Building the VCO Classic

PCB and Schematic:



Area in yellow box contains the optional 4.7k NTC thermistor and matched transistor package (unlabelled on PCB). 2.2k resistor in red should be changed to 1.5k if not using the NTC thermistor. If using the NTC thermistor, it can be thermally bonded to the matched transistors by covering both with thermal compound.


BOM:

Qty
Value
Device
Package
Description
Source 1
Source 2
12100nCapacitor0603Note package! Not labelled on PCB.
14K7NTC Thermistor0603Optional. Replace nearby 2K2 resistor with 1K5 if not using.https://www.digikey.ca/product-detail/en/murata-electronics-north-america/NCP18XM472J03RB/490-2439-1-ND/588653
110nCapacitor0805
110uCapacitor0805
4100KResistor0805
1010KResistor0805
118KResistor0805
22M2Resistor0805
12K2Resistor0805
147RResistor0805
14K7Resistor0805
21uCapacitor0805
1Female Header1X04Header
1Male Header1X04Header
1100RMultiturn Trimmer3296WTrimpot. Must be this exact model to fit panel.http://www.taydaelectronics.com/potentiometer-variable-resistors/cermet-potentiometers/3296w.html
110KMultiturn Trimmer3296WTrimpot. Must be this exact model to fit panel.http://www.taydaelectronics.com/potentiometer-variable-resistors/cermet-potentiometers/3296w.html
1100kPotentiometer9MM SNAP-IN9mm vertical snap-in pot, Alpha / Panasonic style with 6mm built-in knob.
21N4007Power DiodeSMAAlso called "M7" in this package.https://octopart.com/search?q=rectifier&start=0&category_ids=4226&specs2.2.values=SMA&specs2.330.numbers=(50__200)&specs2.313.numbers=1&sort=median_price_1000&sort-dir=asc
3TL072SO08Dual OP AMP.
11n4148DIODE-SOD123SOD123DIODE
1MIC6211SOT23-5Single bipolar opamphttps://www.digikey.ca/product-detail/en/microchip-technology/MIC6211YM5-TR/576-1314-1-ND/771918
1DMMT3904WSOT363Bipolar (BJT) Transistor Array 2 NPN (Dual) Matched Pairhttps://www.digikey.ca/product-detail/en/diodes-incorporated/DMMT3904W-7-F/DMMT3904W-FDICT-ND/717879
33.5mm socketTHONKICONN3.5mm socket for Eurorack modular synths

Assembly:

  1. Clean the PCB with Isopropyl Alcohol.
  2. Solder the surface-mount components to the PCB.
    1. Begin with resistors and capacitors.
    2. If not using the 4.7k NTC thermistor then install a 1.5k resistor instead of the 2k2 resistor (shown in red box above) .
    3. The small unlabelled footprints (0603 size) are 100n capacitors.
    4. The large unlabelled diode footprint above the right OpAmp is the 1N4148 diode. Make sure the polarity is correct.
    5. Add the dual transistor. This is a very small package and is unlabelled on the PCB (it’s in the big box with the 4k7 NTC). Pin 1 is on the upper left (marked with a large white dot).
    6. Finally, add the OpAmps, making sure they’re in the correct position (pin 1 is by the large white dot).
  3. If using hot-air or an oven to solder the surface-mount components then do this now.

4. Solder components, header, and power cable to the Offboard Power PCB. The only devices that need to be mounted are the four capacitors and two diodes.

The power cable and header are mounted on the opposite side and soldered on the component side. The cable should go through the strain-reliefs. If not using the strain-reliefs the cable can be soldered on the opposite side.

  1. Attach the Offboard Power PCB. It can be attached with a male/female pin header combo so it’s removable, or soldered directly to the main PCB with just one male header (but it will be harder to troubleshoot/make repairs later on).
  2. Inspect all of the soldered components for solder bridges between pins and components, especially around the OpAmps.
  3. Solder the front components (pot, two trimmers, and four jacks). It’s a very good idea to put the panel on before soldering to make sure they are aligned. Some pots have a metal support underneath — check that it doesn’t short the power lead pins. Cut the leads flush to the PCB if they are. Double check that the proper value trimmer is going to the right place, and that the heads fit into the PCB cutouts.
  4. Again inspect all of the soldered components for solder bridges between pins.

Testing:

Check that there’s no continuity between the red, white, and black wires on the power lead. If there is continuity then for shorts between the components around the power lead and diodes.

Check that there’s no continuity between the both ends of each of the small 0603 capacitors, located near the OpAmp. If there’s continuity then check all of the soldered connections for shorts.

Plug the module in and check that the output is shaped properly and oscillates.

 

Tuning

Quick tuning on-the-fly is accomplished with the TUNE knob. This will shift (offset) the entire scale up or down a few semitones. It is used during performance to tune the oscillator to other oscillators (or detune for a thicker sound.)

After the oscillator has warmed up and the temperature inside the case is stable — about 15 minutes — the tuning shouldn’t drift much from its initial setting.

The oscillator should always be tuned when warm.

Note: The V/OCT input is grounded when no cable is inserted, so it can be used to accurately set 0V or C1.

V/OCT TRACKING TRIM

For trimming the VCO scaling, you need an accurate source of control voltage (like a calibrated 1V/Octave keyboard or MIDI-CV Converter), and a digital tuner (I use DaTuner Lite for Android, but there are dozens of choices. Find one that shows MIDI octaves like C1, C2, C3 etc.)

  1. Set TUNE knob to the middle position (pointing up).
  2. Connect the calibrated voltage source to the V/OCT input. Set the CV to 0V (C1). Connect the tuner to the SAW output. Adjust the right OFFSET trimmer until the tuner displays C1 (the tuner might display C0 or C2 instead. Just change these instructions accordingly.)
  3. Set the CV to 1V and check that the pitch is C2. If that’s the case then go to Step 4. Otherwise, if the pitch is lower than expected adjust
    the left SCALE trimmer to lower the pitch (It may not seem logical to lower the pitch when it’s already too low — you’re really increasing the scale between octaves which makes it sound like the pitch is decreasing because the offset isn’t being changed at the same time.) And if the pitch was higher than expected, adjust the SCALE trimmer in order to increase the pitch.
  4. Set the CV back to 0V, and the pitch will be lower or higher than C1. Adjust the right OFFSET trimmer until the tuner shows C1. Repeat
    points 3 and 4 until there is a perfect one octave shift when switching the CV from 0V to 1V.
  5. Repeat the same procedure from Step 2, but for 0V and 2V to check C3, then 0V and 3V for C4, etc., etc., until C7 or C8, or until the men in the nice white coats come to take you away to the happy home with trees and flowers and chirping birds.

Alternatively, check out VCOTuner. It makes tuning the tracking much easier and faster.