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Conrad Johnson PV5 Preamp Hifi problem

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  • #1

    Conrad Johnson PV5 Preamp Hifi problem

    I’m having a problem with a Conrad Johnson PV-5 Preamp. This is a Tube Preamp Hifi.

    The output has an unsteady DC voltage. Looking at the output with a scope when it’s idle, the voltage is moving sporadically between around +1 and -1 volt, sometimes more. Then with a 500Hz sine wave at the input, I see the sine wave moving up and down in the same way.
    Is this a bad signal capacitor? It’s happening the same on both channels so maybe that means it's a power supply problem.
    I’ve attached a link to the schematic, which is not very clear and doesn’t show values of resistors and capacitors.
    It’s a stereo amp so the schematic shows one channel. The other channel using the other halves of Tubes V5,6,7,8.
    Attached Files
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  • #2
    Is power supply A bouncing too? HAve you watched your mains voltage to see if it is hopping? Your B+ can move several volts for each volt the mains moves.

    Your sea-of-zeners regulator circuit might not be working well.

    And wher are you grounding your scope?
    Education is what you're left with after you have forgotten what you have learned.

    Comment

    • #3
      Is power supply A bouncing too? HAve you watched your mains voltage to see if it is hopping? Your B+ can move several volts for each volt the mains moves.

      Your sea-of-zeners regulator circuit might not be working well.

      And wher are you grounding your scope?
      Education is what you're left with after you have forgotten what you have learned.

      Comment

      • #4
        So, the power supply A doesn’t seem to be hopping. I tested the mains voltage with the scope and it seems fine. I tested the zeners (while in circuit) and they seem ok. Each showing around .700v in the correct direction with the diode test. But maybe I should take them out of circuit?
        I’m grounding the scope at the ring of the rca outputs, or on the chassis at the big capacitors negative side. The ground all seems well connected, around the chassis and the negative side of the power supply caps.

        So, I have discovered another problem on the phono channel. When I turn the volume up the output starts hoping and there’s a massive low frequency voltage - around 2Hz. With the Volume up half way it’s around 50v. With no volume it disappears and higher than half volume it keeps getting higher. Up to over 100v. I have scoped it at the volume pot and it’s present at the wiper of the balance pot and over where R20 & C22 connect. This low frequency voltage is also at A (plate of V4).

        Doesn’t seem to be a tube problem. I have switched out all the tubes.
        I have tested the cathode resistors and caps of V3, V4 and they seem fine.

        What to do next?

        Comment

        • #5
          Your meter cannot est a zener. it can check it as a diode, but all that tells you is that the thing has a forward drop like a diode. Your meter cannot put a high enough reverse voltage on it for it to exhibit zener action. Taking the part out doesn;t change that. But more important than that, if the A supply is not bouncing, then the zeners that regulate it must be working OK, right?

          You have V5,6,7,8 all in series feeding that output, so go back through the string of them and see if your levels are hopping throughout, or if not, then where does it start?
          Education is what you're left with after you have forgotten what you have learned.

          Comment

          • #6
            Your meter cannot est a zener. it can check it as a diode, but all that tells you is that the thing has a forward drop like a diode. Your meter cannot put a high enough reverse voltage on it for it to exhibit zener action. Taking the part out doesn;t change that. But more important than that, if the A supply is not bouncing, then the zeners that regulate it must be working OK, right?

            You have V5,6,7,8 all in series feeding that output, so go back through the string of them and see if your levels are hopping throughout, or if not, then where does it start?
            Education is what you're left with after you have forgotten what you have learned.

            Comment

            • #7
              Ah. Good info on zeners there. I don’t really understand the difference between them and regular diodes and when they are used. Anyway, that’s another day.

              So, I’m seeing the signal at the grid of V7 and it’s not hopping. And at the grid of V5, also not hopping. but there is around 4vdc at V5 grid. Is that normal? I guess its coming from the cathode via R28,R29, R25.

              Comment

              • #8
                Ok. Sorry, wrong calculations in the last message. I was getting messed up. By having the scope on AC coupling mode it was not seeing the voltage hopping.

                So, it seems to be not hopping at V5 grid. but hopping at V7 grid. So something along V5, V6 is causing this.

                Comment

                • #9
                  Don't skip V5 to V7, what is happeneing at V5 plate, V6 grid, V6 plate.
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment

                  • #10
                    I can’t figure out how to test V5 plate, V6 grid when there’s high dc voltage present. If I use AC coupling mode on the scope this also blocks the hopping.

                    Comment

                    • #11
                      Also it’s curious that this hopping is happening on both channels. So if one of these caps or resistors around V5,6,7 are acting up, that’s just going to be one of the channels. Hmm

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