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Identify cap problem in amplifier help

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    Identify cap problem in amplifier help

    Hi, I suspect that my caps are going bad (already are?) in my amplifier. It is a 1990 Marshall JCM900 4100, so the caps are at least 15 years old. The symptoms are intermittent fuzziness/static accompanied by random drops in volume along with an oscillating hum. There is no visible leaking in any of the caps. For example, I will let a chord ring out for a few seconds and I can hear the oscillation go up and down, this is followed by a static and intermittent drops in volume.

    My question is; are these signs of failing caps? and, what are the audible signs of failing caps?


    Right now there are two LCR 50uF+50uF, 500vDC caps. The tubes are brand new.



    Thanks

    #2
    Re: Identify cap problem in amplifier help

    you can find nos caps for these amps

    has it ever been serviced? there is other stuff to attend to. wait for KC8 to pass by and advise.
    capacitor lab yachtmati techmati

    Comment


      #3
      Re: Identify cap problem in amplifier help

      It has been serviced. As metioned, the tubes are brand new with proper bias adjustment, I remelted questionable solder joints, cleaned all sockets, pots, and switches, and all of the components on the PCB still look good (no burnt out resistors or anything). I just want to know if I should focus my energy on the caps.



      Here is a pic of the two caps I got from online (its not my personal amp)

      Comment


        #4
        Re: Identify cap problem in amplifier help

        Bad caps are very possible, even likely, but they would be located under this chassis.
        Jim

        Comment


          #5
          Re: Identify cap problem in amplifier help

          bad main filters would cause a constant loud hum.
          sound more like oscillation.
          open bypass caps.
          can be an intermittant short in a tube too.

          Comment


            #6
            Re: Identify cap problem in amplifier help

            what would the sound differences be between bad bypass caps vs. bad main filter caps?


            also, where would the bypass be on this schematic?
            Schematic

            *if they aren't on here in can provide other schematics.


            Thanks

            Comment


              #7
              Re: Identify cap problem in amplifier help

              Have you tried carefully placing a hot soldering iron close to the non-polarized capacitors to determine the problem (Don't short the bit to anything)!

              Also, I think you should keep the amplifier on for about an hour (no sound) to reform the electrolytic capacitors.

              In the schematic, there are two electrolytic capacitors under the chassis (100V 10uF) for the bias voltages.
              My first choice in quality Japanese electrolytics is Nippon Chemi-Con, which has been in business since 1931... the quality of electronics is dependent on the quality of the electrolytics.

              Comment


                #8
                Re: Identify cap problem in amplifier help

                Originally posted by japlytic
                Have you tried carefully placing a hot soldering iron close to the non-polarized capacitors to determine the problem (Don't short the bit to anything)!

                Also, I think you should keep the amplifier on for about an hour (no sound) to reform the electrolytic capacitors.
                - no, I've never heard of doing anything like that, what does it do?

                - wouldn't keeping it on for an hour or so only be a temporary solution?

                Comment


                  #9
                  Re: Identify cap problem in amplifier help

                  I just looked at the bias caps you mentioned and there is no visible damage to them that I notice.

                  I just played the amp for a bit and left it on for about an hour with no problems what-so-ever which makes the problem really annoying. It will be fine for days or weeks then out of nowhere it will spuratically go intermittent on me. This is no good because if that happened at a gig, I'd be in trouble.

                  I will order two filter caps tomorrow just to have them on hand.

                  Comment


                    #10
                    Re: Identify cap problem in amplifier help

                    The caps under those tubes would not look bad, but it's likely they are dryed up.
                    It is more interesting that the tubes are new, the caps under them should be replaced.
                    Jim

                    Comment


                      #11
                      Re: Identify cap problem in amplifier help

                      well I ordered 2 new filter caps this morning since I will eventually need them in the future and they don't make the LCR caps I need anymore so I might as well buy them before they disappear........... after doing some more reading and talking to people I believe the problem also might be the tube sockets. they might need to be tightened around the pins a bit. I will try doing this with a small eye-glass screw driver later. If it doesn't work I will start replacing screen grid reisistors.

                      I hope I don't have to replace the sockets, they are a pain in the ass.

                      thanks for your help

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