PAiA 4720/A VCO Module

The 4720A VCO is a linear V/Hz Oscillator with ramp, triangle, sine, and square outputs.  It is designed around a unijunction transistor and has minimal circuitry.  This kit was $34.95 in the 1976 catalog.



This is information for the A version

4720A PAiA VCO Manual

4720A PAiA VCO Schematic

4720A PAiA VCO PCB Reference


This is information for the non-A version.

4720 (non-A) VCO Schematic

4720 (non-A) VCO PCB Reference


There are a few minor differences between the revisions that are indicated on this document

4720-4720A differences


These are the bottom traces for the non-A revision.


I upgraded this 4720 to the A revision, upgraded the op-amp, and replaced the jacks.  I also added bleed resistors on the output capacitors and upgraded the 748 op-amp to a LM301 (it requires the balance input).  Although the power supplies are regulated, the current demand varies with the waveform so the series resistor adds some ripple back into the supplies. I chose to increase the filter capacitor from 100 uF to 220 uF on the VCO modules.  The power cable terminates in a 4 pin MTA connector.  



These are the four outputs from the VCO.



Here is an article from Polyphony February 1978 on modifications for a super VCO.




The customer wanted me to provide better trimmers for keeping the VCOs in tune.  I replaced the Init_Trim trimmer with a 20 turn and brought the Zero trimmer to the front panel also with a 20 turn panel mount.