Friday, 18 December 2020

Synthesizer Build part-37: THOMAS HENRY VCO-555.

One of the best sounding analog VCO's you can build, with 4 waveforms including Sine. It has excellent 1V/Oct. tracking. With verified layout. I also made a layout specifically for EuroRack with a stripboard that is cut in half and folded over. You can find that further down the article. This VCO project has very rapidly become one of the most popular on my website together with the 3340 VCO.

This VCO is a brilliant design by Thomas Henry and he worked on it for a long time. He calls it his best design to date and it sure is that! VCO's of different designs, can sound different from eachother despite them producing the same basic waveforms. This difference is not really that noticeable if you listen to just the basic wave outputs. It starts to become more noticeable when you start playing with the Synchronization and the Frequency Modulation inputs. That's where this VCO really shines and I think that's why it is so popular in the modular synthesizer world. With the Hard Sync and FM functions the VCO-555 produces a very full sound, rich in harmonic content and very musical sounding. And it tracks very well over the octaves. Plus it's very stable with the PTC installed for temperature compensation. You'll find out once you build this module and start experimenting with it. It's the VCO of choise for many hardened Modular Synthesizer aficionados. (You should really have at least two of these in your setup.)

This is a medium difficulty project. Not one I would recommend for beginners and certainly not as a first project. You need to have a reasonable knowledge of electronics for this one and you can not do without an oscilloscope. Please read the entire article before you start building so you are aware of things you need to look out for. Be extremely accurate in copying the layout. Check and double check and mark the components you soldered in on a paper printout of the layout.

ABOUT THE CIRCUIT:
I wasn't going to post the schematic here and instead linked to it on the Electro Music forum but then the forum had occasional accessibility problems so here's the circuit. (The link to Electro Music also has the original parts list if you scroll down.) 
For the schematic below, just click on the image to enlarge it. Then right-click and 'save as...' Then you can zoom in on it. In the lower part there are three resistors named R26. I think Thomas made a little mistake there:


The VCO uses no exotic chips. There's only two TL074's, an LM13700 and a TLC555. You can also use the ICM7555 (which is what I used) or the LMC555. Do not use a normal NE555 for this, you need the CMOS version. Although I'm told it will work with the normal NE555 these chips consume a lot more power and they can have oscillation problems under certain conditions (see my 81 LED chaser article.)
Instead of the LM13700 you can also use the LM13600. I tested this myself and there's absolutely no difference between them in this circuit.
I didn't put in the bypass capacitors for the chips. I almost never do because I have a good powersupply and no noise problems in the circuit. If you want to include them solder them in over the powerrails on the stripboard as near to the chips as possible. The connections are shown at the bottom of the schematic. You can use small ceramic 100nF caps for this. You can solder in the two 10µF electrolytic caps over the power rails on the stripboard too. One from plus to ground (- to gnd) and one from ground to minus (+ to ground). The exact position of the electrolytic caps on the stripboard doesn't matter. Put them where you have room. Note these caps are not included in the Bill of Materials below.
For temperature compensation the circuit uses a PTC Thermistor which I guess is the only exotic component in this VCO. PTC stands for Positive Temperature Coefficient, meaning that when the temperature goes up the resistance also goes up. This temperature dependent resistor has a nominal value of 2K. However, it's not necessary to use a Thermistor. You can get away with just using a 2K resistor. It'll just mean that it isn't as stable as it can be, but many people use this VCO without the Thermistor. If it does go out of tune you can easily adjust the Frequency Fine Control and set it right. It is handy to have a hacked tuner like the JOYO tuner attached to the VCO to keep an eye on the tuning.
Capacitor C4 (2200pF or 2.2nF)  is the timing capacitor for the oscillator and therefore it should be a non-ceramic type like a Polystyrene or a Polyester or Silver-Mica type capacitor for temperature stability.
The VCO doesn't have an extra CV input because actually the Exponential FM input has that function. If you look at the schematic you'll see it is connected to the same pin as the 1V/Oct input and it has an attenuator too. The Frequency Coarse and Fine tune are on that same pin too. 

Here is an amazing Falstad simulation of this circuit made by Fabian Kempe: -- CLICK HERE --

ABOUT THE TRANSISTORS:
The two PNP transistors Q2 and Q3 need to be matched. I matched them using the Hfe transistor tester on my multimeter and this is good enough. When measuring Hfe, give the transistors time to cool off after you touched them because the Hfe value will change with temperature. Some will tell you that the transistors need to be matched on the Base Emitter Voltage (Vbe) and that is correct but I noticed that if you match them on Hfe, the other parameters will be pretty close too. Anyway it works fine this way. 
The transistors need to be thermally connected to eachother on the print. Look at the pictures below to see how I did this. For the first VCO I built I covered them in thermal paste and bent some thin copper sheet around the bodies to keep them together. For the second VCO I just glued them together with some super-glue. That'll work fine too. On the layout below, the transistors are mounted in such a way that you can bend them towards eachother so the flat surfaces connect to eachother. You can then bend the thermistor legs so that the body of the PTC rests on top of the transistors and then glue it in place. Once the glue is dry you can cover the PTC with a little bit of Heatsink Compound if you wish.
 
