Marshall mg30dfx Repair - Gain Not Working

Discussion in 'The Workbench' started by Agwood75, Oct 6, 2015.

  1. Agwood75

    Agwood75 New Member

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    I have always wanted a Marshall amp and I found an MG30dfx at a garage sale for $20, so I thought I would try one out. The only caveat, according to the previous owner, was the speaker would cut in and out. I am pretty handy with electronics, though I have not worked on amps before, but I thought for $20 why not give repairing it a try. I got it home and opened it up and found that the solder connection for the speaker wire had broke loose. I soldered this back up and the speaker no longer cut out.
    I started messing with the knobs and found that all of the inputs, push buttons and knobs work except for the gain. The gain knob felt kind of loose and the pot turned without much resistance, so I decided to replace the pot. After replacing the pot I still have the same issue. What it will do is when you move it from 0 to 1 it will make a sound out of the speaker, kind of like when your first turn on the amp, but not quite as harsh. After this first sound there is nothing, it just sounds like it is still on the clean channel but not as loud. I thought that maybe the channel selector was not working, but the light comes on for the overdrive channel and all of the equalizer knobs work in overdrive so I believe that it is sending me to the overdrive channel.
    I pulled the wiring schematic online and noticed a red and green light in series with the gain pot, so I opened this up and looked at these when I turned the knob. I found that at 0 and in the overdrive channel neither of the lights are on, but when I go from 0 to 1 the green light will come on and I will get the pop sound I mentioned earlier out of the speaker. I would assume that since this light comes on I am getting some kind of response out of the gain pot, but I still do not get the overdrive sound out of the speaker. I looked at the circuit board and could not find any evidence of anything getting burnt up, or any loose solder connections so I am at a loss with this.
    I know this is a low end Marshall amp, but I really like the sound that comes out of it on the clean channel and I do not think I could find anything better for $20. Also, I would like to learn a little more about repairing amps in general so this will be a good tool to do this without destroying one of my more expensive amps.
    Does anyone have any suggestions on where to go from here?
     
  2. Murray

    Murray Active Member

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    Welcome, Agwood75. $20? Deal. Good news is it's a good amp to learn some amp repair skills on. Bad news is it's not all that easy to work on. But for $20 it's a cheap education. And I do not believe in disposable/'do-not-repair' things. Not a bad amp & definitely worth saving from the boneyard.

    You already have the schematic. Clean channel works & OD channel doesn't, right? The green & red LEDs that you see are the clipping diodes. They light in some circuits & don't in others. Depends on a few things but light or not they work as diodes. Take a look at this: MG10 Circuit Analysis

    Not your amp but the analysis describes how the od stage & clipping diodes work, so it might get you started in the right direction. Checking continuity between components is a start. Bad/leaking cap or fried resistor, blown op amp. Checking voltages on the op amps helps id faults too. Really just a matter of tracing out the signal path & seeing where you lose the sucker.

    Solid state amp so it doesn't have the lethal voltages of a tube rig, but it can still bite you with 50v. Be safe, make sense out of the schematic as it relates to the circuit board & learn.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    1. IC5D stuck "ON." This disables the gain pot.
    2. Shorted diode(s) LED3, LED4.
    3. Bad IC 2.
    4. Footswitch jack has corroded switch contacts or broken solder joints.
    Try using a footswitch with RTS plug.

    Probably one of those.
    Schematic:
    http://www.amparchives.com/Amp Arch... & Layouts/MG Schematics/Marshall MG30DFX.pdf
     
  4. Agwood75

    Agwood75 New Member

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    Thank you for your replies. I will try these out when I get home tonight. I do have a few quesitons. If I have a shorted diode or if IC 2 is bad, would I still get sound out of the channel 2 overdrive? I do still get sound out of the channel 2 overdrive and the equilizer functions work, but all i get out of the overdrive channel is a lower volume clean tone.

    I am definetly going to check out the footswitch connection. I did not think about that when I was trouble shooting.
     
  5. Murray

    Murray Active Member

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    If you have a situation where a bad component or inoperative switch is attenuating the signal, yes. Good place to look is one of the spots Ampmadscientist delineated: IC5, which is a DG212-CJ SPST Switch ,if I read the schematic correctly. Sitting atop the clipping circuit, if it's not opening it messes things up. Could be the IC or could be the channel select switch that is connected to it. Could be a lot of things. Easy continuity check to see if the switches are doing their job. Might be just a dirty contact.

    EDIT:
    Don't know what you have as far as test gear but a simple & really cheap signal tracing setup can be had by getting a little Radio Shack amplifier like BillM shows in THIS Article. Couple that with a freeware app like TrueRTA which is a signal generator, o-scope & a few other things that works out of your pc. Set up the sig generator input to the amp & trace it out using the RS amp. Pretty basic but works.
     
    Last edited: Oct 9, 2015
  6. Mal

    Mal New Member

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    Not an answer but another question. The overdrive gain pot on my mg30 is broken, anyone know the value of 5d2 pot?
    I think it's vr4 on the schematic. Can't find a UK supplier yet.
    Looks like someone cross threaded the fixing nut maybe attempting a repair. Amp works fine apart from overdrive gain at max with broken pot connections
     
  7. Mal

    Mal New Member

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    Oh well, answer my own question in case it helps anyone. I replaced the gain pot with a 220k linear (yep should have been log but works anyway). Gain now works satisfactorily and with budget guitars and effects it satisfies my less than pitch perfect ears. plenty of variety in distortion and in conjunction with effects can conjur up a variety of 'noises'. I think the pot value should be 1m log
    The original pot had snapped between the connectors and drum of the pot. and the retaining nut seemed crossthreaded, whether this was a clumsey attempt at previous repair I can't tell.
    Also the occasional cut out of speaker due to the known fault of the headphone jack socket connectors not making a good circuit when the plug is out was cured by, with amp module out of case, put a jack in the socket and push the end of connectors down a bit to bend them and make a more reliable contact when the jacks removed.
    Haven't had any further volume 'cut out' problem and can annoy the neighbours with a bad 'get by with a little help from friends' rendition.
    Guess I'll solder on (soldier on, did you see what I did there?)
     
  8. Jon C

    Jon C Well-Known Member

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    Yep ! Tricky……
     
  9. Zachwagoner2001zcw

    Zachwagoner2001zcw New Member

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    Im having the exact same problem with my amp and was curious if the problem was ever solved
     

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