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Re: [korg_mono-poly] Strange artifacts in filter resonance...

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  • Florian Anwander
    Hello Alain I could imagine, that the output potentiometer of one (or all) VCOs does not shut the VCO ouput completely. Though the VCO itself might be not
    Message 1 of 10 , Aug 26, 2010
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      Hello Alain

      I could imagine, that the output potentiometer of one (or all) VCOs does
      not "shut" the VCO ouput completely. Though the VCO itself might be not
      audible for us, the Filter on high resonance may react on it.

      You may try to disconnect the VCO pcb from the VCF/VCA pcb and check the
      VCF then.

      Florian

      > Hi everyone! Lately I've noticed that when I "play" the filter in self-resonance with all VCOs off, it doesn't produce as pure a sine wave as before. I can hear what seems to be faint harmonics at different frequencies. And when I look at a spectrum analyzer, I can in fact see some "other" frequencies at very low levels.
      >
      > When I sweep the cutoff, they bounce all over the place. A little like aliasing in a way. Don't worry I've eliminated everything else by take the output straight from the synth. So nothing else can add artifacts.
      >
      > Could it be due to failing capacitors in the filter section ? Especially those directly connected to the SSM2044 ?
      > Or could it be something else. I've already replaced the SSM2044 with a more recent one and they both exhibit the same behavior.
      >
      > It's a very minor annoyance since it's inaudible when filtering a signal from the VCOs or white noise.
      >
      >
      > Thanks for any suggestions you might have on this. And I'd be curious to know if anyone else has these artifacts on their MP4.
    • Alain
      Great idea Florian. I hadn t thought of that. I ll try as soon as i get home tonight and let you know what happens. Thanks! Alain.
      Message 2 of 10 , Aug 26, 2010
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        Great idea Florian. I hadn't thought of that. I'll try as soon as i get home tonight and let you know what happens. Thanks!

        Alain.



        --- In korg_mono-poly@yahoogroups.com, Florian Anwander <fanwander@...> wrote:
        >
        > Hello Alain
        >
        > I could imagine, that the output potentiometer of one (or all) VCOs does
        > not "shut" the VCO ouput completely. Though the VCO itself might be not
        > audible for us, the Filter on high resonance may react on it.
        >
        > You may try to disconnect the VCO pcb from the VCF/VCA pcb and check the
        > VCF then.
        >
        > Florian
        >
        > > Hi everyone! Lately I've noticed that when I "play" the filter in self-resonance with all VCOs off, it doesn't produce as pure a sine wave as before. I can hear what seems to be faint harmonics at different frequencies. And when I look at a spectrum analyzer, I can in fact see some "other" frequencies at very low levels.
        > >
        > > When I sweep the cutoff, they bounce all over the place. A little like aliasing in a way. Don't worry I've eliminated everything else by take the output straight from the synth. So nothing else can add artifacts.
        > >
        > > Could it be due to failing capacitors in the filter section ? Especially those directly connected to the SSM2044 ?
        > > Or could it be something else. I've already replaced the SSM2044 with a more recent one and they both exhibit the same behavior.
        > >
        > > It's a very minor annoyance since it's inaudible when filtering a signal from the VCOs or white noise.
        > >
        > >
        > > Thanks for any suggestions you might have on this. And I'd be curious to know if anyone else has these artifacts on their MP4.
        >
      • Alain
        Well, finally I m home. And I ve disconnected the wire harness coming from the VCO board so no signal can come from there at the filter input. And it still
        Message 3 of 10 , Aug 26, 2010
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          Well, finally I'm home. And I've disconnected the wire harness coming from the VCO board so no signal can come from there at the filter input. And it still exhibits the same behavior.

          I've also checked the +15v dc for possible ripples on the oscilloscope and it's perfectly flat, pure direct current at 15.01 volts (at the source on the power supply board). Since VR2 is biased with that same 15v dc, I thought it might have been a source of noise there, but I guess not.

          I've also trimmed down the maximum resonance level a little bit (with VR3 trimpot) since I've always found that it was twice as loud as the loudest sound coming from the VCOs or noise source. That has diminished the harmonics just a little, but they're still audible. Especially around 800hz and higher.

