CEM3340 VCO Help

Hi All,

I want to thank you in advance for reading this and dedicating your time to helping me learn (I can’t believe there are people around that have enough patience to bother explaining what I’m sure are complete basic ideas!) :slight_smile:

I’ve been trying to follow and replicate Sam’s CEM3340 Basic VCO and despite following the layout almost identically, I’m not getting any sound… :frowning:
Instead of a nice, clean sawtooth or triangle wave, I get clicks every couple of seconds. I have triple checked my wiring according to the diagram, none of my soldering on the back has accidentally spread across multiple strips and so I therefore can’t see or imagine what the problem could be.

Looking at past forum posts, the power rails at the bottom are all wired in correctly, the only thing I can find that may be a contributing factor is the top of the CV resistors as identified here: Annotated Simple 1V/Oct Oscillator Stripboard

Using my multimeter, I get next to no voltage up here (literally -0.01V), I assume this isn’t supposed ot be this way as I would need almost 12V for a full range of frequencies?

If there are any ideas, I would love to hear them, many thanks!

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I’ve not checked your circuit but there may be a solder bridge between pins 9 & 10 of the 3340.

A few of solder joints look a little dry, hard to tell.


If you’ve measured the control voltage at the indicated place


(where it says TONE CV, where it says CV I assume you have no CV plugged in and so that should be zero — make sure you’re measuring at the indicated end of the resistor) and are getting zero, there’s almost nothing it could be except a bad pot or bad connections at the pot or on the 12 V rail or on that resistor. Do you have nonzero voltage on the purple wire to the pot? And +12 V on the red wire?

I can see multiple possible solder bridges.

I’m that guy that puts EVERYTHING on breadboard first…so naturally I’d ask if you’ve tried a breadboard circuit, and suggest that you do. It would definitely help with diagnosis of faulty parts as they are easily interchangeable with a breadboard. I made this circuit on breadboard first and even powered it with two 9v batteries in series. Noise beautiful noise (followed immediately by dead batteries)

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From a visual inspection of the solder side of the stripboard I have identified a number of places where there might be shorts between adjacent tracks.

I have highlighted these below using orange circles:


If your 3340 and tl072 are in sockets remove them and do continuity checks between the highlighted tracks, use a low resistance range if you meter doesnt have a continuity tester function. If the chip is soldered in you might get some false positives due to current flowing within the IC’s. A dead short will probably show as a very low resistance and means there probably is a track short.

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If you have some click, maybe it’s just too slow, have you try to adjust the trimpot ?

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I don’t really see, but it seems that your opamp look backward

I sort of agreed at first about the TL072, but on further inspection its OK, you can clearly see pins 1&2 tied together and pin 4 to -ve.

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Hi Everyone,
Thank you so much for your responses.

I’ve taken all of your points and tried to use them in troubleshooting.
Firstly, the big topic that kept coming up was solder bridges - I had checked this before coming to the forum but have since looked at each solder joint multiple times over and still cannot find any unintended connections between rows.

I’ve swapped both chips out for others that I know definitely work, coming from a breadboard version of the circuit that does actually output sound, but to no success, I still only get clicking.
And have also adjusted the pot and the trim pot, both having no effect on the clicks at all.

Going back to solder bridges and the general subject of continuity, I’ve taken my multimeter and done continuity test on every row and the ones above and below it, ensuring that there aren’t any solder bridges anywhere that I’ve missed when looking. I then also did tests on the same row, either side of the IC sockets, just to be thorough and double check that the chips wouldn’t be shorting through 2 opposite pins being connected.

I’ve re-measured all the points, still around -0.01V at the top of the Tone CV resistor and 0V at the top of the CV resistor (the one next to it). The red wire to the pot has +12V consistently and the purple wire to the pot (white in my pictures) has a voltage of -0.02V which in my eyes seems like it should be okay? (Also I’m measuring these readings at the pot end, physically touching the solder points.)

