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JCM 800 2205 Low boost channel output

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  • JCM 800 2205 Low boost channel output

    With nothing plugged into the footswitch jack the amp is in boost mode. The amp has very little volume and the Gain and Master controls have very little effect on the volume. With the footswitch jack shorted, the amp goes to normal mode and works correctly. I changed out the CA3046 for an NTE912 (twice) with no effectMarshall-JCM800-Splitch-50W-2205-Schematic.pdf
    The LED lights like it should in boost mode. The voltage at the cathodes of D2 and D3 is about 4.5 volts (normal)
    I've swapped out all the preamp tubes with no change, and the voltages on the preamp tubes look normal.
    I've looked up threads concerning this amp and switching problems but this particular problem doesn't seem to be covered.
    Thanks for your time!

  • #2
    Normal pretty much just bypasses V2, so look in there for your signal loss. What are your DC voltages for V2 pins 1,3,6,8 ?
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #3
      A quick test can be: with the boost channel activated disconnect one pin on capacitors C11 and C12 (0.22uF). That way you eliminate the mute. If it sounds, it means that the switching circuit does not work well.

      https://drtube.com/schematics/marshall/2205prem.gif

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      • #4
        V2 voltages are 1, 254v - 3, 2.48v - 6, 206v - 8, 1.33v
        I lifted C11 and C12 and the distorted signal seemed to get a bit louder, but still way below what it should be.

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        • #5
          Is the master volume functioning on clean mode?
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

          Comment


          • #6
            Sorry, I asked about the wrong tubes before. DC voltages for V1 pins 6&8, V3 pins 1&3.
            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

            Comment


            • #7
              V1 voltages: Pin 6 - 95.5v, pin 8 - .535v. V3 voltages: Pin 1 - 195v, Pin 3 - 1.58v. I believe the master works in clean mode but I'll check again.

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              • #8
                Found it! Two bad solder connections. One from the gain control in the distortion circuit and one to the chip holder that holds the switching IC. Kind of unusual. They looked good but weren't connecting.

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                • #9
                  Good job
                  are those 3046's extinct?

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