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  • Marshall MA100h

    Got a Marshall MA100h that had FS1 blown. I made sure none of the tubes were shorted, after replacing the fuse it plays fine. I'm wondering what would cause FS1 to blow .

    Thanks,
    nosaj
    MA100___MA50_Circuit_diagram.zip
    soldering stuff that's broken, breaking stuff that works, Yeah!

  • #2
    No replies? I played it for a while through my cabinet and cables no issues, sent it back the other day dead again. I requested the customers cables and cabinet.
    Am I that far off base with the HT fuse blowing?

    Thanks,
    nosaj
    soldering stuff that's broken, breaking stuff that works, Yeah!

    Comment


    • #3
      How did you make sure no tubes were shorted? Chances are real good that is the problem. Tubes do not have to hard short to be bad. A loose piece of grid wire swinging around inside can short every now and then. No tester will find that.


      Here is only one test of other possible ones: Pull each power tube, hold it up next to your ear, and rap on the side with your knuckle. Does it rattle inside at all or make other noises?
      Education is what you're left with after you have forgotten what you have learned.

      Comment


      • #4
        Originally posted by Enzo View Post
        How did you make sure no tubes were shorted? Chances are real good that is the problem. Tubes do not have to hard short to be bad. A loose piece of grid wire swinging around inside can short every now and then. No tester will find that.


        Here is only one test of other possible ones: Pull each power tube, hold it up next to your ear, and rap on the side with your knuckle. Does it rattle inside at all or make other noises?
        I used my tester to see if there was a hard short. I'll rap the tubes to see if any of them are jangly. So I would be way off base suspecting a speaker/instrument cable or cabinet issue?

        Thanks,
        Jason
        soldering stuff that's broken, breaking stuff that works, Yeah!

        Comment


        • #5
          I agree with Enzo. It's more likely an intermittent tube. A tester won't usually show this because it doesn't stress the tubes like an amp does. Sometimes, if you let a tube sit in the tester for a while and warm up a bit, the tester will show the problem. Otherwise, try a new set of output tubes.
          Last edited by The Dude; 04-11-2019, 10:29 PM.
          "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

          Comment


          • #6
            When I was a kid my dad told me to tap the tube while holding the test button down. Sometimes the short light will go on and off and sometimes it will come on and stay on.

            Sometimes when there is a bad rectifier tube, I can plug the amp into a light bulb limiter and then tap the rectifier tube and watch it flash between elements. It can sit and work fine until it is vibrated.

            Comment


            • #7
              Originally posted by The Dude View Post
              I agree with Enzo. It's more likely an intermittent tube. A tester won't usually show this because it doesn't stress the tubes like an amp does. Sometimes, if you let a tube sit in the tester for a while and warm up a bit, the tester will show the problem. Otherwise, try a new set of output tubes.
              tapping the tubes while in the tester an checking shorts tests doesn't reveal anything I can see. Before I mention a quad of EL34's is there anything else I should possibly look at?

              Thanks,
              nosaj
              soldering stuff that's broken, breaking stuff that works, Yeah!

              Comment


              • #8
                Originally posted by nosaj View Post
                tapping the tubes while in the tester an checking shorts tests doesn't reveal anything I can see. Before I mention a quad of EL34's is there anything else I should possibly look at?

                Thanks,
                nosaj
                Although the EL34s are the prime suspects you could check the bias current is steady in all four tubes to eliminate bias voltage drift and/or poor g1 connection on one or more of the sockets.
                Experience is something you get, just after you really needed it.

                Comment


                • #9
                  Intermittent problems can be hard to find.

                  Hook the amp up to a load and run music into it for a few hours, periodically checking bias and tube voltages.

                  Comment


                  • #10
                    Originally posted by drewl View Post
                    Intermittent problems can be hard to find.

                    Hook the amp up to a load and run music into it for a few hours, periodically checking bias and tube voltages.
                    Replaced the fuse then powered it up and caught one of the el34 red plating shut it off an saved the fuse. tried in another socket can't get it to happen again. I think the bias current was sitting around 42 with a bias probe.
                    So we' will go with a new set of tubes for now and see how it goes.


                    Thanks all,
                    nosaj
                    soldering stuff that's broken, breaking stuff that works, Yeah!

                    Comment

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