x0xb0x Build Manual Voltage Controlled Oscillator

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.


Access to an oscilloscope, even a bad one or a 'soundcard scope' is essential! You can learn to make a basic probe for your soundcard scope here.

Now is a good time to check your PCB for any errors, visit the prep page for information on how to do this and what to look for

As usual, check all resistor values and capacitor values before soldering them in, with a multimeter if possible

You'll really want a variable power supply of some sort here, it will make debugging easier. There's some suggestions on how to build one for cheap from parts or a kit here. If you aren't planning on calibrating the VCO now (which is certainly fine) you can probably get by with a battery holder for 2 AA batteries. You can buy these at RadioShack or any hobbyist shop. Make sure you're getting about 3V out of the battery pack.

Click on the images for high resolution photos when you want to see details

Toggle switch 1 SW1
1N4148 3 D25, D30, D31

2.2K 5% resistor

Red Red Red Gold

2 R107, R104

10K 5% resistor

Brown Black Orange Gold

4 R34, R36, R105, R101

22K 5% resistor

Red Red Orange Gold

2 R45, R60

100K 5% resistor

Brown Black Yellow Gold

6 R35, R93, R92, R59, R102, R118

220K 5% resistor

Red Red Yellow Gold

1 R103

1MEG 5% resistor

Brown Black Green Gold

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


Time to solder!

Verify that you have the power supply working well, heat up your iron and get your tools ready

Check the kit to verify you have all the parts necessary
This is where all the soldering is done, in the top right quadrant of the mainboard PCB
First you have to bring power to the mainboard from the IO board. Use the wires you cut earlier, the black wire is ground, the green wire is 5.333V and the red wire is 12V

Use your multimeter to probe the points shown (click on the images for higher resolution versions). Make sure you have 12V and 5.333V. The black probe in both tests is connected to the ground plane. Any ground will do, I just chose ones that I thought were convenient.

Disconnect power and solder in the following parts:

Diodes D25 D30 D31
2.2K resistors R104 R107
10K resistors R101 R105
24.0K resistor R106
100K resistor R102 R118
200K 1% resistor R90
220K resistor R103
Thermistors R100A R100B
Trimpots TM4 TM5
0.001uF capacitor C34
0.01uF capacitor C33
2SK30A-O Q28
2SA733P PNP transistor Q27
2SC536F NPN transistors Q24 Q25
2SC1583 Q26
Dual Op-Amp IC11

On the other side, solder in 50K linear potentiometer VR2

Test the adjustable power supply and verify it's outputting 3V.

Connect power to the x0xb0x and retest the 5.333 and 12V supply. (Please note we may not actually tell you to connect power but you must always test the x0x kit powered! If you think about this for a second it will make sense.)

Apply the Control Voltage using the adjustable power supply. Connect ground of the supply to ground of the x0xb0x, here I simply clipped it to the 7805 tab (which is off screen)

Connect the output voltage to R90 as shown. (Click for higher res picture)

Verify that your control voltage is correct, use the multimeter to measure the DC voltage as shown, it should be 3V

Verify the op-amp is working. Test the DC voltage as shown on pin #7 of the chip.

It should be the same as the control voltage in the previous step, 3V

Use your oscilloscope to probe the sawtooth generator. Connect ground of your scope to a convenient ground (not shown here) and the probe to R105 as shown

You should center VR2 and TM4 and TM5

You should see the following waveform, a 5.5V peak-to-peak saw tooth wave, about 150Hz (although it may be different) on a 8.5V DC bias

If you're using a soundcard scope, you wont see the DC bias but you will see the sawtooth wave.

Either way, adjust the powersupply voltage input, VR2, TM4 and TM5 to verify that you can change the frequency of the sawtooth wave

If you don't have a scope, you should at least measure the DC bias, which is about 8.5VDC

If you dont have a scope or a soundcard oscilloscope, at a bare minimum you can at least listen with a pair of really cheap headphones. The ones I'm using here are $1 earbuds.

Use a alligator clip to connect the sleeve to ground (the 7805 tab).

Then use a 2A103 polyester capacitor and stick one end in R62 (which is the sawtooth output) and touch the other end to the tip of the headphones. The left ear should have a buzzing sound. As you adjust the powersupply you will hear the pitch change up and down.

Once you've verified that sawtooth wave generation is working, you can continue on to the square wave generator.

Disconnect all external power and solder in the following parts:

10K reisistor R34
100K resistor R35
22K resistor R45
0.01uF capacitor C10
1uF capacitor C11
2SA733 PNP transistor Q8

Power up the x0x and apply the 3V control voltage to R90 as before. The probe the waveform switch pin that is second from the top (see picture)

If you have an oscilloscope, you will see a square wave of the same frequency as the sawtooth wave, about 4Vpp and 7V DC bias

Note that the square wave is not really that square

You can also observe the waveform using a PC soundcard scope.

For some reason, the pulsewidth is different. Probably some weird soundcard scope thing.

If you don't have an oscilloscope, or are using a soundcard scope, be sure to test the DC bias on the square wave pin as shown.

It should be around 7V

You can also listen to the square wave with a pair of earbud headphones as before. You dont need to use a capacitor, just poke the tip at the squarewave pin of the waveform switch (in this image its already soldered in). Tweak the control voltage power supply and note the pitch change.

more pix soon... Solder in the remaining parts


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 R90. 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 R90 while turning TM5 ('width') until the frequency at 3.0VDC is twice that of 2.0VDC.

    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.
  4. Now apply 2.0VDC to R90 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.

May 17, 2011 20:07