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  • Marshal DSL 401 help - low/no volume

    Hello all,

    Been a while since I posted here. Have received great help in the past, that's for sure.

    I picked up a broken 2003 Marshall DSL 400 as a project amp. The original owner said he had it fixed twice over the years, and it's now broken again and he'd had enough and wanted to get rid of it.

    It had very low vol on the clean channel (barely audible with everything dimed). On OD1 it had some volume, and even more on OD2. However, there was nothing resembling what should be coming out of a 40w Marshall. Upon disassembly, it looks like the Bridge Rectifier had been replaced (common on these models). I replaced the BR again and stood it about 3/4" off the board. I found a few cold and broken solder joints on the speaker, input and footswitch jacks and addressed those.

    After reassembling the amp and retubing with known good tubes I'm still getting barely any volume.

    I ran the effects loop send into a powered speaker and didn't get much on any channel. Plugging a guitar into the effects loop return I get plenty of volume and it's obvious the PI is working and the power amp section is firing on all four EL84s.

    I measured the plate voltages on V1 pins 1 and 6 and they seem reasonable (I believe they were just over 200v, didn't write it down). The plate voltage on V2A seemed reasonable, however the plate on V2B was only 80v - that seems off to me.

    I started thinking that the effects loop is broken and something in there is sending a lot of signal to ground.

    I carefully jumped from the top of R104 to pin 2 of V3A.

    The amp came alive (noisy from the jumper being long, etc...), but it sounded great.

    So I'm definitely thinking something in the effects loop is the culprit.

    That's where I'm at; I don't know where to go from there. To be honest, I'd be happy to just remove the effects loop from the circuit as I never use an effects loop, but it looks like the reverb is integrated into it, and I like reverb. I'd like to return the amp to be fully functioning as it's a project, and a challenge.

    Can someone please offer some direction that I might start looking to get this resolved? Any help appreciated.
    Attached Files

  • #2
    Had to fix it TWICE in 19 years? Geez how unreliable...

    OK, Most of your preamp seems to work then? SO trace the signal to IC1b, is that working? Looks like IC1a and T16 are OK because the FX return works.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      That seems really strange that the plate voltage of V2b is lower than V2a, it seems like it should be higher. What are the voltages at 3 and 8 of V2? Otherwise there are a lot of FETs that are presumably for channel switching or muting, I'm wondering if one or more of them is stuck partially on.

      Comment


      • #4
        Originally posted by Enzo View Post
        Had to fix it TWICE in 19 years? Geez how unreliable...

        OK, Most of your preamp seems to work then? SO trace the signal to IC1b, is that working? Looks like IC1a and T16 are OK because the FX return works.
        Appreciate the reply.

        I'll look at it tomorrow when I have more time, but, I believe I'm OK up to IC1b as the reverb is working (probably should have mentioned that). The reverb is weak can only be heard on the OD channels, and it doesn't sound very good.

        Originally posted by glebert View Post
        That seems really strange that the plate voltage of V2b is lower than V2a, it seems like it should be higher. What are the voltages at 3 and 8 of V2? Otherwise there are a lot of FETs that are presumably for channel switching or muting, I'm wondering if one or more of them is stuck partially on.
        I'll get accurate readings tomorrow. Thanks for the reply.

        Comment


        • #5
          Hey all;

          I haven't broke out my scope yet, it's deep in a closet (and hopefully still works!). I did have a chance to write down the voltages on the preamp tubes and IC1 though:

          v1a
          1: 176v
          3: 1.3v
          v1b
          6: 175v
          8: 1.3v

          v2a
          1: 187v
          3 1.2v
          v2b
          6: 87v
          8: 4.2v

          v3a
          1: 210v
          3: .83v
          6: 333v (cathode follower)
          8: 211v (cathode follower)

          IC1a
          3: -293mv
          2: -297mv
          1: -295mv

          1C1b
          5: -250mv
          6: -306mv
          7: -306mv
          8: 15.6v (previously 5)
          Last edited by wagdog; 03-20-2022, 12:19 AM.

          Comment


          • #6
            I don't claim to be particularly good at amp fixin', but hundreds of volts on op amps seems off to me. I would want to suspect C72 of shorting out and sending HV to the effects ICs, but I'm trying to square that with the fact the amp worked OK when you jumpered from R104 over to V3 grid.

            Comment


            • #7
              How can an op amp have -300v on it? I mean where in the entire amp is -300v. Where could it have come from, let alone get there?
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Originally posted by Enzo View Post
                How can an op amp have -300v on it? I mean where in the entire amp is -300v. Where could it have come from, let alone get there?
                That's a good point, I wasn't even looking at the polarity.

                Comment


                • #9
                  Originally posted by glebert View Post
                  I don't claim to be particularly good at amp fixin', but hundreds of volts on op amps seems off to me. I would want to suspect C72 of shorting out and sending HV to the effects ICs, but I'm trying to square that with the fact the amp worked OK when you jumpered from R104 over to V3 grid.
                  I know, weird.

                  Originally posted by Enzo View Post
                  How can an op amp have -300v on it? I mean where in the entire amp is -300v. Where could it have come from, let alone get there?
                  I have no idea. I've checked rechecked though, and that's what's on those pins.

                  I checked ZD3 and ZD4, they're fine.

                  At the moment, I'm at a loss.

                  Comment


                  • #10
                    Originally posted by Enzo View Post
                    How can an op amp have -300v on it? I mean where in the entire amp is -300v. Where could it have come from, let alone get there?
                    OK, I'm an idiot, lol.

                    Those are mV. I was wondering why that OP amp wasn't smoked. Now we know. Edited the original post.

                    Yikes.

                    Comment


                    • #11
                      What are the voltages at both sides from R104 ?

                      Comment


                      • #12
                        Originally posted by strato56 View Post
                        What are the voltages at both sides from R104 ?
                        -450mv at R104/C72 and -470mv on the other side of R104.

                        Comment


                        • #13
                          So there is 2.5x the current going through V2b as V2a according to the cathode voltage (assuming the cathode resistor values are not too far off). That would explain the low plate voltage at V2b. Not sure what could cause that, maybe oscillation? Have you tried a different tube there?

                          Comment


                          • #14
                            That would suggest that the 15V you measure at pin 5 IC1b is coming from IC1b itself and not from R67. Not a good sign for IC1.
                            Can you desolder pin5 and then measure voltages at R67.

                            Comment


                            • #15
                              Originally posted by strato56 View Post
                              That would suggest that the 15V you measure at pin 5 IC1b is coming from IC1b itself and not from R67. Not a good sign for IC1.
                              Can you desolder pin5 and then measure voltages at R67.
                              I think if there was really 15V at IC1b pin5, then pin7 would go to +15V also.
                              wagdog , can you double check that IC1 pin 5 voltage (and pin7)? If IC1b voltages previously listed (post #5) are correct, then you must replace IC1.
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


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