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  • Problem with Mesa DC-5 hoping for a little advice/wisdom

    So I have a mesa dc-5 combo amp. I love the thing great tone awesome gain. Here's the problem. I want to use it in my metal/rock cover band and I decided it was time to retire my solid state marshall and give the mesa a whirl. I went to plug in my rack mount to the effects loop and wouldn't you know it it wouldn't work with effects pluged in. I decided it must be the potentiometer for the mix and I did a really simple mod I found that converts it from parallel effects send to series. I pluged everything in and it worked great. Effects sounded beautiful I was a happy man. Fast forward to a few days later when I take the amp to the studio. I pluged it in and I could barely get any noise to come out on either channel. I thought it was strange so I plugged directly in bypassing all effects pedals etc. Still very low volume. I noticed something else the master volume for the channels doesn't affect the volume. Only the gain and the Master (Master) Volume control seem to affect the overall volume. Strange. Next I swapped out all the tubes one at a time to attempt to eliminate the possibility of tubes. Next I used contact cleaner and went to town with on the effects loop input output jacks. Then I tried plugging a patch cable between the return and send. Then I used the recording feature on the amp that bypasses the power stage. Nothing. I have no cards left to play. I am a novice and also saving up to put a transmission in my relatively new mini-van (whole other story). Needless to say I wont be able to take it into Mesa for quite some time. I can use a soldering iron and I have been decyphering the schecmatic as best I can but if anybody has any thoughts or hints I would be all ears.

    Schematic link below:

    http://www.thetubestore.com/lib/thet...-Schematic.pdf

  • #2
    The only problem I have ever had with a Mesa amp is with the pre amp tubes. They seem to eat em up. I would start there and replace them all, not just one at a time. I actually have had a few pre amp tubes not working well at the same time..

    Give this a whirl and good luck.


    _____________________________________
    Faith is taking the first step even when you don't see the whole staircase.

    Join Date: Aug 2001
    Location: N. Adams, MA USA
    Posts as of Jan 10th 2013: 82,617

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    • #3
      It sounds like you're getting some preamp signal. You could confirm that by connecting the effects send of this amp to another amp's input (or effects return) and seeing if your preamp circuit is passing a signal.

      You can also check your power amp by connecting another amps effects send to this amps effects return.

      This will localize the problem down to being either the preamp or power amp.

      Beyond that, what you have to do is signal trace it down to the individual component causing the issue. That is done by feeding the circuit (preamp or power amp) a test tone. Then you use a small battery powered amp to trace the signal component.

      What you use as a probe is simply a guitar cord with the end removed and the wire trimmed back. You connect a long wire with an alligator clip to the shielded ground. This connects to the chassis to provide a ground.

      For the probe tip, you need to connect a .1uf cap to the hot lead of the cable and use the other end of the cap as a probe. What the cap does is pass the test tone without passing high AC and DC voltages which would cook the amp or short the circuit. You also need to have everything well heat shrinked so only the end of the probe can make an electrical contact. Human error and improper operation is the leading cause of all amp failures. You don't want to slip and make things worse.

      From there you feed the circuit with a test tone, it can be a sine wave generator or simply the signal from an FM receiver. You use the schematic and follow the signal flow. If you have a good clean signal going into a component, and nothing coming out, its like sound going in one ear and not making it out the other. It obviously met an obstruction preventing it from passing through. It may be the component itself is bad, or it may be the component fails to receive the voltage needed to make it operate/amplify.

      Keep in mind this is advanced stuff better left to a professional. Its dangerous too. One slip and either the amp dies or you do.

      Until you've narrowed down it being either the power amp or pre amp I cant give you many more pointers. As a tech I'd first focus on where you modded the amp and check the loop connectors. Any place you soldered can have a cold solder joint or short. Most people don't know how to solder properly and don't know how to heat a connection before applying fluxed solder. If the amp has plastic connectors, they are prone to failure. They are welded together with plastic rivets which break loose and the garbage contacts tend to bend and loose contact.