SOME OF THE FEATURES OF THE VCO:
The VCO has four waveforms: Sine-, Ramp-, Triangle- and Square/Pulsewave. 
All the waves have an amplitude of +/-5V peak-to-peak.
It's got a Linear and an Exponential FM input, one Hard Sync input and a 1V/Octave input naturally for the keyboard. 
Frequency range is roughly from 0,1Hz to 28.000Hz (28kHz).
The FM inputs have attenuators. If you connect a signal to the Exponential FM input the pitch of the oscillator will change, with it being connected to the same input that also drives the 1 Volt per Octave control voltage. The linear FM input has its own circuitry and it has a capacitor on the input, blocking any DC voltages but with Exponential FM the VCO pitch will change the moment you open the attenuator. Obviously you have to have some external way of influencing the pitch otherwise you couldn't, for instance, connect a sequencer to it.
There are trimpotmeters for one Volt per Octave tuning (100 Ohm), High Frequency tracking, Ramp Wave connection (this makes sure the ramp wave has a smooth slope. If it is set wrong the ramp wave will have a step in it at the zero Volt level. There are two trimmers for the Sinewave. One for roundness and one for symmetry. The roundness trimmer will also change the amplitude of the Sinewave a little.
You will need an oscilloscope to set these parameters correctly, but a cheap 20 dollar one from eBay will do fine. Make sure you set it to DC when measuring.
At first I used multiturn trimmer potmeters for all but the 1V/Octave trimmer but I have changed that because I found it very tedious to tune the VCO with a multiturn trimmer for HF tracking. It's not necessary. I only use multiturns for the Sawtooth step and Sinewave symmetry and really only to save space on the stripboard.
When I built this module I had set all the trim-pots in the middle position before I soldered them in and when I started the module up, everything was perfect except for the tuning. Even the Sinewave was perfectly symmetrical right from the get go. So was the Rampwave :) Btw, the Rampwave is the reverse from what the 3340 VCO produces. It goes straight up and then slopes down. So I guess that is called a Sawtooth wave officially. I always get Saw and Ramp mixed up anyway so we'll keep caling it a Ramp wave ^___^

PULSEWIDTH MODULATION:
There's a potmeter for the Pulse Width Modulation which goes from 21% to 75% if you use the 330K resistor (R47) as seen on the schematic. I changed that resistor to 190K and now the Pulse Width goes all the way from 1.7% to 95%. You can also put in a 200K trimmer with a 47K resistor in series so you can set the range you want manually. If you don't have a 190K resistor, use a 180K or 200K, just the closest you have or combine two resistors in series to make up the right value. The layout below uses the 190K resistor instead of the 330K of the original schematic.
Make sure the potmeter for External Pulsewidth Modulation has a ground wire that connects straight to the Ext. PWM input socket. I had problems with it, in so far that I couldn't turn the external signal off completely by closing the potmeter. There was still some external modulation going on. It turned out that the cause was that I had the input socket just grounded through the metal of the front panel. When I soldered in a wire from the socket ground to pin 1 of the potmeter the problem was solved. So you can see that good grounding is very important!
Btw, you can still use the internal PWM potmeter when you're using external Pulse Width Modulation. If you want to change that you'll need to install a switch in the PWM connection to the print. You can't use the Ext. PWM socket switch because that connection goes through a different value resistor.

CONCERING THE COARSE FREQUENCY:
When I wanted to measure the range of the Coarse Frequency potmeter I discovered that its range was enormous. When I turned it from the middle position (50 on the decal) to one stripe before that (between 40 and 50 on the decal) I was already 4 octaves down. And turning it up it wasn't long before the frequency was so high I couldn't hear it anymore. I measured the frequency range and it goes from 18Hz to 28kHz. So I decided to tame it a little by increasing the value of R29 from 100K to 300K. This worked out pretty neat at first but I noticed that my VCO would sometimes not work because of this change so I have now changed it back. You can put in a value that's a bit higher than 100K but not much higher. 180K would be the max I would go for but better still, don't do it at all. I tested it so you don't have to. Certainly don't do this when you first build the VCO. If you want to try this, do it after you build and tested the VCO to avoid you having to do any troubleshooting.
To get in the right octave, compare the sound with a VCO you already tuned and then turn the Coarse potmeter so it's roughly the same pitch. Mark that position of the Coarse pot with a little pen stripe. Keep the potmeter there when you start tuning the VCO and don't touch it.

TUNING:
Before I started tuning, I set the 'Frequency Coarse' potmeter in the 11 o'clock position (the pen stripe I mentioned in the text above) to get in the right octave, and the 'Frequency Fine' adjust was set to the 12 o'clock position. Let the VCO warm up for about 15 minutes before you proceed.
Tuning the VCO is just a matter of playing a low C note like C2 and a high one like C5 and turning the trimmer for 1V/Octave and checking it against a good tuner or tuning app on your smartphone. The trimmer is just a 100 Ohm one and I used a normal type for this, not a multiturn trimmer, and it works fine. It's a matter of tuning the C notes and seeing if the higher note is a bit lower or higher than it should be and compare it with the low note. If the one is too high and the other too low and the middle note is spot on then you have to turn the HF Tracking trimmer a tiny bit and also the 1V/Oct. trimmer. In the tuning proces you mainly use the 1V/Oct. and the HF Tracking trimmers but you can also use the Fine Tune potmeter on the panel if you're just off frequency. Changing the 1V/Oct. potmeter also influences the tracking so it's a delicate balancing act. Once you get it right it'll track marvellously over a wide range of octaves. I was impressed. It tracked even better than the Digisound-80 VCO and I didn't even had the Thermistor installed at first, but it was a lot more difficult than tuning the Digisound-80 VCO. I must admit though that I had trouble getting the lowest octave in tune. Octaves 2 upto 5 would be tracking beautifully but octave 1 was a bit high. Anyway I left it like that because it's not bothering me.
After I installed the Thermistor and re-tuned, the VCO was rock solid with temperature changes.
I had my VCO in tune over 4 octaves in a timespan of about 10 to 15 minutes. Before I installed the Thermistor the VCO would go out of tune after a while because of temperature changes but after I put the PTC in it stayed in tune beautifully. It can still be off a little when you first switch on but a slight re-adjustment with the fine tune potmeter and it's all back in track.
What can be really helpful with tuning is to use a sequencer to play a string of C notes from low to high in a slow tempo. That way you can easily hear how the tracking tunes or de-tunes the VCO over the Octaves as you turn the trimmers. (Just an idea.)

