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rjd2
Joined: Sep 02, 2007 Posts: 236 Location: philly
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Posted: Sun Sep 06, 2009 2:42 pm Post subject:
CGS 048 VCO-stuck at scaling. pitch is being self-modulated" |
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hi folks, sorry to be constantly asking for help, but....im not seeming to be able to sort these things on my own! so, here's my question:
CGS VCO-im able to do the first step of setup properly: getting the "zero" trimmer to show 0V at pin 7 of said IC. my problem is that when i go to SCALE, im having 2 problems:
1: when monitoring the SINE out, something seems to be modulating the pitch VERY slowly. it basically drifts up and down. if i apply CV to the V/OCT input, it seems to drift faster when i change the CV. 2: even when it stabilizes a bit, the SPAN trimmer doesnt seem to change the SCALE. it affects the pitch, but not the SCALE.
i figured that i might have a wiring problem, so i wired up a second completed PCB with just the tuning trimmers(coarse and fine), V/OCT jack, and SINE jack. i get the same behavior on both of the units. obviously i did something wrong on the pcb. any ideas? ill try to post a picture if that may help. thanks, rj
EDIT: after sufficient warm-up, the drift issue seems to ALMOST go away. however, the SPAN trimmer still seems to just affect the overall tuning; the scaling stays the same regardless of any of the trimmers' rotation. |
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andrewF
Joined: Dec 29, 2006 Posts: 1176 Location: australia
Audio files: 4
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Posted: Thu Sep 10, 2009 9:29 am Post subject:
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I have found when setting the SPAN trimmer the pitch changes.
just ignore it, keep switching between 0V & 1V and adjusting SPAN until the jump is one octave. If the tone is C when I start adjusting SPAN it might be on B by the time the adjustment is correct.
the important thing is to get the one octave change, it doesn't matter what the initial note is (you set that later)
I use a digital tuner so it is easy to see when the jump is 1 octave as the meter indicator doesn't move......much.
Once you have it for 0V & 1V, try 0 & 2, 0 & 3
then to back to the zero trimmer, to set the base freq.
If you find the drift disappears after a few minutes, maybe it just needs to warm-up....sounds excessive tho
hope this helps a bit |
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rjd2
Joined: Sep 02, 2007 Posts: 236 Location: philly
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Posted: Thu Sep 10, 2009 5:08 pm Post subject:
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thanks alot for your response, andrew. unfortunately, turning of the SPAN trimmer does not affect v/oct scaling at ALL. it stays at a fixed interval, basically a major 6th=one octave. turning the SPAN trimmer will affect the overall tuning, but it stays at that v/oct interval from one side of the trimmer to the other.
im gonna try to post a picture so maybe something will pop out as wrong? it sure would be great to get this working, as i built 3 of them, and the 2 i've wired behave the exact same way, so im obviously doing something wrong.
EDIT: on a whim, i swapped the 10k trimmer at SPAN for a 100k. by sweeping from one end to the other, i can find a difference in scaling-still can't get 1V/oct, but i get a change. if i run it down so the impedance measures 0 across the trimmer, i get 1V=about 14 half steps. still HIGHER than an octave. if i sweep to other side, it obviously goes the wrong way. does this help diagnose? thanks much. |
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