Thanks again everyone for your input and help, it’s really appreciated!

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Have you used the same timing cap on the breadboard version? That looks like a cheap ceramic.

Switch out the tune pot.

Did you use the same pot as the breadboard circuit? If they are dif pots I’d bet the one on the strip board is bad.

That’s not OK, not unless the left pot terminal is grounded and the pot is turned fully counterclockwise. With 12 V at one end and the middle terminal not grounded, it has to be some intermediate voltage. Either the pot is bad or there’s a bad connection to the pot or there’s an unintended connection between the middle terminal or the strip it connects to and ground, and it’s hard to see how the latter is possible since there are no grounded strips anywhere nearby.

Amazing, thank you all so much, I swapped out the potentiometer and it works perfectly!

Today I’ve also built the breadboard one and added on the square wave and PWM functions to this second one also. This worked as intended first time!

One thing I am finding is the volume levels of each waveform vary quite a lot. To control the volume levels of each waveform separately, would it be a good idea to insert a potentiometer between the breadboard and tip of each waveform jack? (And if so would 100K work, or would a smaller resistance value such as 22K be better?)

Thanks as always.


Right on! Beautiful noise! I would use 100k at least on any volume attenuator. You will have a more precise adjustment with a higher value pot. I have built the module with all three outputs, and i did not include any level pots for any of the outputs. i have plenty of mixers with attenuators on all inputs and overall output so i just didn’t need them, but they would prove useful either way.


The 3340 waveforms have different amplitudes; with 12 V power the triangle wave is 0 to 4 V, the ramp is 0 to 8 V, and the pulse is 0 to about 10.5 V. So they have different volumes partly for that reason. (Even with the same amplitude, triangle waves come across as quieter because they have fewer harmonics added to the fundamental frequency.)

I’m not a fan of output attenuators on modules, unless they’re intended to go to an external system; input attenuators on downstream modules are common and they’re my preference. Also, attenuators can only attenuate (truth bomb) and the 0 to 4 V triangle amplitude is kind of low by most standards — 10 V peak to peak is what’s often assumed. If output attenuators are used they should be no more than 1k potentiometers (OK, maybe 10k). Higher resistance can lead to loss of signal amplitude in the downstream module, depending on that module.

It’s possible to add output op amp stages to equalize the waveform amplitudes at 10 Vpp without adding output resistance, and also to center them around 0 V; I did that in my VCO, but it adds considerably to the parts count. I think it’s worth it but maybe you want to keep things simpler to start with.

If you mean a smaller adjustment slope in the middle of the range and higher at the ends, that’s sort of true, but it’s unlikely to be a big enough effect to notice, and the (in mid-pot) 50k output resistance will lead to a factor of 2/3 amplitude loss when plugged into a module with a typical 100k input resistance.


I didn’t realize the difference between input and output attenuation. I’ve used 10k pots as in line passive attenuators (in or out, either, both?), And it just seemed like the 10k had sudden level changes in a short range. So is that because of it being passive? I guess i can see how active circuits would alter the expected result. Is that the case in my observation?

If you have a 100k pot as an output attenuator going into a module with a 100k input resistance, the latter is in parallel with the bottom of the attenuator and it results in a nonlinear response to the knob, but not drastically so:

If the downstream module’s input resistance were only 1k (which would not normally be the case) then the effect is much more drastic:

With a 10k attenuator it’s not so extreme:

And a 10k attenuator into a 100k downstream resistance would be quite linear:

Typical design practice is to have about 1k for the output resistance and 100k for the input resistance, which would get you even closer to perfect linearity than that, not that that’s needed for something like this. When you’re dealing with 1 V/oct control voltages, though, even 1k out and 100k in leads to out of tune behavior, you need more like either 100Ω output or 1M input for that.


Thanks Richard, for this much needed info!

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Sam’s design already has opamp buffers on each output. It’s possible to add just a couple resistors to reconfigure them to set the gain and offset. I did that to fix the levels on my build.

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