12V vs 15V:
I have not tried this circuit on a dual 12 Volt powersupply yet. However there are some notes about this on the Electro-Music forum stating that for 12V you need to change these resistors:  
R13 = 2K This is the 3K resistor in series with the Square- or Pulsewave output from pin 14 of IC4. 
R27 = 22K This is the 39K resistor in series with the Sinewave Roundness trimpot to pin 16 of the LM13700.
R33 = 137K This is the 100K resistor over pins 6 and 7 of IC4. 
That last one is a bit of a weird value for a resistor but the resistor values don't have to be spot on so you can also just put in a resistor closest to that value. It determins the gain of that opamp so a few K's more or less won't be a big deal. The circuit is quite forgiving anyway.
As for the Pulse Width Modulation resistor (R47). I already changed it from 330K to 190K and for 12V operation I guess it'll have to be changed to a lower value still. You'll have to do some experimenting with that to get it to your own liking. My advise would be to use a 200K trimmer with a 47K resistor in series and solder that in temporarily, set it so the pulse duty cycle goes from 1% to 100% or closest to that, de-solder it again carefully and measure the resistance and then put in a resistor of the measured value to replace the trimmer.
Edit: There is now a Eurorack compatible layout down below which has the resistor changes already implemented. 

SYNCHRONIZATION:
The Hard Sync function works like a treat. If I connect a sawtooth signal from VCO-1 to the Hard Sync input of VCO-2 the second VCO will follow the tuning of the first VCO perfectly, even if that second VCO isn't tuned very well of itself. If you then force the second VCO out of tune by adjusting the Coarse Frequency control on VCO-2 you'll get some awesome distortion-like sounds that sound very musical. It totally blew my mind when I fed that through the Steiner-Parker filter. The VCO tries to stay in tune with VCO-1 and you can almost hear it struggling to do that. On the oscilloscope you can see the wave jumping in frequency as it tries to stay in tune and it does stay mostly within the main note played on VCO-1. But hearing the VCO struggle to do that is so awesome sounding. Unfortunately I didn't film this in the demo video below because when I filmed it I only had one VCO built, but you're going to get some great results if you build more then one Thomas Henry VCO-555.
I might make a new demo video soon, but in the mean time you can get an idea of the Hard Sync function by listening to the Fonitronik video I posted below my own demo video. 

LAYOUTS:
Okay, below here is the layout I made for this VCO. The first VCO I built was made with a different layout. That layout was published in this article before and is still visible on the LookMumNoComputer Forum, but it had the transistors and the thermistor quite far away from each-other and it also had some jump-wires. I have since made a new layout and built a second VCO with the new layout to verify it and luckily it all worked first time. It has also been verified by at least 10 people who gave me feedback that they built it successfully. So here is the new and verified layout. Don't forget the 220nF capacitor between the linear FM input socket and its potmeter. 


Wiring diagram:


(Last revised at: 17-Jan.-2021: Made cosmetic changes to layout and changed two trimmers from multi-turn to single turn (also updated in BOM. 25-jan.-2021: Slight cosmetic changes. 24-Aug-2021: Slight cosmetic changes, removed color coding from resistors to make values more ledgible).

Here's the print only view. Note the stripboard used is 56 holes wide (not 55) with 24 strips:



And here's an overview of the cuts that need to be made and the wirebridges that need to be put in. I'm giving you a component side view for the wirebridges. Also mark the cuts on the component side with a black felt pen. I always mark the cuts on the component side first and then stick a needle through the marked hole and mark and cut it on the copper side where the needle comes through. That's my procedure and it guarantees that all the cuts are made accurately.


Here's the 'cuts only' view from the copper side:



ABOUT THE PTC THERMISTOR:
Here is a link to UK retailer Thonk. who has the PTC Thermistors listed. These are the ones I use. They are 3300ppm/°C instead of the desired 3500ppm/°C but it's close enough and will work fine. My VCO's stay in tune rock solid with these. Choose the 2K version: https://www.thonk.co.uk/shop/tempco-resistor-akaneohm/  

Here's an other link for the same item as the first link, this time from the United States. Last time I looked however, the website was offline. (404 error) : - CLICK HERE -

And here's a link to a supplier in Germany, twice as expensive (€2 per PTC) but no VAT. These were sold out but I believe they are now back in stock:. - CLICK HERE - 

There's also a 3500ppm/°C version from the UK Thonk retailer, (they are back in stock) but it is a lot bigger in size. (I would recommend the 3300ppm/°C model above.): - CLICK HERE -

I bought eight of the Akaneohm 2K PTC thermistors from the first link (Thonk) in the UK and they work like a charm. They are also the ideal small size. The VCO is now rock solid on it's frequency. I ordered 8 of them to build up some stock while keeping the total cost below €22 because I'm in The Netherlands. With Brexit, I was afraid I might have to pay extra when the parcel was delivered at the door but that was not the case. But there are some new changes to the EU regulations that go into effect around this time (summer 2021) that make the €22 limit obsolete. So you'll just have to order them and see what happens if you're outside the UK.