          Since this MP4 is now about 28 years old, I thought it might be components starting to degrade. I wasn't rich enough back in 1982 (I was 17) to buy a brand new MP4 then, so I've never heard its filter feedback when it was new. But I did hear an SSM2044 feedback in other circuits than the MP4, and it sounded perfectly sinusoidal with no artifacts whatsoever.

          Alain.




          --- In korg_mono-poly@yahoogroups.com, Florian Anwander <fanwander@...> wrote:
          >
          > Hello Alain
          >
          > I could imagine, that the output potentiometer of one (or all) VCOs does
          > not "shut" the VCO ouput completely. Though the VCO itself might be not
          > audible for us, the Filter on high resonance may react on it.
          >
          > You may try to disconnect the VCO pcb from the VCF/VCA pcb and check the
          > VCF then.
          >
          > Florian
          >
        • Alain
          Problem solved! It turns out that the filter section simply needed to be re-calibrated. I followed (and corrected along the way) the service manual procedure,
          Message 4 of 10 , Aug 26, 2010
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            Problem solved!

            It turns out that the filter section simply needed to be re-calibrated. I followed (and corrected along the way) the service manual procedure, and I discovered there was a small DC offset present at the filter output, and also that the resonance VCA and level were off.

            There are 2 corrections to the service manual and they are as follows:

            All on page 21

            Item 6.2 at some point it reads: "...If oscillation level is off, adjust VR3 to obtain 20V p.p at both sides of KLM-355 R10 (with CUTOFF at 10 and RESO at 10)"

            It should read instead:"...If oscillation level is off, adjust VR3 to obtain 20mv p.p at one side of KlM-355 R10 (with CUTOFF at 10 and RESO at 10)


            Item 7 at some point it reads:"...Then adjust KLM-355 VR4 to obtain 60mv p.p across both sides of KLM-355 R44."

            It should read instead:"...Then adjust KLM-355 VR4 to obtain 60mv p.p on one side of KLM-355 R43."

            These two corrections correspond to the ckeck points "E" and "F" on the KLM-355 board. And following these corrections, I was able to obtain exactly the specified required measurements and the resonance artifacts disappeared completely!

            Oh, and the graph in figure 2 is wrong. The drawing on the left is okay, displaying a 1-1.5v p.p total waveform amplitude. But the one on the right should have read the same with values of 1.5-2.0v p.p. for the TOTAL amplitude of the waveform including the resonance!

            The idea being that the filter is supposed to keep a constant amplitude of the signal regardless of the resonance level.

            Anyway, thanks Florian for the help!

            Alain.

            P.S. Of course these corrections apply to both versions (old production and new production) of the schematics, even though the checkpoints "E" and "F" are only identified on the new production schematics.







            --- In korg_mono-poly@yahoogroups.com, "Alain" <hubertalain@...> wrote:
            >
            > Hi everyone! Lately I've noticed that when I "play" the filter in self-resonance with all VCOs off, it doesn't produce as pure a sine wave as before. I can hear what seems to be faint harmonics at different frequencies. And when I look at a spectrum analyzer, I can in fact see some "other" frequencies at very low levels.
            >
            > When I sweep the cutoff, they bounce all over the place. A little like aliasing in a way. Don't worry I've eliminated everything else by take the output straight from the synth. So nothing else can add artifacts.
            >
            > Could it be due to failing capacitors in the filter section ? Especially those directly connected to the SSM2044 ?
            > Or could it be something else. I've already replaced the SSM2044 with a more recent one and they both exhibit the same behavior.
            >
            > It's a very minor annoyance since it's inaudible when filtering a signal from the VCOs or white noise.
            >
            >
            > Thanks for any suggestions you might have on this. And I'd be curious to know if anyone else has these artifacts on their MP4.
            >
            > Alain.
            >
          • Florian Anwander
            Hi Alain Thanks for the detailled information. May I put your findings on my MonoPoly page? Florian
            Message 5 of 10 , Aug 27, 2010
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              Hi Alain

              Thanks for the detailled information. May I put your findings on my
              MonoPoly page?

