x0xb0x Fabrication Manual: VCO

The first section of the main board to be assembled is the VCO (voltage controlled oscillator). This section converts a logarithmic voltage into a scaled oscillation. The relationship between the voltage and oscillation is called "1V/octave" because every 1V increase corresponds to a 1 octave increase in sound. For example, low C (65.4Hz) is generated by 2.0V, middle C (130.8Hz) is generated by 3.0V, and high C is generated by 4.0V.

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Before: The VCO is in the top right corner of the mainboard

After: The completed VCO connected to the power supply. (The jumper wire should be replaced with plain wire)


Recommended order
  1. Solder in all parts except for S1 and VR2
  2. Solder in S1 and VR2 (on opposite side)
  3. Do basic calibration/testing.
Testing & Calibration

The VCO can be simply tested without the use of an oscilloscope (although it helps a lot). The power supply 5.333V must be calibrated before this step:

  1. Apply power (make sure J4 is connected with wires)
  2. With the tuning knob VR2 centered, apply 2.0VDC using your powersupply to R89 (also pin 5 of IC11). Also center TM4 and TM5 to start.
  3. Use a multimeter with frequency-detection, or an oscilloscope, or a pair of cheap headphones, to probe pins 1 or 3 of the waveform switch. Switch between 2.0VDC and 3.0VDC to R89 while turning TM5 ('width') until the frequency at 3.0VDC is twice that of 2.0VDC.
  4. If you have an oscilloscope, verify also the offset voltages are correct (Vmin = 6V for saw, 5V for square, Vmax = 11.5V for saw, 9V for square), if you have only a meter, the DC voltage should be ~9V for saw ~7V for square, the AC voltage should be 5V for saw, 4V for square.

    Tuning the sawtooth wave to C1. Note that the tip of the saw isnt 'sharp,' also the min and max voltages.
    Tuning the square wave to C2. Note that the square wave droops slightly and isnt 50% duty cycle.
  5. Now apply 2.0VDC to R89 and probe the waveform switch again, adjusting TM4 ('tune') until you see/hear C1, 65.4Hz.

The precise calibration step can be delayed until the rest of the synthesizer is completed, when you can just press a button to generate 2.0V and 3.0V to the VCO.

If you don't have a bench power supply you should center TM4 and TM5 (which will get you close enough) and listen to the square waveform using a pair of cheap headphones: probe the middle pin of the waveform switch while its set to SQR. Now touch the metal leads of the 200K resistors to vary the frequency and turn VR2 and note that the frequency changes.

Toggle switch 1 SW1
1N4148 3 D25, D30, D31
2.2K 5% resistor 2 R107, R104
10K 5% resistor 4 R34, R36, R105, R101
22K 5% resistor 2 R45, R60
100K 5% resistor 6 R35, R93, R92, R59, R102, R118
220K 5% resistor 1 R103
1MEG 5% resistor 1 R91
24.0K 1% resistor (red yellow black red brown) 1 R106
200K 1% resistor 17 R74-R90
5K (502) trim potentiometer 1 TM5
50K (503) trim potentiometer 1 TM4
50K B (linear) potentiometer 1 VR2
1K PTC Thermistor 2 R100A, R100B
.001uF (2A102K) capacitor 1 C34
.01uF (2A103K) capacitor 2 C33, C10
.22uF (2A224K) polyester capacitor 1 C35
1uF electrolytic capacitor 1 C11
10uF electrolytic capacitor 2 C31, C32
4066 analog switch 1 IC12
74AC174 1 IC9
2SA733P, TO-92 PNP transistor 2 Q8, Q27
2SC536F, TO-92 NPN transistor 4 Q29, Q30 , Q25 , Q24
2SK30 JFET, 2SK30AO sub-type if there is one 1 Q28
AN6562 8-DIP dual Op-Amp 1 IC11
2SC1583 5-SIP matched pair with common emitter 1 Q26


May 17, 2011 20:07