Bill of Materials. Please note the decoupling caps and electrolytic caps you can see on the schematic bottom right, are not included in this BOM. 
There's an extra 2K resistor included if you want to put a 2K in, instead of the 2K PTC Thermistor. I advise to order a batch of 100 2N3906 transistors so you can easily find a matched pair. Again, there are two R26's in the BOM but they are both valid resistors, they just got the same number by mistake.



--- EURORACK LAYOUT ---

I made a second layout for stripboard that is 59 holes wide. That can be cut in two halves and folded over to get a smaller footprint and it requires 14 jump wires to connect the necessary copper strips together. It has the resistor changes for operating this VCO on dual 12 Volt already done. I have not built or tested this VCO on 12 Volt but I'm assured it will work fine. As you can read in the comments below I already had confirmation that this layout works like it should but if you have any feedback you think could benefit others then please do share it in the comments below.
Again, this stripboard is 59 holes wide so the standard 24 by 56 hole stripboards will be too small.
Here it is:

Wiring Diagram:



Print only. Once you finished the build and tested the VCO fold the print over with the copper sides facing eachother and glue a little plastic spacer between them with hot glue so they can never touch. (Hot glue works well because it can be removed should you need to solder something.) 
Then you can use the L-Bracket to mount the print onto your panel. One bracket will be strong enough, the wiring will help keep everything in place. Do note that this VCO will have a considerable depth and won't fit into some Eurorack cases if you mount the print at a 90° angle to the panel. It's better to have it parallel with the panel in such a way that you can easily remove the print so you can get at all the trimmers for tuning. Or maybe have some holes in the panel to stick a little screwdriver through. I leave that up to you.


The cuts that need to be made seen from the Copper Side. Naturally, the six cuts that are directly next to the edge of the stripboard cutting line don't need to be made, but I left them in to make it clear where the jumpwires need to be placed.
Of course, instead of jump wires you can also use Pin-Headers like in the Wavetable Oscillator project. You can get extra long ones or push the male pins further through the plastic holders to make them stick out more so they make good contact with the  female strips.
This is the layout for the copper side. So use this as a guide and cut where indicated.


The next layout shows the cuts as seen from the component side!!! (always a good idea to mark the cuts on the component side). So only use this one for marking purposes. 


Bill of Materials. Like in the other BOM there are two resistors designated R26. This is a little numbering mistake but both resistors are needed:




DEMO VIDEO:
Here's a demonstration video, demo-ing the waveforms and especially the Exponential FM option. I put it through the Steiner-Parker filter and I compare it with the Digisound-80 VCO. That comparison is not entirely fair because the DS-80 has no Exponential FM input, only a Linear one. Although, I suppose you could use the normal CV input as am FM input. That should be the equivalent of exponential FM but I haven't tried that. Btw, I forgot to mention the Steiner-Parker filter has a slow Triangle wave on the CV input which accounts for the 'Wah' sound you can hear. This video was made before I altered the Pulse Width Modulation so here you only hear it going from 25% to 75%.
For some odd reason my YouTube embedded videos don't show up on mobile devices so here's the link to this demo video in case it's not visible underneath. - CLICK HERE - 



Here's an other video (by Fonitronik) with a very cool demonstration of this VCO. If you look closely you can see that the Coarse potmeter on this VCO is also set to the 11 o'clock position to hit the right octave (confirming that it is the exact same VCO). There is some reverb on the signal in this video so it sounds a bit fuller than the clean audio you get from this VCO. This video also demonstrates the awesome Hard Sync function:



Here are some pictures from the build proces. I always start by making the cuts and then I put in all the wire bridges. You can see the cuts marked in black on the component side. There are 33 wire bridges to put in. They differ a bit from the layout because since the first build I made some cosmetic changes to the layout:



Here you can see how I bent the two transistors Q2 and Q3 towards eachother and then thermally connected them together with some thermal heat-sink compound and some thin copper. I left some extra copper on there which I intended to use to mount the thermistor to but I decided to put that on top of the transistors, so I later cut the extra copper off.
As you can see from the pictures below it is quite an easy build. Just over 40 resistors, 4 IC's and some other components. If you work methodically you should be able to easily copy this design and have yourself a fantastic VCO for a fraction of the price they cost new.


In the picture below you can see progress of the third VCO-555 I'm building. Here I used super glue to connect the two matched transistors together and I neatly bent the legs so it all fits in place nicely.


Here's a look at the finished product:





Here's the Thermistor installed on top of the transistors, covered with heatsink compound:



Here's a look at the panel I made for it. On the right you can see a 1V/Oct. output socket. It is connected in parallel over the 1V/Oct. input without any buffering. It's just a wire connection. I use that to 'daisy-chain' all my VCO's together and so keep the Dual Buffered Multiple free for other things. This feature is also included in the Digisound-80 VCO in article 18. Do not use this output as a CV input because it has no resistor in series. So that wouldn't work and could even damage your MIDI to CV converter.