              Florian
            • Steve Carter
              Very useful info Alain. It shows once again that you can t take manufacturer s service info on face value. I ve found this time and time again especially
              Message 6 of 10 , Aug 27, 2010
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                Very useful info Alain.  It shows once again that you can't take manufacturer's service info on face value.  I've found this time and time again especially with equipment from the 1970s and early 80s. 

                Thanks for sharing

                Steve



                On 27 Aug 2010, at 00:55, Alain wrote:

                 

                Problem solved!

                It turns out that the filter section simply needed to be re-calibrated. I followed (and corrected along the way) the service manual procedure, and I discovered there was a small DC offset present at the filter output, and also that the resonance VCA and level were off.

                There are 2 corrections to the service manual and they are as follows:

                All on page 21

                Item 6.2 at some point it reads: "...If oscillation level is off, adjust VR3 to obtain 20V p.p at both sides of KLM-355 R10 (with CUTOFF at 10 and RESO at 10)"

                It should read instead:"...If oscillation level is off, adjust VR3 to obtain 20mv p.p at one side of KlM-355 R10 (with CUTOFF at 10 and RESO at 10)

                Item 7 at some point it reads:"...Then adjust KLM-355 VR4 to obtain 60mv p.p across both sides of KLM-355 R44."

                It should read instead:"...Then adjust KLM-355 VR4 to obtain 60mv p.p on one side of KLM-355 R43."

                These two corrections correspond to the ckeck points "E" and "F" on the KLM-355 board. And following these corrections, I was able to obtain exactly the specified required measurements and the resonance artifacts disappeared completely!

                Oh, and the graph in figure 2 is wrong. The drawing on the left is okay, displaying a 1-1.5v p.p total waveform amplitude. But the one on the right should have read the same with values of 1.5-2.0v p.p. for the TOTAL amplitude of the waveform including the resonance!

                The idea being that the filter is supposed to keep a constant amplitude of the signal regardless of the resonance level.

                Anyway, thanks Florian for the help!

                Alain.

                P.S. Of course these corrections apply to both versions (old production and new production) of the schematics, even though the checkpoints "E" and "F" are only identified on the new production schematics.

                --- In korg_mono-poly@yahoogroups.com, "Alain" <hubertalain@...> wrote:
                >
                > Hi everyone! Lately I've noticed that when I "play" the filter in self-resonance with all VCOs off, it doesn't produce as pure a sine wave as before. I can hear what seems to be faint harmonics at different frequencies. And when I look at a spectrum analyzer, I can in fact see some "other" frequencies at very low levels.
                >
                > When I sweep the cutoff, they bounce all over the place. A little like aliasing in a way. Don't worry I've eliminated everything else by take the output straight from the synth. So nothing else can add artifacts.
                >
                > Could it be due to failing capacitors in the filter section ? Especially those directly connected to the SSM2044 ?
                > Or could it be something else. I've already replaced the SSM2044 with a more recent one and they both exhibit the same behavior.
                >
                > It's a very minor annoyance since it's inaudible when filtering a signal from the VCOs or white noise.
                >
                >
                > Thanks for any suggestions you might have on this. And I'd be curious to know if anyone else has these artifacts on their MP4.
                >
                > Alain.
                >


              • Alain
                Absolutely Florian! If it can help someone else, by all means do so. Alain.
                Message 7 of 10 , Aug 27, 2010
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                  Absolutely Florian! If it can help someone else, by all means do so.

                  Alain.


                  --- In korg_mono-poly@yahoogroups.com, Florian Anwander <fanwander@...> wrote:
                  >
                  > Hi Alain
                  >
                  > Thanks for the detailled information. May I put your findings on my
                  > MonoPoly page?
                  >
                  > Florian
                  >
                • Alain
                  Yes Steve, unfortunately most of the service manuals that I ve come across did have mistakes or typos in them. Let s hope that people responsible for fixing
                  Message 8 of 10 , Aug 27, 2010
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                    Yes Steve, unfortunately most of the service manuals that I've come across did have mistakes or typos in them. Let's hope that people responsible for fixing electronic gizmos under warranty have access to more up-to-date versions...

