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Tone Generator Oscilloscope Measurements

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  • Tone Generator Oscilloscope Measurements

    Hi Forum People,

    Just thought I'd share some of my experiences using a T 2221 60 Mhz Oscilloscope to measure the tonewheel generator signal output from my Hammond T-211 organ.

    When I initially tried measuring the peak to peak output in mv (from the tone generator input into the upper manual bus amplifier board) I was having trouble getting good clear readings. The signal was fuzzy and I was having trouble getting a clear thin line. The waveform mostly looked too thick to get an accurate measurement. I wasn't sure if it was a grounding or syncing issue, or something related to the probe or low output levels. I also thought the peak to peak levels were too low. I had added a three prong earth grounded plug to the organ also, so I wasn't sure if I had messed up something and was getting unwanted noise on the ground. I tried a variety of ground signals, none of which improved the clarity of the signal.

    I followed the instructions for measuring this output from the service manuals. Unplug the lead from the tone generator input signal (pin #15 for example) from the upper manual bus amp board, lower preset drawbars switch down, upper drawbar 8' all the way out, upper key # 25 depressed, expression pedal at full, everything else off.

    The first mistake I made was reading my output wrong on the scope and forgetting to look at the 10x reading rather than the 1x reading, and so originally I thought my output levels were way too low. I was using a standard 10x probe and referencing the readings from this data.

    The 10x probe was attenuating the signal, which was on the low end of the scope's ability to read, so switching to a direct output (1X) though a bnc cable with some clip leads, game me a higher amplitude reading. These readings confirmed the overall aptitude was in the ball park of the reference data. However, the live feed from the scope was still making a thick fuzzy waveform, so I messed around with the digital storage modes and got these results. My conclusion is that the frequency of the tonewheels seems to vary a fair bit, so any fixed syncing frequency of the scope would produce a range of data re the sample frequency used for syncing. No amount of hf rejection would fix this, but I did find some interesting info from comparing the sample, average and peak average and peak accumulate modes of the digital storage readings.

    I posted a photos of the set up and readings here @musicaudiostuff on instagram for reference.

    From the photos, you'll see from "sample" mode in digital storage mode that a limited number of samples makes a fairly coherent waveform. It is only over time that the average and peak signals begin to blur, or in the live reading it appears as a thick fuzzy line.

    One bizarre thing I noticed is that when I put the scope channel switch half way between the ac and ground settings I get a very clear yet attenuated signal. Photo also at @musicaudiostuff. I have no idea why this is.

    I may be wrong about any of this stuff, just thought I'd share my process!

    Help / advice / feedback is most welcome.



  • #2
    Wouldn't it be easier to use an AC voltmeter and convert between rms and pk-pk values as needed?

    Allen 965
    Zuma Group Midi Keyboard Encoder
    Zuma Group DM Midi Stop Controller
    Hauptwerk 4.2


    • #3

      I think we should always use the instruments recommended by the manufacturer to make measurements (or equivalent and the choice is very large)

      Your 2221 is too powerful (60Mhz) and also not enough (Sensitivity, No differential imput.)

      If you want to use a tektronix, the 502 that is contemporary of the organ is a better choice : Differential input; 100μVsenstivit√©; Very simple to use; Sufficient bandwidth (50kHz)

      With it, you can eliminate ground loop problems and you will easily get a fine trace.



      • #4
        Thank you jp


        • #5
          It's good to hear all this stuff!
          Years back when I measured my TG I had to borrow a professional DMM which read way down into AC millivolts.
          When I saw a scope come up cheap from a retired Ham radio dude I jumped at it but could never make head or tail of the readings!
          If I recall it didn't have the increments marked off, bt it also didn't come with the correct probes.
          It made a good light display though!

          Might look nice in the T series console as a bit of extra unnecessary bling.
          -1958 Hofner 550 archtop guitar -1959 C3 and PR40- -1964 Busillachio Harmonium- -1964 M101-
          -1967ish Leslie 122- -1975 T500 (modded..chopped, and reassembled!)-
          -DIY 760 FrankenLeslie/rat hideout-
          -1980 Electrokey Electric Piano- -Yamaha electric Harmonium (early 80's?)-
          -1990 Jansen GMF150 amp- -1992 Korg 01W/fd- -1992 G&L S-500 geetar.