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  • Oscilloscope Settings to trace oscillation

    Looking for the best settings on a PC based oscilloscope and signal generator to trace oscillation on a 2010 Laney 50 watt head.

  • #2
    You shouldn't need any special settings to see an oscillation. With normal scope settings it would appear as a smaller wave form impressed on the normal wave form. Or, with an overdriven or square wave test it may show as a wavy line at the top and bottom clip where it should otherwise be flat (ringing). The important thing though is to apply appropriate test circumstances that instigate the problem you're looking for. That is, if you test the amp with an 80mV or 100mV input signal (which is standard) and see NOTHING that may not mean the amp is alright IF you only get oscillation symptoms on hard attack or when pushing the amp with a booster, etc. The test has to replicate the circumstances that instigate the problem. Further, Oscope testing usually involves a resistive load. And this will not always tell you what the amp does under the same circumstances when played into a speaker load because speakers have a variable impedance @ frequency while resistors do not.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    • #3
      Originally posted by Chuck H View Post
      You shouldn't need any special settings to see an oscillation. With normal scope settings it would appear as a smaller wave form impressed on the normal wave form. Or, with an overdriven or square wave test it may show as a wavy line at the top and bottom clip where it should otherwise be flat (ringing). The important thing though is to apply appropriate test circumstances that instigate the problem you're looking for. That is, if you test the amp with an 80mV or 100mV input signal (which is standard) and see NOTHING that may not mean the amp is alright IF you only get oscillation symptoms on hard attack or when pushing the amp with a booster, etc. The test has to replicate the circumstances that instigate the problem. Further, Oscope testing usually involves a resistive load. And this will not always tell you what the amp does under the same circumstances when played into a speaker load because speakers have a variable impedance @ frequency while resistors do not.
      I can get it to oscillate by pushing xpand or boost at almost any volume. Would you be interested in walking me thru the process of tracing oscillation? Also very little experience tracing signal. Thanks

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      • #4
        In no particular order:

        * what brought you here?
        Meaning: what problem do you find, what do you hear, which is the symptom which caught your attention?

        * what do you do / how does it appear?
        Is it constant or varies?

        * Clip scope leads to speaker out.
        in general, you set scope to some scale which allows the full output waveform (say 15-20V RMS or so) to occupy the full screen, with a little margin of course, so you set your volts per division scale to some value which allows for, say, 30 or 40 V peak displayed.

        To confirm, drive amp to clipping and back some into clean: is it too small to see comfortably?
        Is is larger than screen?
        Adjust accordingly.

        * if you injected 400Hz or 1 kHz, set horizontal frequency so you see 2 or 3 waveforms on screen.

        * now that your screen is reasonably showing amp output, what do you see?

        A pure sinewave or weird stuff mixed in?

        If you turn generator OFF but touch nothing else: do you have an oscillation?

        * show us some screen capture.
        Juan Manuel Fahey

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        • #5
          Originally posted by J M Fahey View Post
          In no particular order:

          * what brought you here?
          Meaning: what problem do you find, what do you hear, which is the symptom which caught your attention?

          * what do you do / how does it appear?
          Is it constant or varies?

          * Clip scope leads to speaker out.
          in general, you set scope to some scale which allows the full output waveform (say 15-20V RMS or so) to occupy the full screen, with a little margin of course, so you set your volts per division scale to some value which allows for, say, 30 or 40 V peak displayed.

          To confirm, drive amp to clipping and back some into clean: is it too small to see comfortably?
          Is is larger than screen?
          Adjust accordingly.

          * if you injected 400Hz or 1 kHz, set horizontal frequency so you see 2 or 3 waveforms on screen.

          * now that your screen is reasonably showing amp output, what do you see?

          A pure sinewave or weird stuff mixed in?

          If you turn generator OFF but touch nothing else: do you have an oscillation?

          * show us some screen capture.
          2010 50w Laney oscillation when either xpand or boost switch is engaged. NFB loop was checked, reconnected. Will begin tracing asap. Thanks for your help.

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          • #6
            Originally posted by ca7922303 View Post

            2010 50w Laney oscillation when either xpand or boost switch is engaged. NFB loop was checked, reconnected. Will begin tracing asap. Thanks for your help.
            Oscillation is constant w/xpand or boost engaged. It's a four(4) function piggyback switch. Modern/Bright/Xpand/Boost. All four can be engaged and there is no oscillation, only when either xpand or boost only. Switch was recently replaced. Could be bad switch or bad solder joint?

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            • #7
              Originally posted by J M Fahey View Post
              In no particular order:

              * what brought you here?
              Meaning: what problem do you find, what do you hear, which is the symptom which caught your attention?

              * what do you do / how does it appear?
              Is it constant or varies?

              * Clip scope leads to speaker out.
              in general, you set scope to some scale which allows the full output waveform (say 15-20V RMS or so) to occupy the full screen, with a little margin of course, so you set your volts per division scale to some value which allows for, say, 30 or 40 V peak displayed.

              To confirm, drive amp to clipping and back some into clean: is it too small to see comfortably?
              Is is larger than screen?
              Adjust accordingly.

              * if you injected 400Hz or 1 kHz, set horizontal frequency so you see 2 or 3 waveforms on screen.

              * now that your screen is reasonably showing amp output, what do you see?

              A pure sinewave or weird stuff mixed in?

              If you turn generator OFF but touch nothing else: do you have an oscillation?

              * show us some screen capture.
              Are settings correct for signal generator?

              Comment


              • #8
                Originally posted by ca7922303 View Post

                Are settings correct for signal generator?
                No. You're sending 20 whole volts into the amp. Not what you want.

                Aim for around 100-150 mv. Maybe a little more to push things a bit, but not more than like 500 mv. 20 volts is way too much.

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                • #9
                  Originally posted by Greg_L View Post

                  No. You're sending 20 whole volts into the amp. Not what you want.

                  Aim for around 100-150 mv. Maybe a little more to push things a bit, but not more than like 500 mv. 20 volts is way too much.
                  Having driver issues with PC based oscilloscope, can we use signal generator and DMM on ac volts to trace?

                  Comment


                  • #10
                    Originally posted by Greg_L View Post

                    No. You're sending 20 whole volts into the amp. Not what you want.

                    Aim for around 100-150 mv. Maybe a little more to push things a bit, but not more than like 500 mv. 20 volts is way too much.
                    How bout this?

                    Comment


                    • #11
                      Here is link to schematic if anyone can walk me thru tracing with signal generator and DMM for oscillation. Thanks.
                      https://acrobat.adobe.com/link/track...0-ad65930e0bba

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                      • #12
                        Originally posted by ca7922303 View Post

                        How bout this?
                        That would be ok, but now you're hitting it with a pretty weak signal.

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                        • #13
                          Originally posted by ca7922303 View Post
                          Here is link to schematic if anyone can walk me thru tracing with signal generator and DMM for oscillation. Thanks.
                          https://acrobat.adobe.com/link/track...0-ad65930e0bba
                          DMMs have limited frequency bandwidth and typically don't register HF oscillation.
                          Only a "wide-band" scope allows to discern between signal and oscillation.
                          Last edited by Helmholtz; 03-11-2022, 07:35 PM.
                          - Own Opinions Only -

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                          • #14
                            Originally posted by Greg_L View Post

                            That would be ok, but now you're hitting it with a pretty weak signal.
                            Where do I need to be at on signal generator?

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                            • #15
                              50mv is fine.

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