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Marshall MG15 Hums on Gain Channel

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  • Marshall MG15 Hums on Gain Channel

    I have a Marshall MG15 mini stack that started to make noise on the gain channel. If I move my guitar around from side to side it changes the volume of the noise. The noise sounds like 60 cycle hum. I'm at a loss as to what is causing it.

    Your help is appreciated. Thanks

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  • #2
    Just for a confirmation: does hum disappear when you set guitar volume and tone controls to 0?

    I mean, with the guitar plugged straight into the amp, no pedals.

    I bet it does.
    Juan Manuel Fahey

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    • #3
      This sounds more like a guitar problem than an amp problem. Do you play single coils? Does the hum go away when you touch the strings? Are you near florescent or neon lights?
      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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      • #4
        And how close are you to the amp?

        Justin
        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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        • #5
          I really don't think this is a guitar problem. Between my friend and I, we have played at least five different guitars with it and there is excessive hum with them all. We swapped several cables. We have played it at his house and at my house. The problem first appeared one day when my friend sat down to play.... the noise didn't exist before that day. The amp was not moved. So what I'm trying to explain is that it is not the environment or the guitar or the cable.
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          • #6
            If indeed it's not the guitar, the next thing I would suspect would be cracked solder on the input jack causing a lack of good ground.
            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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            • #7
              It could be a problem in filtering capacitors. Although check the input jack

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              • #8
                I re-flowed the solder joints on the input jack, no better. I have replaced the power section capacitors c23,c27,c28,c31,c32,c33,c34. I replaced diodes D3,d4,d5,d6. I also replaced C9 and IC1(TL072). Still have noise. When I use my scope to look at the signal path, I get a 45mV sine wave at R1 (input). If I measure at c24 (connected to pin 2 of the output audio amp IC), it is about 35mV. On the other side of the output IC on R11 I get around 70 mV. this test point is at the speaker lead W1. I see this sine wave on the power rails too. So is it normal to have this much ripple? Is there something else I should replace in the power conditioning components?
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                • #9
                  Try plugging a cable into the amp and measure from sleeve of jack to chassis ground. Make sure the connection is good. I know you have resoldered the jack, but there is still the possibility that a ground connection is loose or not being made. I'd also make sure the ground lug of the AC cord is at chassis ground potential with a meter. The fact that you can move the guitar side to side and change the noise still makes me think ground problem.
                  "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                  • #10
                    I would have to question exactly where the OP is connecting the ground probe.
                    The LM1875 output ic should not be transfering the output signal to the power rails.
                    Both rails at that!
                    Is the signal also on the + - 15 volt opamp supplies?

                    If the headphone out jack is corrupt, the normally closed ground contact will not be NC.
                    Check that it is indeed fully closed.

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                    • #11
                      It is also on the 15 volt supplies. I get noise on the headphones just the same.
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                      • #12
                        You say it hums on the gain channel - does it do this on the clean channel? The PSU is common to both. If it does it on one and not the other then it's unlikely to be the PSU. Hum can be caused by excessive current draw causing PSU ripple (sometimes a defective output IC), but something will usually get hot and show up.

                        45mv on R1 is very high. Does this disappear with guitar not plugged in (input jack shorted)?

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                        • #13
                          At this point, I would suggest that you remove the output ic.

                          If the signal is no longer 'riding' on the power rails, then the ic is bad.

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                          • #14
                            Finally got back to this issue. I removed the output IC and the "noise" went from 35mV to 16mV. I'm thinking this is still too much? If it is too much then I've traced it all the way back to the rectifier diodes.
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