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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. http://www.dairiki.org/HammondWiki/T...orOutputLevels.
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.
Thanks!
Caesy
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. http://www.dairiki.org/HammondWiki/T...orOutputLevels.
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.
Thanks!
Caesy
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