      If the amp does have plastic connections there's a reason. The chassis may not be used as a common ground and any connection between the plug ground and chassis ground can do serious damage - likely to the power supply.

      If neither the preamp or power amp passes a signal, then the problem is likely the power supply which is common to both. The difference may be, the power tubes may receive a different b+ voltage. If the preamp works and the power amp doesn't then you've likely lost b+ voltage to the power tubes. There's a million other possibilities but I cant narrow them down till you rule some things out by making those tests.

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      • #4
        Originally posted by jimijames31 View Post
        ... I decided it must be the potentiometer for the mix and I did a really simple mod I found that converts it from parallel effects send to series...
        When troubleshooting, my first question to a customer is usually "what has changed?"

        What, exactly, did you do when you did the really simple mod?

        As a human being, you come with the whole range of inner possibilities
        from the deepest hell to the highest states.

        It is up to you which one you choose to explore
        .

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        • #5
          One more thing just to be sure you have all the bases covered...

          Did you check to make sure the Speaker Mute (Silent Recording) switch is not engaged? I used to play through a Subway Blues and sometimes the switch would get bumped when I put things in the back of the amplifier.

          As a human being, you come with the whole range of inner possibilities
          from the deepest hell to the highest states.

          It is up to you which one you choose to explore
          .

          Comment


          • #6
            Wow! I really appreciate the response here. I will try the first suggestion of replacing all the pre-amp tubes since that is the easiest. As for probe testing I think you hit the nail on the head when you said it is better left to a professional however I may toy around with that idea with something less dangerous and see if I can get good at it for the future . As for the speaker mute I am positive it is on. At one point when trying A few things to try and isolate the problem I put the speaker on mute but I was able to catch myself and turn it back on sadly did not solve the problem but a great suggestion none the less. The revisited the mod I made and made sure all the connections were solid. As far as I can tell nothing has changed there that would have affected the amp. However, my gut tells me it is either multiple pre-amp tubes that have gone bad or its the effects loop mod I did. I'm not sure how old this amp is or what happened to it before I owned it. I'll keep working with it and update you all when I figure it out.

            Cheers!

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            • #7
              Can you provide details about the effects loop mod - perhaps a link?


              Have you tried it with another speaker?


              It might also be a good idea to move the EQ Select switch located on the back of the amp to see if there is a bad connection there.

              Last edited by onelife; 04-25-2017, 07:26 PM.
              As a human being, you come with the whole range of inner possibilities
              from the deepest hell to the highest states.

              It is up to you which one you choose to explore
              .

              Comment


              • #8
                I have tried it with another speaker. The mod was based on something I saw on another forum. Here's the skinny,
                1) Remove the 2 orange wires from the send jack. One wire goes to the mix pot, the other goes back to the board. Remove the one that goes to the mix pot completely. It is not needed.

                2) Remove the red wire from the mix pot and connect it to the orange wire that goes back to the board. This eliminates the parallel circuit and makes it series.

                3) Cut the white wire from the mix pot and tape off the end.

                That's it! It took me about an hour and I had no instructions.
                If anyone sees anything wrong with what I did please let me know.

                Anyone who is running effects in their loop should do this mod. The difference is amazing!!

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                • #9
                  What kind of tubes do you all like. I need to get 6x 12ax7's and 2x 6L6's. I'm seeing Tung Sol sets and Sovtek on ebay but I'm not super familiar with the brands. Anyone have preferences?

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                  • #10
                    Again, you may have missed it in my post. Before you buy tubes you want to confirm the problem is in the preamp or the power amp. You're only guessing until you make theses checks.

                    What you need to do is connect the effects send to another amp, any amp that has a line level input. If you get sound, your preamp is fine. If you don't, you could start with new preamp tubes and hope you get lucky.

                    Next you connect a sound source to the effects return. You can try plugging a guitar in but the signal may be low but you should hear something. It can even be a CD player or FM receiver. If you hear sound, you know the power amp is good, if you don't new preamp tubes isn't going to fix the power amp problem. It may be the power tubes you need to replace.

