Building a Eurorack 3340 VCO – Part 4 Assembly and Testing

Finishing the build and testing the VCO module

This is part four of the Eurorack 3340 VCO build guide. In part one, we covered the 3340 schematic and bill of materials. In part two, we built the three boards comprising the module, and in part three, we added the pin headers to connect the boards.

With the three boards built, let’s put them together, do some initial testing, and add the panel before we calibrate our new Eurorack VCO in part 5.

On this page

Photo of the CEM3340 / AS3340 VCO Eurorack module

Testing, Testing, check one two.

It’s good to do some basic tests before you assemble and power up your VCO for the first time. While not extensive, these help keep the magic smoke in the components where it belongs.

Visual inspection

  • Compare your module to the diagrams and schematic in this guide. Do all the components and wires look like they are in the right place? Anything missing?
  • Inspect the solder side of the Eurorack Breadboards and Control Deck.
    • Are any of the pads shorted by solder splashes or untrimmed component leads?
    • Are all the component leads soldered? 
    • Have soldered the bridges indicated on the diagrams been made?


We want to ensure there is no continuity between +12v, ground and -12v rails. We’ll do this using a multimeter.

Put your multimeter in continuity test mode, then, with each of the two breadboards, check the continuity between the following points on the circuit:

  • Connect one of the multimeter’s test leads to the +12V rail and the other to ground. There should be no continuity
  • Connect one of the multimeter’s test leads to the -12V rail and the other to ground. There should be no continuity
  • Connect one of the multimeter’s test leads to the -12V rail and the other to +12V. There should be no continuity

Final Assembly

1. Insert the 3 TL072 op-amps and CEM3340 or AS3340 chips into the DIP sockets, ensuring you have them orientated correctly.

2. Carefully insert the long pins of the IO board’s stackable pin headers into the corresponding female headers on the VCO core board, making sure all the pins are aligned, including the four 1×2 pin headers. The four 1×2 headers act to space the boards correctly as well as passing power between boards.

3. Insert the Control Deck’s six pin headers into the corresponding female headers on the VCO IO board.

Power Up

If your new module passed the continuity tests and visual inspection, it’s time to power it up. If you have a bench power supply, it is good practice to use this for the first power-up of a DIY synth module so that it is a minimum safe distance from your other modules.

The Eurorack power connector format is sadly a little open to interpretation, and many a module has lost its life to the specification’s vagueries.

N8 Eurorack prototype boards follow the most common convention. A white stripe is printed next to the -12V end of the power connector. Typically, this is where the red stripe of the power cable should be aligned. BUT not every manufacturer follows this convention, and this is a DIY synth tutorial, so chances are you made your cables, right?

If you used a box-type power connector, also check it is correctly orientated.

Always check that your power supply is supplying -12V at the red stripe before connecting power to your synth module and that the red stripe is connected to the -12V pin on the module.

Bench Testing & Debugging

If the solder gods smile upon you, your module will work the first time. You’ll have a new VCO module with four wave outputs and controls that control what they’re supposed to, and you are ready to calibrate the VCO. However, if you’ve angered the solder gods and things don’t quite work as expected, here are some tips for debugging your VCO.

Which board is fubar? 

The first thing to check is which of your boards isn’t working. With some care, the VCO core board can be tested in isolation, and then the other boards can be added for further testing. The diagram shows the appropriate connections.

  • Separate the VCO core from the other boards. The saw output is fully buffered on the VCO board, so connect this to your oscilloscope. If there is no waveform, double-check that no components or wires are missing, no pads are shorted with solder splashes, etc.
  • The triangle wave output isn’t buffered but can be viewed on an oscilloscope.
  • No voltage is applied to the 3340 PWM input with the VCO core board isolated. As such, you may be able to view a pulse wave on your scope, but it will have a very small duty cycle.
  • A variable voltage source ( like a function generator)  can be used to provide a CV input for the oscillator’s frequency. Conenct a 0-5V source via a 100K resistor to vary the frequency.

If the VCO core looks good, the IO board can be added, and the above tests repeated.

Diagram of the setup for calibrating the Eurorack 3340 VCO
Test set up for 3340 VCO Core

Making your VCO module look beautiful

Before we move on to calibrating our module, let’s spend some time making it look like the part next to those commercial Eurorack modules. The good news is you can do this with nothing fancier than an inkjet printer and some sticky-back plastic.

We have a complete guide to making labels for Euroack panels here. If baby blue is your thing, you can download our finished label, ready for printing.

Download the 3340 VCO label template.

We’d love to see your build. Share your pix with us on Facebook and Instagram.

Photo of the finished Eurorack AS3340 VCO


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