Building a Eurorack 3340 VCO – Part 2 Constructing the VCO Breadboards

This is part two of the Eurorack 3340 VCO build guide. In part one, we covered the 3340 circuit schematic and bill of materials. In part two, we will build the three boards comprising the completed module.

Our completed VCO module consists of two Eurorack breadboards and one 10HP Control Deck. We’ll first add components to the three boards and then add the pin headers which connect them in part three

Printing out the diagrams and marking off the wires and components on paper as you build can be helpful.

On this page

Photo of the CEM3340 / AS3340 VCO Eurorack module

The 3340 VCO Core Board

The core of the VCO circuit is built on an N8 Eurorack breadboard. If you’ve used a sprung breadboard before, you should be right at home, as the layout is basically the same. If you need a quick overview of the design and features of the N8 Eurorack breadboard, this guide provides a diagram of the layout and connections.

As with any PCB construction, we want to build low to high. Starting with the wires and then adding components in reverse order of height. Beginning with the lower profile components makes our life easier, giving us more room to manoeuvre without the taller parts getting in the way. Additionally, we will add a few wires ( the “flying leads” – see diagram )  last, as it is more convenient to route them through the placed components rather than place components around them. The pin headers will be added later when we assemble the module in part 3

Step 1

1. First up, we add the wires. Referring to the Step One diagram for placement, strip around 3mm of insulation from each wire end and feed the conductor through the appropriate holes. It can be helpful to work in sections and use masking tape to hold several wires in place before flipping the board for soldering. We’ve used multiple wire colours to indicate signal flow, but this is optional, and any colour of 24 AWG wire will work just fine.

2. Position and solder diode D2 and the resistors that are mounted flat to the board – R19, R25, R29, R30, R31, R32, R34. Again, holding components in place with masking tape can be helpful or bending the pins slightly to stop the component sliding out of the holes.

3. Add the IC sockets, taking care to orient them correctly. Solder one pin of each socket, check that it is square to the board, adjust if needed, and then solder the remaining pins.

4. Add the ceramic capacitors and C23, the Polypropylene film timing cap.

5. Add R37. This resistor is critical to the oscillator’s ability to track a volt per octave. It should be as close to 1.8K as possible by either using a 0.1% tolerance component or by using a multimeter to find the resistor closest to 1.8K from a batch of 1% tolerance resistors.

6. Add the remaining resistors and 2N3906 PNP transistor, Q1, followed by the three electrolytic capacitors C20,C21, and C22. Ensure these are fitted with the correct polarity.

7. Add the two trimmers, RT3 and RT4. If you’ve opted for multi-turn Bourns 3296W style trimmers, bend the pins at 90 degrees to the body and mount as illustrated. These can be substituted for RM-063, 6mm vertically mounted trimpots, at the cost of some precision when it comes to calibrating your oscillator.

Step 2

1.  With the core components mounted, add the final wires illustrated in the “step two” diagram. As can be seen in the photos on this page, these wires can be routed between components but are shown crossing components in the diagram for clarity

Put the VCO core board to one side for now; you’ll add the pin headers and power connector (J10) later.

The 3340 VCO IO Board

The I/O board of the 3340 VCO module is also built on an N8 Eurorack breadboard, specifically the Bread & Butter B1. This variant does not include a power header and is designed to be stacked on the powered variation. Again if you need a quick overview of the design and features of the N8 Eurorack breadboards, the quick start guide provides a diagram of the layout and connections.

On the input side of things, this board handles setting the oscillator’s frequency by combining the control voltages from the coarse and fine controls with the two CV inputs. These are summed internally by the 3340. Similarly, the pulse width modulation inputs are summed and scaled.  The triangle wave is buffered for output and also used to generate the sine wave. 

As before, we want to build low to high. Starting with the wires and then adding components in reverse order of height. The 1×10 and 1×2 pin headers will be added later when we assemble the module.

1. Referring to the diagrams for placement, start by adding the wires. We’ve used multiple wire colours to indicate signal flow, but this is optional, and any 24 AWG wire will work. There are quite a few wires on this board, so it would be helpful to print the diagram and mark them off as you add them. A highlighter pen works well for this.

2. Add the resistors that are mounted flat to the board – R7, R8, R14, R42, R45, R44, R49.

3. Add the IC sockets, taking care to orient them correctly. Solder one pins of each socket, check they are square to the board, adjust if needed, and then solder the remaining pins.

4. Add the ceramic capacitors. IMPORTANT: Add a small amount of insulation stripped from the wire to insulate the pins for capacitor C3 as it is mounted across the edge connector – see photo. 

5. Add R1 & R2. These two resistors are critical to the oscillator’s ability to track a volt per octave. Both should be as close to 100K as possible by using either a 0.1% tolerance component or a multimeter to find the resistor closest to 100K from a batch of 1% tolerance resistors.

6. Finally, add the remaining resistors, two 2N3904 NPN transistors, and trimmers RT1,RT2, RT5, and RT6. If you’ve opted for multi-turn Bourns 3296W style trimmers, bend the pins at 90 degrees to the body and mount as illustrated.

Put the VCO IO board to one side for now; you’ll add the pin headers later.

The 3340 VCO Control Deck

N8 Synth Eurorack Control Decks have a grid layout with each grid cell able to host a 3.5mm jack socket, 9mm potentiometer, sub-miniature SPDT switch or an LED. We refer to these as JPS cells – short for Jack-Pot-Switch. Each cell has vertical and horizontal connections labelled A, B, & C; these connect the components’ pins to the edge connector or, in the case of the 10HP control deck, the two edge connectors. Each component only uses the vertical or horizontal pads allowing us to use the unused pads to make wired connections between components directly on the Control Deck.

In addition, each JPS cell has a fourth connection to the D-Bus. The D-Bus is not connected to the edge connectors but can be used to create a common ground on the Control Deck. We’ll use it for this purpose here. 

Control Decks have a logical front and rear. The front is the side with the screen-printed boxes indicating the positioning of components. For this VCO design, we stick with convention and mount our components on the front.

More information on the layout of the 10HP 3×6 control deck can be found here

Wiring the Control Deck

1. We start by adding the wires illustrated to the rear of the Control Deck. The rear doesn’t have the screen printed boxes for aligning components. Each of the 3.5mm jack sockets has its sleeve pin connected to the ground. We achieve this by connecting the horizontal C-pad to the corresponding D-pad with a short wire. When connected to the breadboards, the D-pads will form a common ground.

2. Pots RV1 and RV4, RV5, RV6 also have pin one grounded. We achieve this by connecting the vertical A-pad to the corresponding D-pad

3. Next, add the yellow and purple wires. These connect the tips of the CV2 and PWM  jacks ( J2, J3 ) to their corresponding trimmer pots, further reducing the number of connections we need to make on the breadboards.

3. Finally, connect the grey wire from the horizontal A-pad of JSP10 to the equivalent on JPS5. Here we are making use of the unused JPS5 cell to route a sync signal to a convenient location on the edge connector and then the breadboard.

Mounting the components

1. Double-check that all the wires on the rear of the control deck are in place. Some of these are difficult to fit after the components are added to the front of the Control Deck.

2. If the pots have metal support tabs at their front and rear, these should be bent up under the body ( see diagram and  details here ). Do this before mounting them.

2. Position the pots and jacks on the front of the Control Deck and attach the 10HP panel using the hex nuts and washers supplied with the pots and jacks.

3. With the panel attached, check that the pots and jacks are flat on the control deck and centred in the panel’s holes, and then solder them into place.

4. Jacks J1-J5 have their sleeve and switch pins connected. We achieve this with an elegant solder blob on each, as illustrated

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