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  • Amp plays on standby!

    I am working on a Peavey MX VTX hybrid guitar amp (soild state preamp and tube output). The amp has a high / standby / low power switch. When the switch is in the middle position (standby) there is a low distorted sound when a guitar is played. There should be no sound.

    When you do put it in standby the volume and LED light slowly decreases probably due to discharging capacitors. But, there is still some voltage (about +30v) on the plus side of C3. I cannot figure out where the voltage could be coming from. Could C1 or C2 be leaky? I thought it might be the switch so I pulled it out of the circuit and there still is voltage! Very puzzling.

    Here is the schematic: Peavey-MX-VTX-Series-Schematic.pdf

  • #2
    Weird. Standby switch only disconnects half of the full wave bridge rectifier. Seems obvious it would still be creating DC voltage.
    --
    I build and repair guitar amps
    http://amps.monkeymatic.com

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    • #3
      Originally posted by Axtman View Post
      Could C1 or C2 be leaky?
      That's my guess. That is the only way I see the secondary side seeing a current path, even though the side with the F1 fuse is still connected to the rectifier.

      Does it behave the same way if you power up in standby (starting with discharged caps) vs going into standby from Hi or Lo?

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      • #4
        Originally posted by xtian View Post
        Weird. Standby switch only disconnects half of the full wave bridge rectifier. Seems obvious it would still be creating DC voltage.
        What do you mean?

        In standby the HV voltage is disconnected from the rectifier, so no supply current.

        - Own Opinions Only -

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        • #5
          I think it may just be capacitive coupling and may be normal. Even if C1 were leaky, I don't see how that could do it. There needs to be some path to ground for the secondary of PT.
          Or the PT is faulty and there is leakage from the winding to it's case (chassis).
          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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          • #6
            I don't see how a leaky C1 or C2 could be responsible, i.e. if the schematic is correct.
            - Own Opinions Only -

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            • #7
              Originally posted by Helmholtz View Post

              What do you mean?

              In standby the HV voltage is disconnected from the rectifier, so no supply current.
              Yeah, ok, agree.
              --
              I build and repair guitar amps
              http://amps.monkeymatic.com

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              • #8
                Originally posted by g1 View Post
                Or the PT is faulty and there is leakage from the winding to it's case (chassis).
                Now that would a safety issue.

                Otherwise I don't think it's normal.

                A few hundred pF of coupling within the PT couldn't deliver real power.
                Last edited by Helmholtz; 02-12-2024, 08:41 PM.
                - Own Opinions Only -

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                • #9
                  Originally posted by Helmholtz View Post
                  A few hundred pF of coupling within the PT couldn't deliver real power.
                  How much would be real? The standby is only for the HV on the power tubes. All the SS voltages are still up, the heaters are still running. I think all we need is a little voltage at the collectors of the cathode drive transistors.
                  I get the impression the amp is only putting out a very faint distorted signal.

                  There are some amps that do not totally mute on standby, so I'm not sure we can call that idea a 'rule'.

                  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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                  • #10
                    Is R65 in good working order?

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                    • #11
                      Originally posted by g1 View Post
                      How much would be real?
                      I measured a few PTs for capacitance between HV winding and heater winding or frame.
                      Results were between 100pFand 1000pF.
                      1nF means an impedance of 2.7M at 60Hz.
                      - Own Opinions Only -

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                      • #12
                        Originally posted by Axtman View Post
                        But, there is still some voltage (about +30v) on the plus side of C3.
                        Relative to what "GND"?

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                        • #13
                          X-pro: Yes relative to ground.

                          Here's the crazy part. I pulled fuse F1 and I still have voltage. I am thinking now that it is getting back fed somehow.

                          BTW, there are currently no tubes in this amp while I am making these tests.

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                          • #14
                            "GND" - 2 pieces!
                            Attached Files

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                            • #15
                              GND on (-)C4 has undefined potential at middle position of S1. Floating level relative to the second GND (+/-C6,C7).
                              R65=47- rather high value.​

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