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Oscilliscope Question

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  • billdeg
    I have a digital pocket scope. DSO QUAD....When I use it to generate a 100Hz sine wave I read it comfortably in the AC / 2V / AUTO / 10mS range. On my older
    Message 1 of 9 , Jun 6, 2014
      I have a digital pocket scope.  DSO QUAD....When I use it to generate a 100Hz sine wave I read it comfortably in the AC / 2V / AUTO / 10mS range.

      On my older analog Tek 5103N, to duplicate the screen I see on the digital scope I have to use the following settings - volts/div = -2  Secs/div 5m. The differences in the two oscilloscopes' settings, to see the same wave, have something to do with the probe's capacitance, right?  I know about 10x probes, this on has no markings.  I think it may be 50 years old. 

      I never heard from anyone about my question from before, I'll restate simply making a few logical assumptions to get to the point...
      Is it stange that a cap labeled 590 would read ~850uF - can they increase capacitance as they go bad?
      2) For a 850uF cap, an ESR of .13ohms is too high/marginally leaky yes?
      3) If it was really a 590uF an ESR = .13ohms would have been "more OK".

      NOTE - I replaced the cap and now the circuit works.  The original problem of a ~11.6v on a 12v line did not turn out to be the problem, it was the performance of the cap I removed with characteristics stated above.

      Thanks for your help, my lunch is getting cold.
      Bill


    • billdeg
      Here is the picture of the scope controls and probe (different settings than described in my post)
      Message 2 of 9 , Jun 6, 2014
        Here is the picture of the scope controls and probe (different settings than described in my post)
        http://vintagecomputer.net/cromemco/system_one/repairs/Tek-5103N_.settings.jpg

         

      • Dave McGuire
        ... First, I want to be sure I know what you mean when you say to duplicate the screen . I assume you mean you ve read the voltages on each by counting the
        Message 3 of 9 , Jun 6, 2014
          On 06/06/2014 11:43 AM, billdeg wrote:
          > I have a digital pocket scope. DSO QUAD....When I use it to generate a
          > 100Hz sine wave I read it comfortably in the AC / 2V / AUTO / 10mS range.
          >
          > On my older analog Tek 5103N, to duplicate the screen I see on the
          > digital scope I have to use the following settings - volts/div = -2
          > Secs/div 5m. The differences in the two oscilloscopes' settings, to see
          > the same wave, have something to do with the probe's capacitance,
          > right? I know about 10x probes, this on has no markings. I think it
          > may be 50 years old.

          First, I want to be sure I know what you mean when you say "to
          duplicate the screen". I assume you mean you've read the voltages on
          each by counting the number of vertical divisions between waveform
          positive and negative peaks, and multiplying by the scale factor.
          That's the only way to compare apples to apples.

          Next, probe capacitance, typically in the double-digits of pF, will
          not really come into play at the very low frequency of 100Hz.

          I've forgotten what vertical plugins you have in your 5103, but I'm
          pretty sure it's a 5A20 or similar. Assuming that's the case, the BNC
          input connectors on that plugin have an isolated metal ring around the
          base of the connector. This is for probe type auto-sensing. Some
          Tektronix probes have a little spring-loaded metal contact peg that
          sticks out a bit from the connector ring. This contacts that outer ring
          on the plugin's connector and tells the plugin that there's a 10x probe
          connected to it. The scale factors on the screen (if any) change
          accordingly, as does the backlight for the knob to tell you which type
          of probe is in use.

          The little pocket scopes typically don't have such functionality.

          > I never heard from anyone about my question from before, I'll restate
          > simply making a few logical assumptions to get to the point...
          > Is it stange that a cap labeled 590 would read ~850uF - can they
          > increase capacitance as they go bad?

          That would surprise me a *lot*, but I suppose it isn't impossible.

          > 2) For a 850uF cap, an ESR of .13ohms is too high/marginally leaky yes?
          > 3) If it was really a 590uF an ESR = .13ohms would have been "more OK".

          It's a tad high, but not ridiculously so.

          -Dave

          --
          Dave McGuire, AK4HZ
          New Kensington, PA
        • Systems Glitch
          ... If your volts/div are the same as the pocket scope for the same sine wave to have the same apparent amplitude (numbers of divs on the graticule it actually
          Message 4 of 9 , Jun 6, 2014
            > On my older analog Tek 5103N, to duplicate the screen I see on the digital scope I have to use the following settings - volts/div = -2 Secs/div 5m. The differences in the two oscilloscopes' settings, to see the same wave, have something to do with the probe's capacitance, right? I know about 10x probes, this on has no markings. I think it may be 50 years old.

            If your volts/div are the same as the pocket scope for the same sine wave to have the same apparent amplitude (numbers of divs on the graticule it actually crosses), then the probe is not a 10x probe (unless the pocket scope comes with 10x probes...but if it's the one you bought at HOPE it should have 1x probes). You may have a different aspect ratio or graticule scale between the two scopes. As long as you have the same quantity of time between the peaks of the sine wave (should be 10 mS for 100 Hz) then everything is OK. You should be able to grab the Secs/Div knob and compress/stretch the graph of the sine wave with the Tek scope.

