MRG VCO (Bare PCB) Assembly Guide

This is the assembly guide for the MRG VCO bare PCB build. If you have a SMD presoldered PCB, check the MRG VCO presoldered assembly guide.

This build should be considered of medium-to-high difficulty given the number of components and steps required.

Please note that this is the assembly guide for the second version of the MRG VCO (marked as revision #7a on the PCB). The assembly guide for the previous MRG 3340 VCO is here.

Theory of operation:

  1. If you bought the PCB+Panel+IC only, acquire needed components (See Bill of Materials. All components used in this guide are already included in the full DIY kit.
  2. Build the Main PCB.
  3. Build the Front PCB.
  4. Install the front panel.
  5. Tuning.

Main PCB build

The suggested order is to start soldering the front of the PCB (the one with all the resistors), and later the back, that contains the main IC, the timing capacitor, the trim pots and the eurorack 2x5 pin headers.

Main PCB Front

Step A1. Start by soldering all the horizontal resistors.

vco pcb

Step A2. Solder the Zener diode, being careful of orientation.

vco pcb

Step A3. Solder all ceramic capacitor.

vco pcb

Step A4. Solder the voltage regulator.

vco pcb

Step A5. Solder the vertical 470 Ohm resistor, the two 22uF electrolytic capacitor, and the TL072 IC. If desired, an 8-pin socket can be soldered on the PCB, instead of directly soldering the IC.

vco pcb

Main PCB Back

Step B1. Solder the 10-pin header.

vco pcb

Step B2. Solder the trim pots.

vco pcb

Step B3. Solder the timing capacitor and the AS3340 IC. If desired, a 16-pin socket can be soldered on the PCB, instead of directly soldering the IC.

vco pcb

Everything apart from the two 14-pin headers are soldered on the board now. It is time to solder the front PCB.

Front PCB

Step C1. Check the alignment of the PCB (label U1 is the top row), arrange the jack sockets and the potentiometers.

vco pcb

Step C2. Solder the components. Tip: in order to be sure to align the components to the front panel, install the front panel as in photo below and solder the components on the back.

vco pcb

Step C3. Solder the socket headers. Tip: In order to align the Main PCB's pin headers to the Front PCB's socket headers, assemble the two PCBs together, as in picture, and solder them in place.

vco pcb

All soldering is now done! The next step is installing the front panel.

Final Module Assembly

Install the front panel, using nuts and washers to keep it attached, and add the knobs to the potentiometers.

vco pcb


Congratulations! You have concluded a fairly complex build!

The next step is tuning the oscillator.

You can find the tuning guide here

Feedback? Suggestions? Contact me!.

Bill of Materials

This is the required bill of materials if you bought the PCBs+Panel+IC only. All components listed below are already included in the full DIY kit.

Component Quantity Package





Ceramic Capacitors:

220nF (224) 1
100nF (104) 4
10nF (103) 3
1nF (102) 1
10pF (100) 1

Other Capacitors:

22uF Electrolitic Capacitor 2 25ZLG22MEFC5X7
680pF Film Capacitor 1 PP film reccommended


10K1 Vertical, THT, inline
20K1 Vertical, THT, inline
100K1 Vertical, THT, inline

Other Components:

10V Zener Diode 1 DO-35
79L05 -5V Voltage Regulator 1 TO-92
14 Pin 2 Row Header2
14 Pin 2 Row Socket2
10 Pin 2 Row Header1
B100K3Alpha 9mm T18 Potentiometer
PJ398SM 3.5mm Jacks6