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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
CGS 048 VCO-stuck at scaling. pitch is being self-modulated"
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rjd2



Joined: Sep 02, 2007
Posts: 236
Location: philly

PostPosted: Sun Sep 06, 2009 2:42 pm    Post subject: CGS 048 VCO-stuck at scaling. pitch is being self-modulated" Reply with quote  Mark this post and the followings unread

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

PostPosted: Thu Sep 10, 2009 9:29 am    Post subject: Reply with quote  Mark this post and the followings unread

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

PostPosted: Thu Sep 10, 2009 5:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

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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