                    If you get no sound from either section - then your problem is likely related to the power supply and you'd need professional help to fix it.

                    Comment


                    • #11
                      Originally posted by WRGKMC View Post
                      Again, you may have missed it in my post. Before you buy tubes you want to confirm the problem is in the preamp or the power amp. You're only guessing until you make theses checks.

                      What you need to do is connect the effects send to another amp, any amp that has a line level input. If you get sound, your preamp is fine. If you don't, you could start with new preamp tubes and hope you get lucky.

                      Next you connect a sound source to the effects return. You can try plugging a guitar in but the signal may be low but you should hear something. It can even be a CD player or FM receiver. If you hear sound, you know the power amp is good, if you don't new preamp tubes isn't going to fix the power amp problem. It may be the power tubes you need to replace.

                      If you get no sound from either section - then your problem is likely related to the power supply and you'd need professional help to fix it.
                      ‚ÄčI think the problem is most likely related to the modification done on the effects loop.

                      If so then it would render the test suggest above unreliable.

                      The fact that the loop has been changed at all makes anything to do with its function questionable.
                      As a human being, you come with the whole range of inner possibilities
                      from the deepest hell to the highest states.

                      It is up to you which one you choose to explore
                      .

                      Comment


                      • #12
                        I agree that the modiication is the most likely candidate for the issue. However, it was working great for awhile after the mod. If it was a botched mod and it stopped working when I modded it I would obviously know it was what I did. I went back and re-did the connection for the mod again and tested to make sure the connection was strong and I am still suffering the same issue. I am a newb and not to proud to admit that it is probably the mod to the effects loop. I may have a spare potentiometer and if so I can attempt to restore it to the original configuration. I do understand how to solder properly.

                        Comment


                        • #13
                          Originally posted by WRGKMC View Post
                          Again, you may have missed it in my post. Before you buy tubes you want to confirm the problem is in the preamp or the power amp. You're only guessing until you make theses checks.

                          What you need to do is connect the effects send to another amp, any amp that has a line level input. If you get sound, your preamp is fine. If you don't, you could start with new preamp tubes and hope you get lucky.

                          Next you connect a sound source to the effects return. You can try plugging a guitar in but the signal may be low but you should hear something. It can even be a CD player or FM receiver. If you hear sound, you know the power amp is good, if you don't new preamp tubes isn't going to fix the power amp problem. It may be the power tubes you need to replace.

                          If you get no sound from either section - then your problem is likely related to the power supply and you'd need professional help to fix it.

                          Thank you for these suggesitons I didn't miss them I just haven't had a moment yet to try these isolation tests. And I completly agree it would be stupid to purchase tubes without know where the problem actually is. I am praying to sweet Jesus that it isn't the power supply. I am assuming a repair of that magnitude would probably be worth more than the amp at this point?

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                          • #14
                            Originally posted by jimijames31 View Post
                            I agree that the modiication is the most likely candidate for the issue. However, it was working great for awhile after the mod. If it was a botched mod and it stopped working when I modded it I would obviously know it was what I did. I went back and re-did the connection for the mod again and tested to make sure the connection was strong and I am still suffering the same issue. I am a newb and not to proud to admit that it is probably the mod to the effects loop. I may have a spare potentiometer and if so I can attempt to restore it to the original configuration. I do understand how to solder properly.
                            I've learned to be grateful when I discover that problems like this are a result of something I've done wrong rather than something serious like a failure in the power supply.

                            Can you post a picture of the inside of the amp where you did the mod? I tried to compare your description of the mod in post #6 with the schematic you posted but couldn't really follow it because the wire colors are not marked on the diagram.
                            As a human being, you come with the whole range of inner possibilities
                            from the deepest hell to the highest states.

                            It is up to you which one you choose to explore
                            .

                            Comment


                            • #15
                              Here's a few photos.

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