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.
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.
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. 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.
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.
PINHEADERS:
Below are two pictures showing how the pinheaders connect the board 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.
The finished product:
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. Then you can start tuning the waveforms so they look like 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. 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.
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.
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.
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.
- 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 your transistor pair might not be matched correctly.
You probably won't get it perfect though. This VCO is notorious for not being the most accurate with octave tracking.
If you can't get the VCO in tune you can lower the capacitance of C3 (10nF). Put in a cap of around 8.5nF.
Changing the value of the 390 Ohm resistor won't help with tuning at all. Believe me, I tried that.
You can lower the value of R15 (10K) to 4K7 to help with tuning but the most likely cause of not getting this VCO in tune is either a bad 4046 or a mis matched transistor pair.
If the higher octaves go progressively more out of tune you can try and use the HF Trim to set that right but HF Trim does not have much influence. You will only see it work on octaves 5 and higher but the lion share of the tuning is just done with the V/Oct trimmer and the frequency potmeters on the front panel. The Coarse potmeter should end up at the 11 o'clock position.
The CD4046 chips can differ in the results you get too. Out of four CD4046 ICs from Texas Instruments I had in stock, some worked well and others didn't eventhough they were all from the same batch. Beware of that. The make of your 4046 chip is important.
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. I got it in tune between C2 and C5 but inbetween it was up to +18 cents high. I think my transistor pair may be a bad match eventhough I matched them myself but I could have made a mistake. There's definitely something off in the scaling of the V/Oct. reach or span. I had no such trouble with the previous two I built and they use the same KiCad schematic file so it's not a mistake in the circuit.
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.
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.
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