This article is especially for those of you who bought the 4046 SUPER VCO module. It explains how to build it up and tune it and what to look out for.
I have one ready made module left for sale but the PCBs have sold out. The ready made module is fully calibrated and tuned and costs € 150,- incl. shipping.
BUILD INSTRUCTIONS:
The build instructions are all in the image below which you can easily download and print out.
Instructions on how to tune the VCO are at the bottom of this article.
Please read the entire article before you start assembling the boards.
BILL OF MATERIALS:
Below is the Bill of Materials. It's important that you buy the correct potmeters, trimmers and audio sockets because these are the only ones that fit the PCBs. They can all be bought from AliExpress and don't worry, the quality is good enough. I advise you however to buy the Toggle Switches from a reputable source because the Ali ones are rubbish. They are not made from heat resistant plastic so the plastic melts as you heat the lugs with the soldering iron. Now you can get away with it, if you solder them in quickly but don't risk it. Buy good switches. They last longer too.
The 50K trimmers may also be 100K in value. So you can order eight 100K trimmers and one 100 Ohm trimmer.
I have links here for the black aluminium knobs I use for the VCO. The top two, for Coarse and Fine frequency control can be bought here (ebay) https://tinyurl.com/2p3d72z
The smaller knobs are 10 x 15mm, I use those for all the other potmeters (ebay):
You can buy the PTC 2K resistor here as long as the stock lasts. They're not being made anymore so there won't be any new stock.
Here's another retailer based in Germany that has a good stock of these: --- CLICK HERE ---
You can also just use a 2K resistor for TH1 but then the VCO won't be temperature stable.
Make sure you get all your IC's from a reputable source, especially the CD40** chips.
ALWAYS KEEP IN MIND THAT THE MAXIMUM SPACE BETWEEN THE PCBs IS 10.7mm.
Keep that in mind when soldering in capacitors. They can't be too high or the boards won't fit together.
I've tested the VCO's sub-oscillator with the Hard Sync function and the sub oscillator totally reacts to it amazingly! You get the weirdest sounds out of it from bell like rings to the Atari computer noise and weird stuff inbetween. It really is amazing.
If you need to know how to match your 2N3904 transistors then please check out article 66, the TB-303 filter. It has a section about the Ian Fritz methode of matching. You can easily set up a little matching circuit on a small piece of stripboard or on a breadboard. Go here for the full explanation. (scroll down)
Once you soldered in your matched transistors, use superglue to glue the flat part to eachother and mount the PTC resistor on top and use a little bit of heat sink compound to thermally connect it to the transistors. (You can see it in the second picture down).
PINHEADERS:
Below are two pictures showing how the pinheaders connect the boards together. Each of the PCBs has a number of small and long pinheaders and with each pinheader there's a text indicating on which side of the PCB it should be mounted. BE ACCURATE HERE because once soldered in, these are impossible to desolder without making a mess of the PCB.
Make sure you fit the first two boards together with the pinheaders inbetween to make it all fit, and secure it with paper masking tape before soldering everything in place. Then do the same with the 2nd and 3rd PCBs. In the picture below I pulled the boards appart so you can better see the pinheaders.
The finished product.
In the picture below on the middle board top right, you can see some heatshrink tubing. That's my 2M2 resistor. I didn't have one, so I made one up by putting a 1M and a 1M2 resistor in series and I put some heat shrink over it to make sure it didn't touch any other components. My C3 timing cap is soldered onto the backside of PCB 2. I did this for spacing reasons. I also changed it out a few times to experiment with different values for tuning.
If you want to see detailed pictures of a certain section of the vco just ask.
Schematics are at the bottom of this page.
TRIMMERS:
With the third PCB make sure the trimmer screws line up with the holes in the PCB so you can get at them to calibrate the VCO and the waveforms. All trimmers must be soldered in with the screws facing outward.
The rectangular solderpad for the trimmers shows where the screw should be facing. It's important that you solder in the pinheaders first and then you can test-fit the two boards together and see if the trimmer screws line up with the holes. They don't have to be exactly in the middle but you must of course be able to turn them with a small screwdriver.
If you solder the back leg of the trimmer first, you can then check and adjust the positioning of the screw before soldering in the other two legs.
In the picture below notice how the trimmer screws line up with the holes in the PCB:
Once you have everything put together you can start testing.
The first step must be to check the power connector for short circuits, with your multimeter in continuïty mode. Now take your oscilloscope and you can start tuning the waveforms so they look how they should look.
Start with the Triangle wave. Use the two trimmers Triangle Connect 1 & 2 to line up each half of the wave to get a symmetrical triangle wave. The Triangle wave has big spikes coming out of the top and bottom of the waveform. This is normal !! Don't worry about it, you can't hear it.
Now use the triangle offset trimmer to trim it so the 0V line goes straight through the middle. Once you've done that you can trim the Sinewave. Use the appropriate trimmers to shape the sinewave. This must be done AFTER the triangle wave because the sinewave is made from the triangle wave. Now set the offset for the ramp wave and you're good to go.
The Triangle wave is made up of two sawtooth waves put together so the Ramp or Sawtooth wave is half a Triangle wave and therefore double the frequency of the other waveforms. (One octave higher).