Here's how it's installed in my synth. A Digisound-80 VCO flanked on both sides by a Thomas Henry VCO (the second and third TH VCO-555 I built.). Note the fine tune buttons. It's the first time I used knobs with a number scale on them, like the ones LMNC uses only smaller, and they fit very well here. I used big knobs for the Frequency Coarse potmeters and they leave only the numbers of the decal visible. I somehow like that, but that is just a personal consideration and may change in time:



ADDING LEDs:
Lately I installed some 3mm LEDs in the front panel to have a visual indication that the VCO gets proper power. I had some trouble earlier with dodgy contacts on my powerbus system so I thought this would be a good idea to detect any trouble. The LEDs each have a 15K current limiting resistor and they are connected straight to the +15V and the -15V on the stripboard. I made the current limiting resistors a high value to make sure they wouldn't pull much current and with a 15K resistor each LED pulls 0,882mA. So less than 1 milli amp at 15 Volt. The Cathode of the positive LED is connected to the Anode of the negative LED and from there goes a wire to ground on the print. The plus and minus are connected through the resistors to the power rails on the print.



Finally a look at some scope images of the VCO. The picture below shows the basic waves and the Duty Cycle of the squarewave with the Pulse Width potmeter fully counter clock-wise and then fully clock-wise. The somewhat limited range of the PWM was the only drawback of this VCO and it was naging me so I changed resistor R47 from a 330K to a 190K (after experimenting with a trimmer) and now the PWM has a nice range all the way from 1% to 95%. You can see the exact values in the image below. 



In the next picture we see the Sinewave FFT or Fast Fourier Transform at the top left. This shows the main peak in the middle at approx. 332Hz and then the harmonic frequencies as the peaks to the right of the middle. As you can see the harmonics are at least 30dB attenuated compared with the main wave so well suppressed.
The rest of the pictures show the waveforms being 'Hard Synced' by an other Thomas Henry VCO. Here you can see how it influences the different waveforms. (The picture at the bottom right shows the output of my VCA after the Hard Synced squarewaves have gone through the Steiner-Parker filter (not important)).



TROUBLESHOOTING TIPS:
I've had a few people who built this VCO and then it didn't work. Most of them eventually got it working though. Here's a little summation of the most common causes of trouble that I came across. This list can be used for any of the projects on this website:

- Forgetting a cut or a wire bridge. I think this must be the number one cause of the VCO not working.
- Grounding problems. This is something I experienced myself. If you rely on grounding the sockets through the metal front panel you are asking for trouble. It can cause the VCO to just not work. This design is particularly sensitive to grounding errors I found. So make sure everything is grounded with wires. You can connect all the grounds of the sockets together by weaving a copper wire through them, soldering the connections and then connect it to the print.
- Short circuits between strips. Solder whiskers. This is also a leading cause of modules not working. (This has actually happened to me and also a lot of other people posting problems on the Facebook group) A tiny bit of solder can be the cause of the short circuit or sometimes even bad etching of the stripboard itself so two strips are connected by remaining copper. Take a sharp iron pin or small screwdriver and scrape the area between the copper strips to make sure there are no connections. Measure strips that are next to eachother for continuity before you start building.
- Chips not working. Seems obvious but it has occurred. Make sure your chips are good and from a reputable source (in other words not fakes from China.)
I've had feedback where someone had static noise in the audio outputs. This turned out to be caused by a bad 7555 chip. You can also see in the comments below a comment about a bad TL074 chip being the cause of the rampwave not working properly. 
- Small bits of wire getting into potmeters and causing a short circuit. This is also something that happened to me and it is almost impossible to find. I discovered this once when I couldn't find a short circuit which I knew was there and decided to put the full voltage of the powersupply on the short to see where it would start smoking (after first taking out the chips). A big flash came from one of my potmeters as the wire evaporated and that solved the problem. The potmeter wasn't even damaged.
- Powersupply issues. The VCO is not getting power or intermittent power on one or both of the power rails. Check your powerrails with an oscilloscope while in use and while you're at it, check the powercord or ribbon-cable to the VCO. 
- Bad solder joints. Bad soldering can be a difficult to spot problem. A wire or component might seem to be connected but that isn't always the case. When in doubt, re-flow your solder joints to make sure. Also make an effort to solder neat and tidy. Use thin solder (0.5 or 0.6mm) with rosin core and don't use too much. A good solder joint looks from the side like a christmas tree not like a snowball. (I also prefer to use lead solder 60% tin/40% lead). Make sure your soldering iron is at the right temperature and that is HOT! (about 370°C) Better too hot than too cool because with a hot iron you can solder more quickly and expose the component to heat for less time.
- Faulty patch cable. A no brainer right? It happens though. Use good patch-cables!

< this list will be updated as more causes and solutions come in >


Okay, that's another one done. 

For questions and other help you can use the comments below but I also advise to check out the EddyBergman Discussion and Help FaceBook group. You can also find the schematic of the VCO in the Files section of that group

If you like what you see and want to help to keep this website up and running and to support new projects you can buy me a coffee. There's a button for that underneath the main menu if you're on a PC or Mac. Otherwise you can use this PayPal Donation Link (that's a bit easier too) if you can spare a few bob. It would be a great help and all donations go towards the purchase of new components. Thank you!!

DISCLAIMER: The author of this article does not accept any responsability for the correct functioning of this, and any other, module/project on this website. What you build, you build at your own risk. All project layouts are thoroughly tested before publication, it's up to you to replicate them and the author can not be held responsable for any mistakes made.

