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1980's Marshall Bass 12 - Hum Problem

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  • 1980's Marshall Bass 12 - Hum Problem

    Hi All,

    Hope you are doing well!

    I've just bought one of these off e-bay - in a terrible state cosmetically, but it has cleaned up ok. The 80's solid states are great
    little amps (I also have the Lead 12, Reverb 12 and Master Lead Combo). On this amp straight off I have a problem with hum and
    some interference noise.

    A more detailed description of the hum:
    Amp switched on, no lead attached. volume and gain at zero, amp humming through speaker. Gradually rotate the VOLUME
    control, the hum quietens down (but still there), then goes back up to the original volume with VOLUME control at 10.
    With guitar attached, same thing happens but with the VOLUME at zero there is a sound like shortwave radio interference.
    Some sound clips here to help:

    https://soundcloud.com/user-339899961

    Volume control swept from 0 to 10 a few times on each clip. As you can hear, the sound diminishes at mid point, then re-appears
    at 10. With the guitar plugged in the crackly interference sound only appears when the volume is at zero.


    I have replaced the electrolytic filter caps C18, C19, C15, C16, C13 and C14. On advice from another forum I have reflowed the
    solder joints for the jacks, pots and connections from the power supply (anything which could carry stress onto PCB solder connections).
    Pots have all had a good clean with contact cleaner. Are there any other capacitors that usually suppress hum and that may be faulty
    and need to be replaced?


    This Japanese chappie seems to have had the same problem:

    http://guitarder.fc2web.com/other2/o2-28-bass12.html

    He solved it by switching out R17, I've tried a 1k in parallel and it didn't do anything.
    I also tried powering the Bass 12 off the Lead 12 transformer, but there is no difference.

    These amps as you can see by the photos are pretty simple, so I figured that I could (with a little help from some of the great internet
    resources and various forum members) fix it myself and along the way use it as a learning experience. I have limited electronics
    experience so if it gets too technical I like to ask dumb questions if I don't understand!

    Click image for larger version

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    Click image for larger version

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    If anybody could offer some advice or assistance, I would really appreciate it.


    Attached Files

  • #2
    Is this the correct schematic?
    (notice there are 3 preamp versions)
    Attached Files

    Comment


    • #3
      I think that is the correct one (it's the only one that I have been able to find online).
      The middle pre-amp at the bottom is the Bass 12.

      Comment


      • #4
        How does circuit ground connect to chassis ground?
        "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


        • #5
          Do you have a scope?
          "Yeah, well, you know, that's just, like, your opinion, man."

          Comment


          • #6
            Originally posted by g1 View Post
            How does circuit ground connect to chassis ground?
            Grounds are located as shown below:

            Click image for larger version

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            Comparing this to my Lead 12, the ground should be connected as shown below (fed from below the board through the top hole then
            through the hole next to "BK" and soldered there).

            Click image for larger version

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            I'm not sure if it makes a difference. The traces are connected and are quite wide. In the top photo the ground is soldered
            to the trace that the cap is connected to.

            Comment


            • #7
              Originally posted by The Dude View Post
              Do you have a scope?
              I have an old one that my wife had for teaching in the garage. It hasn't been used for a while, You're gonna have to talk me through what to do with it though!!!

              Comment


              • #8
                Nuts and threads look corroded, so ground connection may be compromised.
                Disassemble and clean all contact faces with Deoxit or even sand them if necessary..
                - Own Opinions Only -

                Comment


                • #9
                  Originally posted by Helmholtz View Post
                  Nuts and threads look corroded, so ground connection may be compromised.
                  Disassemble and clean all contact faces with Deoxit or even sand them if necessary..
                  Good call!!!

                  Comment


                  • #10
                    Yes, and clean all the pot to chassis connections as well if that was not implied by Helmholtz reponse.

                    The black wire connection closest to the filter cap grounds is the best place. Suggest leaving it there.
                    "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


                    • #11
                      Originally posted by Barnsleyboy View Post

                      Grounds are located as shown below:

                      Click image for larger version  Name:	2021-07-30_225351.png Views:	0 Size:	1.06 MB ID:	937894

                      The black transformer centre tap wire should really go directly to the cap (where the circuit ground arrow is pointing) not to chassis so that the existing wire between circuit ground and chassis doesn't carry the rectifier current pulses which could cause a ripple voltage between circuit ground and chassis ground.
                      Last edited by Dave H; 07-31-2021, 03:52 PM.

                      Comment


                      • #12
                        Originally posted by Dave H View Post


                        The black transformer centre tap wire should really go directly to the cap (where the circuit ground arrow is pointing) not to chassis so that the existing wire between circuit ground and chassis doesn't carry the rectifier current pulses which could cause a ripple voltage between circuit ground and chassis ground.
                        Then also run a separate wire from circuit ground to chassis, yes?
                        "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


                        • #13
                          You just beat me to it, I was going to ask the same question!

                          Comment


                          • #14
                            Originally posted by g1 View Post

                            Then also run a separate wire from circuit ground to chassis, yes?
                            Yes that's right but it already has the black wire with the green tape there so I thought that could be used. Just disconnect the PT CT wire from chassis and connect it to circuit ground at the cap.

                            I can't see how the trace is routed so I wouldn't connect the PT CT to point "BK" on the second picture. Connect it directly to the cap.
                            Last edited by Dave H; 07-31-2021, 06:48 PM.

                            Comment


                            • #15
                              Now then...... Some progress has been made! The crackly interference sound when the lead is plugged in has gone - bravo Dave H, I think that has fixed that problem. Interestingly the lead 12 that I have is wired in the way that you suggested!
                              I still have the hum problem at zero and 10 though, and I'm keen to eliminate this. Is it relevant that the hum is heard through the speaker regardless of whether a lead is plugged into the input jack?

                              Comment

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