            If you want to make double sure it's not a 10x probe, put the Tek scope in DC coupling mode and put the probe across a 1.5V battery (AA, AAA, C, D, whatever). At 2V/div, you should see the flat line jump up 1.5 divs.

            > Is it stange that a cap labeled 590 would read ~850uF - can they increase capacitance as they go bad?

            Usually electrolytics are rated with extremely wide margins on the positive/over end. It's not uncommon to see +80/-20 tolerances. Typically, a much-higher-than-labeled capacitance means it's in pretty good shape when dealing with large electrolytic filter caps.

            > 2) For a 850uF cap, an ESR of .13ohms is too high/marginally leaky yes?

            As Dave said, maybe a little high, probably nothing to worry about though.

            Thanks,
            Jonathan
          • David Gesswein
            ... Did you hook up the probe ground? I suspect the pocket scope is battery so without hooking up the Tek ground you will get very little signal. Otherwise
            Message 5 of 9 , Jun 6, 2014
              On Fri, Jun 06, 2014 at 08:43:55AM -0700, billdeg wrote:
              > I have a digital pocket scope. DSO QUAD....When I use it to generate a
              > 100Hz sine wave I read it comfortably in the AC / 2V / AUTO / 10mS range.
              >
              > On my older analog Tek 5103N, to duplicate the screen I see on the digital
              > scope I have to use the following settings - volts/div = -2 Secs/div 5m.
              >
              Did you hook up the probe ground? I suspect the pocket scope is battery so
              without hooking up the Tek ground you will get very little signal.
              Otherwise test your probe as suggested.

              I think your other questions were answered.
            • billdeg
              I see the same wave on both sides, looks the same height relatively off the center AC baseline when I have the settings as I have described on both scopes.
              Message 6 of 9 , Jun 6, 2014
                I see the same wave on both sides, "looks the same" height relatively off the center AC baseline when I have the settings as I have described on both scopes.  I wanted to understand why I have to use -2 as the voltage on the TEK scope in order to achieve a same-same wave on both scopes. 

                The pocket scope is generating the wave and I am splitting the signal into both scopes.  The scopes are grounded to each other.

                b
              • Systems Glitch
                ... -2V/div doesn t make sense as a unit -- can you take a closer picture of the actual setting on the dial? Thanks, Jonathan
                Message 7 of 9 , Jun 6, 2014
                  > I wanted to understand why I have to use -2 as the voltage on the TEK scope in order to achieve a same-same wave on both scopes.

                  -2V/div doesn't make sense as a unit -- can you take a closer picture of the actual setting on the dial?

                  Thanks,
                  Jonathan
                • billdeg
                  http://vintagecomputer.net/cromemco/system_one/repairs/Tek-5103N_.settings.jpg http://vintagecomputer.net/cromemco/system_one/repairs/Tek-5103N_.settings.jpg
                  Message 8 of 9 , Jun 6, 2014
                    http://vintagecomputer.net/cromemco/system_one/repairs/Tek-5103N_.settings.jpg

                    This is a picture of the scope.  If you were to match my settings to what I was talking about in my original post turn the left knob counter clockwise until the -2 at "2 o'clock" is at "10 o'clock".

                    this might be a "you'd have to be there" kind of thing.  there may be a clue from the fact that the input itself is 47pF which is unlike the other scope I have.  The 5A20N is custom or something.  I think Herb was asking me about this too.  whatever, no biggie.  I think the point here is get a good scope and don't mess with a bad one.

                    ----


                    About my other question, a cap that reads 841uF is just that, they do not degrade from a lower rating to a higher rating as they fail (a cap that is labeled 590uF does not typically degrade to 800uF eventually.  Instead, the ESR goes up and out of spec.

                    BIll

                     

                  • Systems Glitch
                    ... I m guessing, from the placement on the dial, that the Volts/Div you have selected is 0.2V/div. If that s what you select to get the same amplitude as the
                    Message 9 of 9 , Jun 6, 2014
                      > This is a picture of the scope. If you were to match my settings to what I was talking about in my original post turn the left knob counter clockwise until the -2 at "2 o'clock" is at "10 o'clock".

                      I'm guessing, from the placement on the dial, that the Volts/Div you have selected is 0.2V/div. If that's what you select to get the same amplitude as the pocket scope, then you likely have a 10x probe attached. From the picture, it looks like you have one of the old Tek 10x probes that lacks the pin to let the scope know it's 10x. Those can be quite confusing, especially if unmarked.

                      > this might be a "you'd have to be there" kind of thing. there may be a clue from the fact that the input itself is 47pF which is unlike the other scope I have. The 5A20N is custom or something. I think Herb was asking me about this too. whatever, no biggie. I think the point here is get a good scope and don't mess with a bad one.

                      The old Tek modules are pretty full-featured (I see those are even differential amp modules) and the controls are sometimes nonintuitive on the older units. They're good scopes once you get used to them. I've had a number of scopes over the years, but I currently only have my Tek 7000-series mainframe, which is similar to your 5000-series but higher bandwidth (and bigger!). It also displays the units for each module on the screen, so there's never any doubt as to your V/div or S/div settings.

                      If you want, I can bring my Tek function generator and a set of known probes, and we can check it out at the next get-together.

                      Thanks,
                      Jonathan
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