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Marshall JCM600 drive channel noise incorrect component on circuit board?

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  • #1

    Marshall JCM600 drive channel noise incorrect component on circuit board?

    Hello,

    I have a Marshall JCM600 I picked up that im working on. Clean channel is great, sounds amazing by the way, but the
    drive channel is very "static" sounding very loud, as I turn up the gain knob static gets worse. I have swapped preamp tubes,
    still same problem, and have checked the gain pot with a multimeter which is working fine, so im thinking there might be a bad
    cap somewhere, leaking DC where it doesnt belong. So Im looking at the schematic C50 according to the schematic should be
    a 2n2 50v cap. But in my amp there is a Resistor in there? it has colored bands red red red black silver, which should be 222 0hm,
    but it measures 386k ohms when i put a multimeter on it.... Im totally confused here... If anyone has a JCM600 2x12 combo they
    might have pictures of their circuit board, that would be extremely helpful. Its the area around VR5 which is the gain pot area of the
    circuit board.

    Included are the schematic, and a picture showing the resistor where the cap should be...

    Any and all help greatly appreciated
    Attached Files
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  • #2
    Have you considered that it might be a capacitor? 222=2200pf=0.0022uF

    Comment

    • #3
      Originally posted by 52 Bill View Post
      Have you considered that it might be a capacitor? 222=2200pf=0.0022uF
      Thanks for the reply kind sir.... If you read the post, My confusion lies in the fact that there is a resistor soldered in place where the schematic shows there should be a capacitor... strange right?

      Comment

      • #4
        It's a capacitor. If you have a cap. meter, check it and you will find it measures around 2n2. There are modern caps that look just like resistors. Often they have green or pink bodies, but sometimes like this case, they look just like resistors.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

        Comment

        • #5
          ok thanks for your input, I get it, its a capacitor! holy smokes... why? why? ;-) blush... first time running into a cap looking like a resistor, I did test the cap my multimeter tests caps.. thank God, and the value is waaaaaay off reads 331000pf when in should be 2200pf, im going to replace it and hope for the best,

          thanks for your input gentleman I will post my results after swapping caps..

          Comment

          • #6
            Did you test it with at least one lead disconnected from the circuit?
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            ​

            Comment

            • #7
              Originally posted by g-one View Post
              Did you test it with at least one lead disconnected from the circuit?
              Hello, just learned "by experience" once again. I pulled the cap measured and it was in spec, replaced it with a new one anyhow..

              Any idea what it could be? I havnt replaced the pot, but it seems to work ok when I measure with a multimeter it seems ok, I tried tube swapping
              that didnt work, replaced that cap, that didnt work.... hmmmm stumped.... maybe another cap down stream? not sure how to proceed, again

              Also, for debugging purposes

              This noise is present weather guitar plugged in or not.

              when gain knob is off, no noise issue. turn gain on even a little bit and static noise appears. loud same volume throughout the
              entire gain pot travel.

              when volume knob for that chanel is off. no noise.
              any suggestions would be greatly appreciated...

              thanks for your help
              Last edited by tonefiend; 08-09-2014, 12:12 AM.

              Comment

              • #8
                1) Marshall very often uses "capacitors that look like resistors"
                2) cheap multimeter "capacitance meters" actually pass some audio frequency current through it and measure impedance, then *assume* it´s capacitive reactance.
                It works ... on an actual capacitor.

                If you put a resistor there, it *will* have an impedance (its resistance) and the multimeter will translate that and show a "capacitance" which is not actually there.

                Besides, agreeing with g-one, I also guess you measured it in circuit, which twists results.
                Juan Manuel Fahey

                Comment

                • #9
                  If turning down the gain reduces the noise, then concentrate your efforts only on things ahead of the gain pot in the circuit. Plate resistors are often sources of noise, it looks like V101B is the only tube stage before that pot, so check R120.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  ​

                  Comment

                  • #10
                    Which pot, by designation (vr1, vr5 etc) makes the difference?
                    I will repeat my usual suggestion.......do not replace parts unless you have proven them to be bad. Replacing random parts can introduce mistakes, intermittent joints, wrong value or ratings and lower the value of the unit. Fix what is bad. Isolate the problem and identify the characteristics of the noise that helps pin point the source. Is it steady hiss or is it granular like snits, pops, crackles. Are you familiar with this amp or others of the same model and know the noise is abnormal(after all, you are are hearing it only on the very high gain channel). Are you positive that the cap you installed is the correct value and type? Why not make sure by reinstalling the good original cap? What caused you to diagnose the problem originally to be C50?

                    Comment

                    • #11
                      Originally posted by km6xz View Post
                      Which pot, by designation (vr1, vr5 etc) makes the difference?
                      I will repeat my usual suggestion.......do not replace parts unless you have proven them to be bad. Replacing random parts can introduce mistakes, intermittent joints, wrong value or ratings and lower the value of the unit. Fix what is bad. Isolate the problem and identify the characteristics of the noise that helps pin point the source. Is it steady hiss or is it granular like snits, pops, crackles. Are you familiar with this amp or others of the same model and know the noise is abnormal(after all, you are are hearing it only on the very high gain channel). Are you positive that the cap you installed is the correct value and type? Why not make sure by reinstalling the good original cap? What caused you to diagnose the problem originally to be C50?
                      The gain pot is VR5

                      Well, I messed up and thought C50 was a resistor... oops. So I pulled it out, had a brand new cap correct value I soldered in place, same issue loud hiss with some crackle. I havnt tried another amp like this, so i suppose it is plausable that its inherent in its design.. but it seems very loud, and unusable to my ears.

                      thanks for your help

                      Comment

                      • #12
                        Hello,

                        I checked plate load resistors R120 and R121 for v101B and v102A respectively, and they tested out fine.... well maybe its just a noisy drive chanel. Im officially stumped..

                        Comment

                        • #13
                          Your problem is noise. I suspect when your resistors "tested out fine" that means the resistance was measured? Your ohm meter will not detect noisy parts. I vote R120.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment

                          • #14
                            What he said ^^^^^. I was thinking more of testing it with a heat gun or freeze spray on R120 to see if it impacts the noise.
                            If you look at the block diagram at the top of the preamp schematic (JCM 6-60-02), there is not much between input and VR5.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            ​

                            Comment

                            • #15
                              ok, I see, that yes although R120 was tested in spec in terms of its resistance in ohms, I believe it measured around 98k, the component could
                              still be a "noisy" component.. well Im going to change that out and see what happens, I will post my results...

                              thanks very much for your input

                              Comment

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