Synthrotek VCO - DIY Build

As you get more and more involved in the modular world, you will hear the statement “you can never have enough VCO’s” We have found that this statement is very true and it was time to add another to our rack… the Synthrotek VCO from their slider series of eurorack modules. VCO’s or otherwise known as Voltage Controlled Oscillators are a must have for any modular artist as they are often used for drones or other audio sources.

Compact and backed by the Curtis CEM3340 reissue chip, the Synthrotek VCO is an analog voltage controlled oscillator with a wide frequency range and 8 trackable octaves. Let’s get onto the build…

The Build...

One thing you will notice with the Synthrotek VCO kit is that it is a partial kit and not a full on DIY kit. A “partial” kit usually means that most of the tiny stuff has been already soldered and you only have to solder the bigger hole-through components; this kit is just that.

Please note …. this doesn’t mean that the kit is for beginners though!

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Synthrotek VCO ... Resistors and Diodes

As with all kits, you usually start by placing and soldering the resistors and diodes first. This is because there are usually a lot of these (resistors usually) and bending and placing eats up a lot of time. There are only 2 diodes in this kit and their placement has to match that of the silk screen. Resistors on the other hand do not have a polarity, so you can mount them any way you like. Work your way around the board according to the included BOM (link below) and before you know it, its time for capacitors.

VCO Time Lapse Build

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Mounting the IC

Next, insert the IC into the PCB after carefully bending the legs inward a tad to make them fit. You will want to make sure to match up the notch on the IC with the notch on the PCB silkscreen.

If these do not match up, you could cause serious damage to either the module or the IC. Once the IC has been matched up and placed, turn it over and solder.

Capacitors and those other sparky things..

Now it’s time to populate the ceramic capacitors. These are non-polarized and can be inserted either direction. Take note that if the capacitor coating runs down the legs a little, make sure the coating doesn’t poke into the pads as this can disrupt the circuit and it makes it very difficult to track faults.

Once they are in, carefully flip over your project and solder them in place, clipping the excess leads.

Note: Be sure to get C5 and C6 fairly close to the board, otherwise they will get in the way of the logic board mounting.

Electrolytic capacitors are polarized so make sure when you mount the 2 included caps that you match the silkscreen accordingly. Hint: The long leg (+) will me mounted into the square shaped pad. Flip the board over and tack everything down.

90 degree pin header

Start with the 12×1 right angle board to board headers. Make sure when you populate these that the side of the header without the bend goes through the control board. Place the PCB on a flat surface and carefully set the header into its location. The non-bent pins must sit flush with your flat surface so that the header is raised a few millimeters from the back of the board (use masking tape to keep the pins from protruding if you want).

Solder one of the pins on the back side of the board to keep the header in place. You will be soldering between the board and the plastic part of the header. Reflow the solder joint as needed and make sure the header sits at 90 degrees.

Synthrotek VCO Power Header

The 10-pin power header is next on the list. Place the header according to the silk screen and then put a dab of solder on a pad and then carefully set the header on the pads. Heat the underside of the pad up that has the blog and the power header will just “click” in.  Solder the rest of the leads and trim these flush with the PCB. Time to move on. 

Before you proceed…

It is very important that you trim leads fairly close to the PCB so that the slide pots will fit flush on the board. They shouldn’t be fully flush (except for the 12 pin header), they should be small mounds.

Check your work by inserting the slide pots; they should be able to lay flat against the PCB. Then remove the pots from the board.  If you have any leads that need touch up, now is a good time to reflow your solder joints. Slide pots will cover up all the solder joints and therefore you won’t be able to access them again.

Slide Potentiometers, Jacks and the Hex Standoff

First populate the aluminum hex standoff. You can do so by inserting a metal-finish 2.5mm screw through the bottom of the board, and then spinning the hex standoff a little to get it started. Then use a Phillips head screwdriver to gently tighten the assembly together.

Next up are the slide potentiometers and jacks. Start with the pot sliders by carefully inserting them into the board. The pots will only fit in the PCB one way, but the legs can get a little bent during shipping, so take your time and make sure they all get fitted properly into their respective solder pads. 

Lastly, place the jacks in their respective places and do not solder anything yet. Lay the front panel over the control board and hand tighten the nuts and the standoff screw. We used our DIY synth tool set to tighten everything up after soldering the lugs. 

Connecting Main Board to Control Board

The main board is the part of the kit that has been completed prior to landing on your workbench. The only thing we will be doing with it is attaching it to the 90 degree header pins that we previously mounted. It helps to just solder one pin first then straighten out as needed.  Once in place, solder the rest of the pins and call this module done. Now the only thing left to do is calibration and testing. 

Synthrotek VCO - Completed Module

Features and Specifications


  • Simultaneously available triangle, saw and square wave outputs
  • Frequency range: 5Hz to 28kHz
  • Frequency Modulation (FM) input and attenuation control
  • Pulse Width Modulation (PWM) input and attenuation control
  • 1 V/o input, tracks 8 octaves


  • Width: 4HP
  • Depth: 53mm
  • Current draw:
    • +12V: 42mA
    • -12V: 31mA
  • Includes power cable and module screws.

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Synthrotek VCO - Purchasing Options

Synthrotek has a couple different options when it comes to purchasing the VCO module. The prices are excellent and the quality of this build is way worth it! If you are interested in buying the kit or the assembled module, click on the images below and you will be redirected to their site to purchase! Make sure and tell the gang at SR that we sent you!

Our Final Thoughts . . .

After completing the Synthrotek VCO, we are very well pleased with its overall tone and warmth. The Curtis CEM3340 is a little beast and its great to see it reissued and utilized in such a small module.

At only 4HP, the VCO packs a little punch and is aesthetically pleasing just like its slider series kin and while we have plenty of VCO’s… one can never have enough! 

Comments or Suggestions?

If you have questions about the build process or suggestions/feedback about this article, feel free to let us know by contacting us  We look forward to hearing from you!

Until the next build … 

~ f i N

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