                    After all, the versions available freely on the web might be re-typed or even outdated. But, hey, they're free...

                    Anyway, even if most are imperfect, they're still a precious source of info for such vintage pieces of gear. As long as we can keep wonderful synths like the MP4 going strong, that's the main goal!

                    Alain.

                    --- In korg_mono-poly@yahoogroups.com, Steve Carter <synth@...> wrote:
                    >
                    > Very useful info Alain. It shows once again that you can't take manufacturer's service info on face value. I've found this time and time again especially with equipment from the 1970s and early 80s.
                    >
                    > Thanks for sharing
                    >
                    > Steve
                    >
                    >
                    >
                    > On 27 Aug 2010, at 00:55, Alain wrote:
                    >
                    > > Problem solved!
                    > >
                    > > It turns out that the filter section simply needed to be re-calibrated. I followed (and corrected along the way) the service manual procedure, and I discovered there was a small DC offset present at the filter output, and also that the resonance VCA and level were off.
                    > >
                    > > There are 2 corrections to the service manual and they are as follows:
                    > >
                    > > All on page 21
                    > >
                    > > Item 6.2 at some point it reads: "...If oscillation level is off, adjust VR3 to obtain 20V p.p at both sides of KLM-355 R10 (with CUTOFF at 10 and RESO at 10)"
                    > >
                    > > It should read instead:"...If oscillation level is off, adjust VR3 to obtain 20mv p.p at one side of KlM-355 R10 (with CUTOFF at 10 and RESO at 10)
                    > >
                    > > Item 7 at some point it reads:"...Then adjust KLM-355 VR4 to obtain 60mv p.p across both sides of KLM-355 R44."
                    > >
                    > > It should read instead:"...Then adjust KLM-355 VR4 to obtain 60mv p.p on one side of KLM-355 R43."
                    > >
                    > > These two corrections correspond to the ckeck points "E" and "F" on the KLM-355 board. And following these corrections, I was able to obtain exactly the specified required measurements and the resonance artifacts disappeared completely!
                    > >
                    > > Oh, and the graph in figure 2 is wrong. The drawing on the left is okay, displaying a 1-1.5v p.p total waveform amplitude. But the one on the right should have read the same with values of 1.5-2.0v p.p. for the TOTAL amplitude of the waveform including the resonance!
                    > >
                    > > The idea being that the filter is supposed to keep a constant amplitude of the signal regardless of the resonance level.
                    > >
                    > > Anyway, thanks Florian for the help!
                    > >
                    > > Alain.
                    > >
                    > > P.S. Of course these corrections apply to both versions (old production and new production) of the schematics, even though the checkpoints "E" and "F" are only identified on the new production schematics.
                    > >
                    > > --- In korg_mono-poly@yahoogroups.com, "Alain" <hubertalain@> wrote:
                    > > >
                    > > > Hi everyone! Lately I've noticed that when I "play" the filter in self-resonance with all VCOs off, it doesn't produce as pure a sine wave as before. I can hear what seems to be faint harmonics at different frequencies. And when I look at a spectrum analyzer, I can in fact see some "other" frequencies at very low levels.
                    > > >
                    > > > When I sweep the cutoff, they bounce all over the place. A little like aliasing in a way. Don't worry I've eliminated everything else by take the output straight from the synth. So nothing else can add artifacts.
                    > > >
                    > > > Could it be due to failing capacitors in the filter section ? Especially those directly connected to the SSM2044 ?
                    > > > Or could it be something else. I've already replaced the SSM2044 with a more recent one and they both exhibit the same behavior.
                    > > >
                    > > > It's a very minor annoyance since it's inaudible when filtering a signal from the VCOs or white noise.
                    > > >
                    > > >
                    > > > Thanks for any suggestions you might have on this. And I'd be curious to know if anyone else has these artifacts on their MP4.
                    > > >
                    > > > Alain.
                    > > >
                    > >
                    > >
                    >
                  • Kaleb
                    Hello, I recently started getting what sounds to be a similar, although more severe, problem on my M/P; a warbling seemingly-random filter drop out sound.
                    Message 9 of 10 , Sep 21, 2011
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                      Hello,

                      I recently started getting what sounds to be a similar, although more severe, problem on my M/P; a warbling seemingly-random filter "drop out" sound. These artifacts are noticeable when playing the VCO's, but most noticeable when bringing the LPF into self-oscillation -- at which point dramatic shifts in pitch occur.