62 comments:

  1. Very nice project!
    I am sort of doing what you are doing, collecting ideas and schematics from here and there to build a modular synth. Using modules from Yusynth, MFOS, Thomas Henry and an Echo Module from Scott Bernardi.
    I ordered PCBs from JLCPCB for the following modules: Yusynt VCO (L394 and 2SC1583), Yusynth Noise+S&H, Yusynth ADSR, Yjomas Hentry VCA, MFOS Dual LFO, Ring Modulator and Scott Bernardis Echo (PT2399). I also made PCBs for the Polivoks VCF.
    If it is of interest, I can ship you the Gerber files and if you like to solder, I have some spare PCBs for the Polivoks filter.
    More odd modules are from Cat Girl Synth and from Barton Musical Circuits I got an arpeggiator and quantizer.
    Building everything in a home made standard - 7" height and width of multiples of 1/2".
    Some little more details (and photos): http://www.pastisch.se/ror/modular.htm
    Also working on the MFOS 32-steg sequenzer and the Soundlabs Mk 0.0 and Mk 2.
    I will keep an eye on your progress and really try to get som ordnung und rednung with my project. A PSU wopuld be nice so I can check my modules as I install them in the chassis.

    PS The cheap oscilloscope was a GREAT idea, but I bought a cheap Joy-It scope from Elektor.

    /Magnus

    ReplyDelete
    Replies
    1. Hi Magnus. Because I already have all the modules built I don't really need the Gerber files but thank you for offering them. I am interested in the PCB for the Polivoks filter. That's one I don't have yet. Also the Quantizer would interest me. Maybe you can contact me via a private message on Facebook. Thank you very much. -Eddy-

      Delete
  2. Hello Eddy,
    wanted to give you or all of you out there feedback, I built the Eurorack stripboard and it sounds amazing, got the OSC working on my Yamaha CS-10 and it sounds great! Thanks Eddy !!! Now all I have to do is calibrate it, hope it's going well, I've never done this before!

    ReplyDelete
    Replies
    1. Thank you so much for the feedback Maik! That's very good to know. It's the same layout as the big one but you never know if issues arise by cutting it in half. And I haven't built that version myself so this is excellent news. Good luck with tuning. It's a bit fiddly but you should get it working without trouble. Thanks again!!

      Delete
    2. Hello Eddy,
      I am very surprised, I tuned the VCO within 3-5 minutes, it was child's play, had imagined it to be more difficult. However, it only runs on 3 octaves, but it seems to be enough for me. Unfortunately, I don't have a low C, it starts with me one note higher "D"! Is that the same for you? Or can I still adjust that! at least a great VCO!

      Delete
    3. Hi Maik. Thse same thing happened to me when I first started to tune the VCO. You need to use the HF trim potmeter to get that lowest note in tune and then try to tune again. At least that is what I remember from my first efforts. You should be able to get that low note in tune.

      Delete
  3. Hi Eddy

    Thank you for all of your efforts in creating content here! I am thinking of building a 12v version of this VCO and I stumbled on this weird value for a resistor 137K. The closest I can find for a reasonable price is 130K is that close enough? On the other hand a series of 3x47K is even closer. Would that work well?

    ReplyDelete
    Replies
    1. You can use a 147K aswell I think. It's a feedback resistor determining the gain of an opamp. The value 137K was a calculated value but you can use approximate values. No problem.

      Delete
    2. I meant a series of 47K + 47K + 47K which gives a total of 141K.

      Delete
    3. I know what you meant :) but 3 resistors in series is a bit clumsy. You can use a 139K (that's a regular value from the E24 series of resistors (1% tolerance)) or a 147K which is also readily available.

      Delete
  4. Hi Eddy
    I made the eurorack version of this module. It works great and sounds amazing but I have a problem. The ramp wave isn't working properly it is just a really loud square wave. I checked the layout twice and I couldn't found any problem. What could I do?

    ReplyDelete
    Replies
    1. I'm sorry you're having problems with it. I know the layout is ok because I had confirmation of that. Check your stripboard for solder wiskers between strips. Maybe there's a short somewhere.

      Delete
    2. I am having the same problem with my build. Everything works except the saw out is a loud square wave. Did you ever determine the problem?

      Delete
    3. @Magrathea No I haven't. I haven't come across this problem myself. Why don't you post about it in the FaceBook group for this website and maybe someone there has come across a solution.

      Delete
    4. @Magrathea The only thing that springs to mind is that your 7555 chip is not the right one for the job because the sawtooth is connected to that chip. Maybe try some others. Maybe even try a normal NE555 just to see what happens. I wouldn't know what else to suggest.

      Delete
    5. I was also thinking the problem could relate to my 555 chip, I'll try out some others. Thanks for all your work! I have made many of the modules on your website and I greatly appreciate your stripboard layouts!

      Delete
    6. You're welcome! Glad you're enjoying it :)

      Delete
    7. I found the problem. It was a faulty tl074 chip.

      Delete
    8. I'm glad you found the problem. A faulty chip will do it every time ^^ Good work! Enjoy the VCO!

      Delete
  5. To expand tbe scale of the coarse pot, instead of changing R39 value, keep R39=100k and modify coarse pot only. Instead of 100k pot, use 47k or 50k pot, and add two 27k resistor in series on both pin 1 and pin 3. You'll use the central part of the previous range with 2x accuracy. E.g. if it changed 4 octaves with slight movement it will only change 2 octaves now with same movement. But you loose range on extremes. By tweaking the 2 series resistors values (e.g. 12k+39k instead of 27k+27k) you can position the expanded scale where you prefer, until you have the control range extend where needed/wanted, removing parts you don't need from the scale and "expanding" the part of the scale you need to the pot/knob movement range.

