The VCF creates the high-frequency sub-oscillations, as controlled by the cutoff and resonance knobs.

Again, solder in the large potentiometers last.

At this point one can test whether resonance/cutoff affect the note.

1. Power up as described in the power supply section, connect 3VDC (using a power supply or some other method as discussed in the prep section) to R90. (See below, the red hook-clip.)
   
2. If you have one, use a scope to probe the potentiometer, twist the cutoff (VR3) and resonance (VR4) pots all the way down and examine the almost sinusoidal notes.
   
3. Or twist them all the way up and note the high frequencies.
   
4. Or, lacking a scope, use a high-impedance (32ohm) headphone to probe the same pin. Connect the sleeve to ground and use the tip to touch the point you want to hear. Make sure that its a pair of really cheap headphones: high quality ones are as low as 2ohms and will pull the signal low. Sinusoudal notes will sound "soft" whereas the high resonance notes are 'acidic.'
   

There is an 'official' tuning for the VCF. According to the Roland TB-303 manual, when playing C1 (65.4Hz) with the cutoff frequency knob set to 50%, saw waveform, and resonance knob at 100%, the resonance frequency should be 500Hz (give or take 100Hz depending on how it sounds to you). You can perform this tuning by either applying 3.0VDC to the VCO or finishing the entire assembly and using something like Keyboard mode to trigger the note. The cutoff can be adjusted by tweaking TM3.