[sdiy] VCO that goes SHARP in highest octaves??

Ian Fritz ijfritz at comcast.net
Tue May 10 14:43:25 CEST 2022

This is most often due to hi freq oscillations. Other causes, too, but I suggest starting there.


> On May 9, 2022, at 10:11 PM, Neil Harper via Synth-diy <synth-diy at synth-diy.org> wrote:
> so this one is pretty strange to me...
> i breadboarded a few of Thomas Henry's VCO-1 (i'm sure that design is
> familiar to most, if not it is here:
> https://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html) with 3080
> replaced with a TI 13700 that worked worked exactly as intended, i was
> able to adjust the scale tracking and both needed a little bit of
> high-frequency compensation as expected - with C8 initially being about
> 10-20cents flat. no problem. built a few thru-hole pcbs, worked exactly
> the same.
> my most recent project was to design a SMT version using the exact same
> schematic - but with SMT equivalents. I received 5 boards back and all
> of them go SHARP at C8 (10-15 cents) without any hf-compensation.
> what's going on?
> my first suspicion was the hf-compensation trimpot never 'went to zero',
> so I removed the trimpot completely and the diode in the Rossom
> Compensation circuit - no change.
> as another test, I removed C1 ("C1 is a speed-up capacitor; its duty is
> to drain away any accumulated charge from the base of Q2. This permits
> the transistor to change states more rapidly"). the oscillator required
> re calibration, but C8 was now only 2-4 cents SHARP. i feel like this
> isn't the solution though, and that i was just crippling the
> high-frequency linearity of the oscillator to compensation for another
> non-linearity somewhere. but i can't think of anything that could be
> different going from breadboard to TH PCB and then to SMT PCB.
> --
> /// Neil Harper
> /// Every Wave is New Until it Breaks
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