    ReplyDelete
  6. I don’t have a 190k resistor, is there another value I can use?

    ReplyDelete
    Replies
    1. Just find the nearest value you have like 182K or 200K or put some resistors in series to get the value or use a trimmer pot. Lots of roads lead to Rome here.

      Delete
  7. Hello, I have a problem with this vco.
    I dont have sound to triangle out, and the frequency pot dont run.
    where does the problem come from ?

    ReplyDelete
    Replies
    1. I wish troubleshooting was that easy. "Where does the problem come from?" I don't know, you must have made a mistake some where. Maybe you forgot a cut somewhere. Check your print closely with the layout and stripe off everything you checked. That way you should find the mistake.

      Delete
  8. Simulation of the oscillator :)

    https://tinyurl.com/y6s7use9

    ReplyDelete
    Replies
    1. That is amazing!! You have been busy. I will post this in the article! Thank you for this!!

      Delete
    2. haha! it's really fun to do! I'm planning to make many of these modules IRL so why not make them as simulations first? then I will really understand them as well. Thanks for putting it up, I think people will appreciate!

      Delete
    3. Well if you make any more from the projects on this website let me know and I'll put them in the articles. They are great for understanding the circuit.

      Delete
  9. Hi! Thanks for ur project!
    I am thinking of building a 12v Kosmo module with this info, but I've a question. ¿Is the tolerance of all resistors 1%?
    Carlos.

    ReplyDelete
    Replies
    1. Hi Carlos. Yes the tolerance of all resistors is 1%. I used metalfilm resistors of 1/4 watt.

      Delete
  10. Hello Eddy,

    I started making one of these and noticed that the layout pics above denote 56 holes in the stripboard. The ones I've received through AliExpress are all 24 x 55 Holes 65 x 145mm.

    What I've done is to move the right hand outputs one row inwards on the layout picture, then resized the image to fit onto the stripboard. I used some stick glue on the stripboard and put the printed on paper picture on top. All I have to do now is pierce the paper with a pin and solder the components in.

    I can send you a copy of the pic if you or anyone else would like to have it, contact on my blog/site thing https://photosynthvideo.blogspot.com/

    Cheers

    ReplyDelete
    Replies
    1. Hi Jim. This is weird. I usually get my stripboards from AliExpress too and they're always 56 holes wide. But your solution sounds very creative. If you like you can post it in the Facebook group. Might be useful for others too.

      Delete
  11. Hello Eddy,

    I don't have an FB account

    I've put a scan of the board with the print glued on here: https://photosynthvideo.blogspot.com/p/vco.html please click on the image to see it larger and you'll see the count ends at 55 albeit upside down.

    It'll make the construction job really easy.

    Cheers

    ReplyDelete
  12. Hello Eddy,

    Just looked on AE and yes the 56 hole ones have an extra hole squeezed in and no writing or numbering on the horizontal borders.
    Odd that I should get the wrong ones none of them state the hole numbers, not that it matters you'd left room room to move that last row inwards :-)

    Cheers

    ReplyDelete
    Replies
    1. Excellent job Jim. I just had a look. Don't rely on it completely but check you're hitting the right strips while you're building it up tho. (I don't have to tell you that right haha ^__^) Let me know if you run into any problems.

      Delete
  13. Hello Eddy, i hope you r doing alright. Im now building the VCO 555 from your tutorial and i would like to know if 2 x 1k ohm PTC in serie would do the job or if i can go with a different value. Sorry for this noob question. cheers

    ReplyDelete
    Replies
    1. Hmmm that's a very good question. Well, firstly there's the problem of mounting two PTC's on those transistors but I guess that can be done but I don't know if you can add temperature coefficients together or which coefficient each of them should have. I can't answer the question for you but if I were you I'd order 2K ones from Thonk.

      Delete
    2. Hi Eddy, thank you for your reply, i found a shop from Germany with fair shipping costs, so ill go for it. A last question i have is about the -12V and +12v wires leaving coarse and fine pots legs. Do i have to connect them wherever i want as soon as i get the proper voltage ? cheers

      Delete
    3. Yes connect those wires to the strips where the voltage comes in. Somewhere on strip N for +12V and strip P for -12V. The exact position isn't important, just put them where you have room.

      Delete
    4. Me again, i think im experiencing the socket ground problem you mention about using a metal front panel.. have you fixed this by using a plastic panel ? i was thinking about put small plastic rings as discret as possible on the front and the back but the thread is probalby going to touch the metal.

      Delete
    5. No the outside of the socket is supposed to touch the metal but the connection isn't always good so the best way is to also solder a wire from the socket(s) ground to the ground on the stripboard.

      Delete
    6. alright i get it, ill try that tomorrow morning. I can't thank you enough for the time you take to help me with advices. cheers

      Delete
    7. Hi Eddy, hope you r doing alright. Im working on the VCO 555 today. And surprisingly, i only get it to work if i push the 7555 in his socket with my finger but even with a plastic pen.. thinking it was a bad contact i decided to solder it right on the veroboard but still same problem,

      Delete
    8. Then it is probably a bad solder joint somewhere near to the chip or a break in one of the copper strips near that point. I suggest re-flowing all the solder joints in the area of the 7555.

