      When measuring small voltages the length of the scope lead and the choice of ground are important. I suggest you use R18/C18 as your ground and keep the scope ground as short as you can.

      I guess connecting V1B grid the the junction of R18/C18 is in effect the same as lifting all the components going to the grid provided the link is er, well, short! So try that again with this ground.

      Can we confirm another couple of things That is is the clean vol, pre and post are all zero you still get the bleed through? Also, is it true that in this situation the channel selection has no effect?

      Experience is something you get, just after you really needed it.

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      • #33
        Originally posted by nickb View Post
        I'm not screwing with you at all.

        What I was trying point out was that you said nothing changed. Earlier, the V1 end of R20 had 7mV while U1 end had 20mV. Is that still true with the U1 end of R20 lifted? I'd find that hard to believe unless there were some kind of HF oscillation present or the ground changed. So what really happened? I don't understand why if the signal was bigger at one end and it's now lifted that you still have exactly the same problem.

        When measuring small voltages the length of the scope lead and the choice of ground are important. I suggest you use R18/C18 as your ground and keep the scope ground as short as you can.

        I guess connecting V1B grid the the junction of R18/C18 is in effect the same as lifting all the components going to the grid provided the link is er, well, short! So try that again with this ground.

        Can we confirm another couple of things That is is the clean vol, pre and post are all zero you still get the bleed through? Also, is it true that in this situation the channel selection has no effect?
        When I said nothing changed, I simply meant that there was still bleed. I admit that I didn’t take precise measurements to compare to my earlier measurements; I let my frustration get to me and I wasn’t thinking systematically.

        I need to take a short break from this amp, but I will go back and get better measurements. I can tell you that there is still a significant signal at the input of U1A, and still signal at the grid of V1B, with R20 lifted on the U1 side.

        Also, I wanted to clarify what you said about the grid of V1B. Do you mean for me to short the grid to the ground where the cathode resistor and bypass cap are connected?

        And yes, all of the the controls you mentioned are zeroed, and channel switching has no effect when the controls are zeroed.

        Thanks again for your input, and I hope you are well.


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        • #34
          Then I have to conclude the PCB itself is bad.
          Experience is something you get, just after you really needed it.

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