If you don't see a pulse wave on the pulse wave output make sure the PW potmeter (Pulse Width) is set to the 12 o'clock position. If you still don't get a pulsewave output, check resistor R55 on the 2nd board. You may have put in a 2K2 instead of a 2M2 resistor. (Happened to me). That's why it has a ! next to it.
Test the sub oscillator by connecting the scope or your synth to the Pulse+Sub output and turn the Pulse/Sub Blend potmeter fully clockwise. Try all the switches to make sure everything works.
NOTE!! If you switch to -2 Octaves and you get a flat line or small pulsewave with no negative phase then your CD4013 chip is most likely a fake. I had this happen to me. I switched to -2 Octaves and I got a small ripple on the scope and then I smelt something getting hot. The CD4013 actually burnt up. That happened twice until I realized my stock of CD4013 and CD4070 chips were fakes. So I ordered new ones from my local electronics store and then everything worked like it should.
Once you finished soldering everything in and you're putting the boards together with the pinheaders, check the underside of the boards for any component legs that are sticking out or have not been cut short enough. You can have sharp metal sticking into components on the board underneath causing potential short circuits. Especially the top board with the controls on it can have things sticking out from underneath. Check for that and cut legs off shorter if need be.
TUNING THE VCO:
Tuning the V/Octave tracking of the VCO is a straight forward process of going between high and low notes. I always use the C2 and C5 notes.
Use the Triangle wave for tuning.
- Press key C2 on your keyboard and use the front panel coarse and fine tune potmeters to tune to C2.
- Now press C5 and see if you are higher or lower than C5.
- If you're lower, trim the V/Oct up a little bit. Don't make big changes and remember if you tuned up or down.
- Now both notes will be higher so press C2 again and use the front pots to tune to C2 and then press C5 again and see if you're closer to it.
Repeat this process until both C2 and C5 are in tune.
If all is well the notes inbetween shouldn't be too far off either. You'll always be a few cents off on C3 and C4 but that's normal with this VCO.
If the notes in between are way off then you can bet your CD4046 is to blame.
You probably won't get it perfect though. This VCO is notorious for not being the most accurate with octave tracking.
I've tried different CD4046 chips (all Texas Instruments) and here is a graph with an impression of the results I got:
With one chip I got a very steep curve like the red line represents and with an other I got the VCO almost perfectly in tune with just a few cents between the octaves as seen in the purple curve. That's about the best result you can expect. The difference between 0 and 5 cents is practically impossible to hear but when the octaves in between the well tuned ones are 18 cents off you'll definitely hear it.
I tuned my VCO by going between C2 and C5. It's better to have these octaves in perfect tune than the lowest and highest (C1/C6) as you'll mostly use the C2 to C5 range normally.
If you can't get the VCO in tune before you're at the end of the throw of the V/Oct trimmer, you can try lowering the capacitance of C3 (10nF). Put in a cap of around 8.5nF. I've tried changing all the other components in the tuning circuit but nothing had any influence on the tuning curve except the capacitor C3 and the CD4046 itself and of course if the transistor pair isn't matched because they determin the exponential curve of the Volt per Octave scale.
Changing the value of the 390 Ohm resistor won't help with tuning at all. Lowering the 10K resistor won't help either and the HF trim potmeter I found to be all but useless. It has a little influence if you're on octave 6 or higher but it's minimal.
The make of your CD4046 is critical with tuning. Motorola MC14046 is the best but you won't be able to find them anywhere, National and Fairchild are also good if you can find them but most likely you'll only be able to find Texas Instruments CD4046's so buy a batch of 5 or 10 and test them all in this VCO to see which gives the best results.
These are the results Thomas Henry himself got with tuning the X4046 VCO. He must have used Motorola chips because 4 octaves spot on is pretty much impossible to achieve:
My tuning experience:
I must admit I had some trouble getting this one in tune. I had to lower C3 from 10nF to 8.5nF for the oscillator to get the high octaves without running out of throw on the V/Oct trim pot. I got it in tune between C2 and C5 but inbetween it was up to +18 cents high. I thought my transistor pair may be a bad match eventhough I matched them myself but I that wasn't it. I put in a new matched pair but the results were the same. I took a CD4046 out of a previously built VCO which was well tuned and put it in this one and immediately the tuning was much better. So it was the chip that was at fault.
The results I got and which I can live with are as follows:
Note C1= -10 cents, C2= spot on, C3= +3 cents, C4= +3 cents, C5= spot on, C6= -6 cents.
Good tuning app for your smartphone:
Use a good tuner on your smartphone like the one I use which is the AiryWare Tuner. I saw this tuner actually being used in the Rhodes Electric Piano factory on a YouTube video. Get the paid version, it's really cheap, only about $ 5,- and it's a once only payment:
If you use this app and you're stuck on some page and don't know how to get out, swipe right on the screen in landscape mode. Some people get stuck there and think the app sucks but it doesn't. It's awesome. You just need to learn how to use it.
It has an enormous amount of instruments you can tune with it (I also use it for my Bass) and also a great many ways of displaying the frequencies, like stroboscopes and waterfall displays etc.
I really like it and you will too. I'm not affiliated with it in any way, I just wanted you to know about it.
SCHEMATICS:
Here are the schematics for this VCO. There's one for the main oscillator and one for the sub oscillator.
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