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

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  • #16
    I was able to run the send to one of my other amps and I did get signal out of it, however, I was picking up an radio signal that was very loud along with my instrument. On the plus side the master volume for that channel was working during the test which it hasn't done since this problem started. Next I ran my phone into the return and played some Pearl Jam. It came through loud and clear. I also used my multimeter to test the eq switch on the back of the amp and it appears to be working correctly same with the speaker mute switch.

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    • onelife
      onelife commented
      Editing a comment
      what happens when you use a guitar cable to connect the effects send directly to the effects return?

    • jimijames31
      jimijames31 commented
      Editing a comment
      Nothing. I tried that awhile back, it was in an earlier post.

  • #17
    You got a signal from the preamp so you know that's working. You got a signal through the power amp so you know that's working.

    That's good news.

    Your problem is likely something to do with the mod you did.

    My instincts tell me, when you converted the amp to a serial effects loop and that pot was taken out of the circuit, something changed with the gain staging the amp doesn't like.

    First be sure the jumper you're using to connect the send and return is good. If it is, I'd suggest you reverse it and see what happens.

    Ideally, the mod should have been done so when nothing is plugged into the send and return - the switched jacks provide switching to connect the send and return together.

    The pot may be key to proper gain staging. It may reduce the gain and balance the feed level between the pre and power amp. Without it the signal may be too strong and somethings breaking down, or there's a voltage imbalance between staging that shuts a component down.

    The mod worked for awhile yes, but was it good for the circuitry. Tubes can take months to weaken. Each stage may function when that are separated and don't have the rest of the circuity loading them down. Or like I said, that pot missing from the circuit may have created an imbalance.
    You wont know until you reverse things and set them back to factory specs. If the amp works, then its best to abandon that mod and find another way of connecting your effects. (Or fix the amp, sell it and buy something that can do what you want)

    The radio signal isn't a big deal. Its likely an issue with your guitar. I Have a strat that does that when the signal is gained up high and the conditions are right. Under the right conditions guitar pickups and potentiometer can act like a fox hole radio and pick up AM Radio stations. It may also be the connection between one amp and another not being properly grounded. You can add ferrite beads to choke that off if it still persists after the amp is working properly.

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    • onelife
      onelife commented
      Editing a comment
      When there is nothing plugged into the effects jacks, the the FX MIX pot has no impact on the signal path.

  • #18
    From the before and after photos you posted I can now see your problem.

    The effects send jack is known as a "shorting jack" like the ones used for headphones that turn off speakers when you plug the phones in.

    In this circuit, with nothing plugged into the send jack the white and orange wires would be connected together. This allows the signal from the preamp to pass through to the power amp. By removing the short orange wire, you severed this connection.

    The fix is actually quite easy. You have connected the orange wire and the red wire together and taped them up. If I was doing it, I would simply undo the tape then connect both the red wire and the orange wire to the terminal on the send jack that had the short orange wire connected to it. This will restore the shorting function of the send jack so that, with nothing plugged into the effects loop, the amp will function as it did before the mod.

    It seems the problem you are having with your rack mount effects is a different issue - perhaps something as simple as a bad cable. I suggest you try it with your other amplifier and see if it works there. I would also suggest testing the effects loop on your DC-5 with a completely different effects setup - perhaps a single stomp box with a different set of cables.
    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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    • #19
      I have tested the effects loop with not only the effects rack but also a stomp delay. I have also tried several cables. Its simple to connect the wires up as you suggested so I may give that a shot but I am leaning toward the imbalance idea as well. I may ty both suggestions here and see which bears fruit. I am hoping to get sometime to try it out tonight. I know fo a fact that the potentiometer for the effects loop that I removed is non-functional. I tested it with my multimeter and it is completly shot. I have to try to figure out a good replacment. I'm not sure what the resistance on that particular pot is but im sure it wont be hard to figure out. Thank you both for the attention to this issue.

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      • #20
        FWIW - from the schematic you can see the FX MIX pot is 25 K Ohms with a Linear Taper

        Also from the schematic - you can see that, when there is nothing plugged into the effects loop, the pot makes no difference whatsoever to the circuit. It is not necessary to replace it unless you want to use the mix function.