      Delete
    9. after spending hours.. guess what it was a broken track in the veroboard... the copper layer on my veroboard is very thin, that means if you desolder something or even solder something leaving the heat too long, you r good to broke the track sometimes without seeing it.. no more less than calibrating the vco tomorrow, then the LFO with a nice tektronics scope we have at school. cheers

      Delete
    10. AAAH OK, I had it right!! :) I thought it had to be either a broken strip (track) or a bad solder joint. Excellent work finding it! I hope calibrating won't be problematic but it should be fine.

      Delete
    11. Hi Eddy ! Calibrating is ok but there few things im missing because im clearly at beginner level. if i turn on the osc, i have all waves going at around 500 hz with pot tune at the beginning, and 540hz at the end clockwise. The 1v/oct trimmer will only affect around 10hz at where the pot tune is set. I can't go lower than 500hz so i don't get it. If i play a sequence with the Baby 8 step, the note play inside one octave only. I think my situation is a noob situation. I thought it would not work that way.

      Delete
    12. Just to let you know im using a 2k 1% resistor, i haven't glue it to the transistors.

      Delete
    13. That 2 K should be fine. That's a weird problem. I can only suggest to look and measure for mistakes because there's obviously something wrong.

      Delete
    14. Hi Eddy, i spent 2 days rebuilding the whole circuit clean on a veroboard and still no chance... so bad it made it work previously, that's make the situation even more depressing :D... once again my lm13700 and tl074 are from china, they all receive the right voltage.. i built the exact copy of the 12V eurorack version you present. My ground socket are all linked to the ground on the veroboard as shown.. I don't know if the mach transistor has face to face each because i put the 2k resistor between them like a sandwich.. i put thermal paste and thight them together.. :(.. in last chance, i think i have to build a vco with one of the AS3340 i have.. do you know if i can go with either CEM3340 layout i find ? cheers

      Delete
    15. My LM13700 get pretty hot. From my PSU, the +12V is consuming 120mA when the negative is 40mA.. I have zero bad contacts on the board.. the trigger PIN2 from the ipa7555 never show the triangle waveform as it's supposed since the circuit stopped working, it's always -1V even with the rebuilt circuit.. i tried many IPA7555 and my 4 LM13700 in stock and nothing.. the TL074 has been checked regarding to the inputs and outputs expected.. I may open a facebook account to find some help.

      Delete
    16. Yes opening a Facebook account to ask for help might be the thing to do coz I'm stumped by this. The fact the LM gets hot is enough to suggest there's something seriously wrong. I wouldn't know what to suggest. As for the AS3340 yes you can use it in any CEM3340 schematic. They work the same. Good luck. Sorry you're having so much problems with it.

      Delete
    17. It's quite frustrating as on the multi fixed previous board, it was working.. but not 100%. Now with a nice veroboard, and all matched, all the waveforms output give me a dc signal from -2v to -10v but not oscillation.. It's my bad because i own 3x as3340 as birthday present and as im stubborn as hell in my "a cheap as possible" project, i didn't go for the easiest VCO build possible i guess. Im going to build your VCO on "real good vco" page, i start this evening with jumpers and cuts and be ready tomorroxw morning to solder the rest of the bom. Cheers

      Delete
    18. It's weird, I built 3 555 VCO's with those plans and they all work fine. Well the 3340 VCO is quite straight forward. It's a better version than the datasheet schematic. Just be patient and accurate and thorough. Check your components for the right value, check the stripboard for shorts between strips before you start and make sure your potmeters and chips are good. Build everything up, panel and all before you start testing.

      Delete
    19. Hi Eddy, im building the digisound vco circuit and i was wondering something. As for this VCO, to run it under +/-12V, you recommend to change the timing capacitor value. Would it be the reason why my vco 555 is not working as i m using the same capacitor value as for +/-15V circuit ? Obviously i should test it myself but asi want to preserve the integrity of the thin copper tracks of my cheap veroboard, i prefer ask you first. CHeers

      Delete
    20. That's a logical thought but I don't think that's the problem. Even if the capacitor were too big it should still oscillate. The capacitor is changed for a smaller one when it is impossible to tune the VCO to 1 Volt per Octave. It's weird but this is not always necessary with the 3340 VCO on 12V. Some people need to change it and others leave in the bigger capacitor and it works fine. With the Thomas Henry VCO I have not heard of anyone needing to put in a smaller capacitor.

      Delete
    21. Hello Eddy, as comments are unables on the digisound version, i only would to ask you something about a sentence that my english don't understand well. Im trying to tune the vco and you say "turn trimpot A up about 3 quarters of the way." and 3 quarters down for B.... what do you mean by quarters ? like 4 quarters = one turn ? because im facing a problem as my trimpot pots arrives a the end at one moment that make the tuning looking like impossible.. cheers

      Delete
    22. Good question and I can see why it might be confusing. Those trimpots have a total of 30 turns to get from zero Ohm to maximum. What I mean is turn the one trimmer until it is 3 quarters of the total resistance, so that would be 30/4*3=22 turns counting from zero Ohm upwards and the other one turn it to 25% of maximum resistance so that's 30/4=7 turns. But listen, those are recommendations. It's the way I did it. You don't have to do it like that but I hope this explains it for you.

      Delete
    23. I adjusted that text a little in the hope of making it clearer.

      Delete

Note: comments are moderated and do not appear straightaway. Your first comment may not contain any links.