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VCO 4069 strange sawtooth output
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author VCO 4069 strange sawtooth output
Hello all--

I have a bit of experience with DIY circuits, but I would definitely still categorize mysefl as a novice. I've put together Rene Schmitz's 4069 VCO (see; I've added the "Sawtooth buffering amplifier" portion as well.

It sounds interesting and there's no smoke coming out of it, but I'm seeing some strange output (this is "saw out" and "pulse out" from the schematic):

Basically, the sawtooth wave has a discontinuity corresponding to the falling edge of the square wave; adjusting the duty cycle of of the square shifts the phase of discontinuity as well.

As far as I can tell, my circuit matches the schematic in the above link; I checked it several times, but it's a breadboard and there are plenty of wires running everywhere, you know how it goes...

Has anybody seen this behavior before, or have some suggestions about how to troubleshoot/mitigate it? Thanks in advance
Try placing a 100NF capacitor as close as possible to the power pins of the 4069 (between pins 14 and 7) it is not in the schematics, which is an error imho.
Apart from power decoupling cap, looks like interaction in the circuits.
The basic oscillator here is the Saw, so make sure that works right on its own.
Try temporarily disconnecting the pulse circuit by...
Lifting its 220nF output cap.
Lifting its 47k input resistor.
I'd assume the board layout puts the 3 inverters on one side of the chip into the oscillator and the 3 on the other side in the pulse modulator, but if they are scrambled side to side, make sure there isn't the slightest contamination between pins. CMOS inputs are very high impedance and easily nudged by crosstalk.
Thank you both for taking a look. Overall it looks like a dodgy component that I probably would not have found without your suggestions.

I tried adding a decoupling capacitor between the power pins and it didn't make a noticeable difference (although it seems like a good idea anyway).

I had already arranged the circuit such that gates 8-13 on the 4096 are handling sawtooth oscillator, and the gates 1-6 the pulse.

Removing the 47K resistor obviously kills the pulse output, but improves the shape of the sawtooth output. Interestingly, the "pw adjust" potentiometer has a small effect on the frequency, even with the decooupling capacitor in place.

Removing the 220nF capacitor on the pulse output also kills the pulse output, "pw adjust" still has the same effect on the sawtooth discontinuity.

Based on this, it looks like there's some cross-talk between the gates but my electro-fu is not sufficient to diagnose. I swapped the 4096 for another one I had on hand and the problem went away; I thought that maybe I had jiggled something else on the breadboard while doing so, so I put the original back in and the problem returned.
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