# Help making the 'My first VCO' module with the CEM3340 **URL:** \ **Category:** DIY STUFF\ **Created:** [December 23, 2023, 6:16am UTC](https://lookmumnocomputer.discourse.group/t/help-making-the-my-first-vco-module-with-the-cem3340/6815 "2023-12-23T06:16:45Z")\ **Posts on this page:** 1\ **Showing post:** 27
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [December 26, 2023, 1:33pm UTC](https://lookmumnocomputer.discourse.group/t/help-making-the-my-first-vco-module-with-the-cem3340/6815/27 "2023-12-26T13:33:37Z")
No, you should not expect anything useful from measuring the red dots. The control voltages should be measured from the other ends of the resistors as shown in the Annotated Simple 1V/Oct Oscillator Stripboard link I posted above. There are two control voltages, the CV Input one and the one produced by the tuning knob, each with its own 100k resistor. Measure them separately. The sum of these two voltages is what determines the pitch, and should be up around +9 V. > [@Need some help with the kosmo vco w/ tuner. Please! I’ve lost weeks](https://lookmumnocomputer.discourse.group/t/need-some-help-with-the-kosmo-vco-w-tuner-please-i-ve-lost-weeks/691/50): > > One more thing to note is that with the typical resistor/capacitor values, the 3340 is calibrated to behave nicely from ~1 Hz all the way to 20 kHz and beyond, and you have to feed in quite a bit of voltage to get an audible signal. Assuming nominal resistor/capacitor values, the 10k tracking pot set to the middle position, and that I didn’t completely screw up my algebra, the CV for a given frequency is: cv = 1.463 \* ln(freq) + 0.239 so for e.g. 440 Hz you need to feed in just over 9 V in to… It’s also not very useful to stick a voltmeter on the triangle (or square, or ramp) outputs, either, because these are or should be oscillating at audio frequency. Better to use a scope to look at them, or lacking a scope use an audio probe: > [@Need some help with the kosmo vco w/ tuner. Please! I’ve lost weeks](https://lookmumnocomputer.discourse.group/t/need-some-help-with-the-kosmo-vco-w-tuner-please-i-ve-lost-weeks/691/30): > > I think you should find out which parts of the circuit are working and which are not. And I agree with @analogoutput that you should concentrate on the analog side of the circuit first. One way of tracing AC signals, if you do not have any specialised equipment like an oscilloscope, is to use an audio amplifier. It basically boils down to that you probe the circuit and listen whether you can hear a signal where you would expect one. This should work with an oscillator as all it will produce (mo… That would tell you at least whether the 3340 is producing a tone that’s not getting to the output or not, which would help isolate the problem. --- _[View the full topic](https://lookmumnocomputer.discourse.group/t/help-making-the-my-first-vco-module-with-the-cem3340/6815)._