        You mentioned in the OP that you can use a soldering iron and, from the photo, it doesn't look like the mod caused any damage. I'm confident your amp will work fine if you solder the red and yellow wires (together) to the third lug on the effects send jack - without any need to replace the mix pot.
        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


        • jimijames31
          jimijames31 commented
          Editing a comment
          Im working on this as we speak. I'll let you know the result

        • jimijames31
          jimijames31 commented
          Editing a comment
          SHE LIVES!!! Oh man it feels so good to hear a beautiful tone pouring out of this baby again. I don't understand what difference it makes having it soldered to the lug though. I need to go back to the other forum I got the mod from and post an update so some other poor sap doesn't make the same mistake. It's still a very simple mod. Thank you so much for the help!!! I can't thank you enough.
          Last edited by jimijames31; 04-27-2017, 10:20 PM.

      • #21
        WRGKMC and Onelife you are both life savers. I really hope this post helps someone in the future. I'm sure I saved a couple of hundred dollars not having to take it up to Mesa Boogie in Petaluma.

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        • #22
          Originally posted by jimijames31 View Post
          WRGKMC and Onelife you are both life savers. I really hope this post helps someone in the future. I'm sure I saved a couple of hundred dollars not having to take it up to Mesa Boogie in Petaluma.
          I am glad you got it to work.

          I don't understand what difference it makes having it soldered to the lug though.
          It's late now and I must get some sleep but tomorrow I will write up an explanation of how the circuit works using the schematic as a reference.

          It helped when you posted the photos - I was able to compare them to the schematic and see where the problem was. As I mentioned in a previous post, the first place to look when you have these problems is at what has changed. In this case, it was the 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
          .

          Comment


          • #23

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            • #24
              Click image for larger version

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              Notice the arrowhead just above the word MIX. It represents the centre lug on the pot and had two orange wires attached to it. One of the wires went to the send jack as you can see on the diagram, The other orange wire goes to the main circuit board and, through the 470 Ohm resistor, connects to V3b.

              The white wire from the send jack goes to the circuit board and comes back to connect to one end of the pot. This is represented on the diagram with the line from the word SEND, over the word FX LOOP and connects to one end of the pot near the 0% label.

              The red wire that comes up from under the small circuit board connects the other end of the pot to the RTRN jack as you can see on the schematic.

              On the diagram, notice the difference between the SEND jack and the RTRN jack. The difference is the third lug on the SEND jack and when there is nothing plugged in to it, the white wire and orange wire are connected together. This allows the signal from the 1uf coupling capacitor to pass through the circuit to the next stage (V3b). When a cable is plugged into the SEND jack, the connection is interrupted so the signal has to pass through whatever is plugged into the effects loop before it can get to the next stage.

              Before the mod, turning the MIX POT to 0% connected the input to the send jack, through the 470 Ohm resistor to V3b.


              When you did the mod, which is essentially removing the MIX POT and the wire on the diagram that goes from the SEND jack to the 0% end of the pot, you still needed to have the third lug of the SEND jack connected via the orange wire to the 470 Ohm resistor to V3b in order for the amp to work without anything plugged into the loop. The RTRN jack also needs to connect to the 470 Ohm resistor which is why the red wire and orange wire need to be connected together. Connecting both the orange and red wires to the third lug of the SEND jack is what allows the signal to pass through whether or not there is anything plugged into the loop.




              btw, were you able to solve your initial problem with your effects rack not working with the DC-5?


              Last edited by onelife; 04-28-2017, 12:43 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


              • jimijames31
                jimijames31 commented
                Editing a comment
                Yeah the effects rack problem was because the Fx loop wasn't working. I have now got it all working together. I actual built a box and transferred the dc-5 into it and have my FX unit in it as well. Now I'm dialing the effects for each song my cover band does.
                Last edited by jimijames31; 05-05-2017, 10:13 PM.

            • #25

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              • #26
                It's all working like a charm. My drummer was worried that the head wouldn't be powerful enough to run my 4x12. Right up until I hit the first chord.

                Comment


                • onelife
                  onelife commented
                  Editing a comment












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