                      I must admit, the random warbling sounds pretty awesome, in its own right, but it is driving me crazy that I cannot shut it off. The pure tone of my oscillators can no longer be achieved or held for any period of time.

                      How does one recalibrate the filter section? Was it a particularly difficult or involved procedure? Do you have the documentation you mentioned in a PDF form?

                      I am, by no means, an electronics expert. I have done some simple things to my Korg over the 15 years of my owning it, like cleaning contacts, etc, but have not needed to replace any components yet. Hopefully this filter recalibration will do the trick though!

                      Thanks for your guidance,

                      -Kaleb




                      --- In korg_mono-poly@yahoogroups.com, "Alain" <hubertalain@...> wrote:
                      >
                      > Problem solved!
                      >
                      > It turns out that the filter section simply needed to be re-calibrated. I followed (and corrected along the way) the service manual procedure, and I discovered there was a small DC offset present at the filter output, and also that the resonance VCA and level were off.
                      >
                      > There are 2 corrections to the service manual and they are as follows:
                      >
                      > All on page 21
                      >
                      > Item 6.2 at some point it reads: "...If oscillation level is off, adjust VR3 to obtain 20V p.p at both sides of KLM-355 R10 (with CUTOFF at 10 and RESO at 10)"
                      >
                      > It should read instead:"...If oscillation level is off, adjust VR3 to obtain 20mv p.p at one side of KlM-355 R10 (with CUTOFF at 10 and RESO at 10)
                      >
                      >
                      > Item 7 at some point it reads:"...Then adjust KLM-355 VR4 to obtain 60mv p.p across both sides of KLM-355 R44."
                      >
                      > It should read instead:"...Then adjust KLM-355 VR4 to obtain 60mv p.p on one side of KLM-355 R43."
                      >
                      > These two corrections correspond to the ckeck points "E" and "F" on the KLM-355 board. And following these corrections, I was able to obtain exactly the specified required measurements and the resonance artifacts disappeared completely!
                      >
                      > Oh, and the graph in figure 2 is wrong. The drawing on the left is okay, displaying a 1-1.5v p.p total waveform amplitude. But the one on the right should have read the same with values of 1.5-2.0v p.p. for the TOTAL amplitude of the waveform including the resonance!
                      >
                      > The idea being that the filter is supposed to keep a constant amplitude of the signal regardless of the resonance level.
                      >
                      > Anyway, thanks Florian for the help!
                      >
                      > Alain.
                      >
                      > P.S. Of course these corrections apply to both versions (old production and new production) of the schematics, even though the checkpoints "E" and "F" are only identified on the new production schematics.
                      >
                      >
                      >
                      >
                      >
                      >
                      >
                      > --- In korg_mono-poly@yahoogroups.com, "Alain" <hubertalain@> wrote:
                      > >
                      > > Hi everyone! Lately I've noticed that when I "play" the filter in self-resonance with all VCOs off, it doesn't produce as pure a sine wave as before. I can hear what seems to be faint harmonics at different frequencies. And when I look at a spectrum analyzer, I can in fact see some "other" frequencies at very low levels.
                      > >
                      > > When I sweep the cutoff, they bounce all over the place. A little like aliasing in a way. Don't worry I've eliminated everything else by take the output straight from the synth. So nothing else can add artifacts.
                      > >
                      > > Could it be due to failing capacitors in the filter section ? Especially those directly connected to the SSM2044 ?
                      > > Or could it be something else. I've already replaced the SSM2044 with a more recent one and they both exhibit the same behavior.
                      > >
                      > > It's a very minor annoyance since it's inaudible when filtering a signal from the VCOs or white noise.
                      > >
                      > >
                      > > Thanks for any suggestions you might have on this. And I'd be curious to know if anyone else has these artifacts on their MP4.
                      > >
                      > > Alain.
                      > >
                      >
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