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74' Marshall JMP 50w broke

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  • 74' Marshall JMP 50w broke

    This amp is 34 yrs old.Sat unused for 10 yrs at friends house. When I got it back I had tech check it out{I guess he brought it up slow} and started to use it. Had some noise problems(excessive hum) and used something else for 6 months. I made up 2 bias-rite sockets and hooked them up , when I turned amp on I hear a pop like something arced in one of the bias -rite sockets. Turned amp off immediately. Now when I flip standby on I get loud humming & crackling with\without volume up or zeroed. I get this with/ without any tubes. I've only been turning standby on momentarily since this happened.Almost sounds like someone holding the end of an input cord ,a short somewhere. Speaker is hooked up ,ohms are correct,plugging input cord in,same thing. Nothing visibly out of order,burnt components,arced sockets,bare wires etc.

  • #2
    Thats an old amp, i would start by replacing the can caps . There should be 3 or 4 on that one , probably LCR brand, tall blue ones that stick up out of the amp.

    I would also replace the 2 screen resistors on it while its apart if they are the originals

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    • #3
      Pull the chassis, and no power tubes. Put on bench and dim th room lights. Watch the empty sockets closely and turn it on. See any blue sparks on the sockets? FLip the chassis over and watch the underside as well. Arcs are easier to see in dim light.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        You have two of these posts up. So I'm reposting my response here as well...

        Hum with no tubes in is a bad thing. The speaker should be isolated without tubes in. So nothing should get to the speaker unless there is a short in the output transformer, a problem with the chassis ground or voltage backing up through the feedback loop from a bad decoupling cap.

        Short story is that because you don't have much electronic experience this will be a hard problem for anyone to dianose from a remote location. I think you should take it to a tech. A different one than you did last time. The way your story reads, that amp was in need of new filter capacitors when you took it to the last guy. And yet he sent it home anyway. The initial symptoms you describe sound like filter cap failure. Then the whole thing $h!t the bed. He should never have sent that amp home. Now you likely have new and more expensive problems to deal with.

        Sorry

        Chuck

        P.S. What Enzo said too. But I would think it would be more of a popping or fizzing sound from the speaker if the sockets were arching. I think your OT is toast and the "tech" that sent that amp home should be told. If I ever had a return because I was too lazy or impatient to do a routine up keep on an amp that needed it, I would fix it for free.

        P.P.S. I have one of those OTs if you want to keep it stock. It's been painted black but it IS a mid 70's 50 watt JMP OT (originally with 6550's). Otherwise there are better replacement OTs available.

        Chuck
        Last edited by Chuck H; 11-29-2008, 06:39 AM.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

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        • #5
          JMP 50w broke

          I borrowed a LCR meter from a friend and measured filter caps.I went to drain caps with a NTE 10w 270k cement resistor and I think they were empty. I only had the resistor hooked up a few seconds and then unhooked it, used a multi meter and had no voltage. I lifted the hot sides and measured w/ LCR from each + to - on caps individually. I got 30 to 38u from either side of the large blue cans.These are Daly 50u/50u. The small green 330u on the pc board also measured about 20% low. Can I install my output tranny ( still have the one that was put in then taken out,I think it is Hammond}and see if this works,reform caps ? Or should I just recap and put ot in. Do I need to form new caps? Can I do this w\out a variac? I don't think I can get one.Thanks all

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          • #6
            Any hum (even without the tubes) is caused by bad filter capacitors. If you power it up with the tubes, and hear the hum get louder as the tubes warm up, it is a bad filter capacitor.

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            • #7
              BMD,
              On the loud pop from the bias-rite when you powered up...sounds like the 1 ohm resistor in the tester's socket took a dump...probably from a shorted output tube.

              You didn't mention if the power tubes had been replaced.

              Also I don't see where you mentioned if you got hum with the power tubes out...if you did, you're ususally looking at a bad output transformer. With the power tubes removed even with bad filter caps you won't hear hum as with no differential voltage across the primary you can't create any sound in the secondary.

              Substitution is the best indicator of the output xformer...sounds like you have one to hang in there.

              You don't want to get confused as you could have both bad filters AND a shorted/leaky output transformer.

              I agree that with an amp that old if you want to keep it you should ultimately should replace the capcans anyway.

              glen

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              • #8
                Originally posted by kor View Post
                Any hum (even without the tubes) is caused by bad filter capacitors. If you power it up with the tubes, and hear the hum get louder as the tubes warm up, it is a bad filter capacitor.
                Referring to the power tubes, not the rectifier tube or solid state rectifier.

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                • #9
                  Originally posted by kor View Post
                  Referring to the power tubes, not the rectifier tube or solid state rectifier.
                  I hate to disagree, but there are many possible causes of hum in an amp besides defective filter caps.
                  Warning! Some Electronics devices contain lethal voltages that can kill you. If you do not feel qualified to work with dangerous voltages, refer your repairs to a qualified technician. By giving you online advice, I am assuming no liability for any injury or damages you might incur through your own actions.

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                  • #10
                    Originally posted by bnwitt View Post
                    I hate to disagree, but there are many possible causes of hum in an amp besides defective filter caps.
                    I'm sorry, but I guess I have to be very discreet. The 120 cycle hum is what I'm referring to! I know all about ground loop problems, bad connections, touching the end of the guitar cord, etc. etc., so I won't try to make bedside diagnostics from what I read!

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