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Acoustic Model 116 Bass Combo DC on Output Help

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  • #16
    I have a scope.

    This is getting more interesting.

    I have been hearing the pop and crackling when turning the amp on or off.

    Amp is ON, guitar plugged in and volume around 3 (same as when video was made).

    I banged the front top of the chassis and hear the crackling. I fist bang it about 5 times and each time it gets more quiet and finally is gone. I bang and chopstick all over the place. No noise now. I shut off the amp and there is no longer a pop and crackle. I turn it on and no longer hear a pop or crackle.

    Low notes are now playing fine.

    There is a cap across the power switch which I now suspect could have something to do with the noise I was hearing. I plan to reflow it. Could this cap have been creating the crackle sounds?


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    • #17
      I believe that cap is used to control the turn on/off pop or surging across the power switch. So once the amp is on, I don't know if it would have any effect. If it were me, I would disconnect the cap and see if it has any effect on the amp.

      If you have a wooden chopstick, poke around in the amp, on each PC board, and see if you can find a spot that causes the crackle.

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      Acoustic_114_115_Schematic.pdf
      It's not just an amp, it's an adventure!

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      • #18
        That capacitor (C101), is not in circuit when the amplifier is switched on! Neither will it be the switch being intermittent as C101 will stop any arcing.
        You will find a dry joint, if it is preamp side of the volume control, it will be affected by the volume control and reduce/increase accordingly.
        I would check the large smoothing capacitors first. Reflow the contacts with a little flux rosin solder then check for any heavy components and do the same.
        The clue is, vibration and that is usually a sign there is a problem with a larger or heavy component

        Have you cleaned the edge connectors.
        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
        If you can't fix it, I probably can.

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        • #19
          Originally posted by Jon Snell View Post

          Have you cleaned the edge connectors.
          What are the "edge connectors"? Are those the wires connecting on the edge of the boards? I have not but I will.

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          • #20
            Originally posted by TomCarlos View Post
            If you have a wooden chopstick, poke around in the amp, on each PC board, and see if you can find a spot that causes the crackle.

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            Yes but will try again.

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            • #21
              Originally posted by misterc57 View Post

              What are the "edge connectors"? Are those the wires connecting on the edge of the boards? I have not but I will.
              Correct, they have a possibility of bad connections.
              Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
              If you can't fix it, I probably can.

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              • #22
                The chassis is back on my bench. I could create noise by touching the B+ wires at the large filter cap. I re-did that connection and it no longer makes noise.

                I do have noise on the pre-amp board. Mostly near the input wires. I reflowed all the edge connectors on the pre-amp board. Reflowed every connection near the input wires. Reflowed the green wire coming from the positive side of the input jacks. Checked ground continuity at the input jacks and continuity on the green wire.

                The green wire (positive input jacks) still makes "scratch" noises when touch with a chopstick.

                Testing the amp things are quieter but there is still some scratchiness noise after suspending a note. Tapping the pre-amp board can still cause some noise, intermittently..

                I am thinking to reflow every connection on the entire pre-amp board. I also wonder if the pots could be contributing to the noise. They have all been cleaned and lubed. The bass pot looks to have been replaced in the past.

                Any thoughts? Thank you

                Pics below


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                • #23
                  I suggest replacing each of the regulators with an assembly of two constant resistors. The accuracy of these resistors is not so important. You should first determine at which position of the variable resistors distortion occurs, measure each half of the variable resistor, and solder the assembly in.
                  Then check again for distortion.​

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                  • #24
                    The story continues. Was having a lot of noise on the preamp board when taping with a chop-stick. The first input jack was self grounded to the chassis. I added a ground wire from the first jack to the preamp board (at the same edge connector as the ground wire from the second input jack). This got rid of all the noise when tapping the preamp board.


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                    I reconnected the chassis to the combo cab with the 15" Scorpion speaker. I now hear terrible note "farts" on all the low notes.

                    I forgot that I have a beefy Yamaha PA cab loaded with a 15" and horn. I connected the chassis to that cab and no longer hear note "farts". So hopefully I am now left with a speaker issue only.

                    This amp is not worth buying a new speaker. I was given another used 15" a few days ago and will see if that one plays okay in the cab.





                    Last edited by misterc57; 11-22-2023, 02:17 AM.

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                    • #25
                      Did you try the combo speaker with the chassis moved away from the cab? If not, it could be a vibration issue.
                      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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                      • #26
                        Originally posted by g1 View Post
                        Did you try the combo speaker with the chassis moved away from the cab? If not, it could be a vibration issue.
                        I did.

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                        • #27
                          Brief moments of joyous amp followed by noise.

                          There are moments when the amp is playing fine with the Scorpion. Chassis was sitting on top of cabinet with several layers of cushioning.

                          Still some noise around the input section.

                          I did more tweaks to the input jacks. Jack one positive signal no longer pass through jack two. New wire from input jack one to the pre-amp board.

                          Chassis is back installed in the cab. Improvements for a moment then it all goes to crap again.

                          This amp picks up a lot of noise from the room and other electronics, including my phone. But it could be the pickups in my bass guitar was picking up my phone (entering pin to access phone).

                          Tried a guitar instead of the bass to see if there is noise. There is. Here is a video with guitar (not bass) plugged into jack one. Volume around 3.


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                          • #28
                            I'd still be suspicious of all the other connections on that edge connector.
                            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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                            • #29
                              Originally posted by g1 View Post
                              I'd still be suspicious of all the other connections on that edge connector.
                              The edge connectors on the main board and the main board itself make no noise.

                              I have reflowed all the edge connections on the preamp board. There are 3 at the input and 2 at the output. The 2 at the output make no noise.

                              The sweet spot for noise is tapping at the input area of the preamp board about 1/2" from the edge connectors. All components in that area were reflowed. All wires to edge connectors have continuity even when moving the wires around.

                              I had noticed that rubbing the input wires from the input jacks to the board causes a scratching sound. This is why I replaced jack one wire to board. Wondering if these wires need to be shielded?

                              Please keep suggestions coming.

                              Thank you

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                              • #30
                                I replaced the first two disc caps on the pre-amp board with new non-disc, and have had no noise since. Fingers crossed.

                                C401 with a 220p (closest value I had to the 150p)
                                C402 .01

                                Can someone tell me the purpose of C401 in the circuit?

                                Thank you

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