PAiA 2720-2 VCO Tests

I ran into an individual online who was having some problems with his 2720-2 PAiA VCO. Among the problems with his VCO was that it was not tracking at all and was incredibly high pitched (even when adjusting the internal range trimmer). So I agreed to help him in any way I could.










The first thing we did was compare VCOs. In examining our VCOs, we judged that there appears to be at least two revs of 2720 VCOs. One, of course, is detailed in the PAiA 2720 VCO schematic which is widely available online (and pictured to the left).




Another rev seems to be detailed in Brice Ward's (1975) Electronic Music Circuit Guidebook. There's a few subtle differences between the two and it appears that our VCOs were closer to Ward's schematic (on the left) than the one that came from PAiA.

The red numbers are areas of the circuit, which were labeled by the owner of the faulty 2720-2, that I am going to test so we can do some comparative troubleshooting.



To help in troubleshooting, I was given a spreadsheet with some test values. The numbers correspond to the areas marked on Ward's schematic (pictured above). The VCO in question is on the left side of the spreadsheet and my VCO's readings are on the right side (and in red). You can see that there's a bit of a difference at D3 and D4. Well, the infamous Scott Lee was asked about this and he confirmed that these should be nearer to 5v.


After the diode swap, I'm told that the VCO functions better but is still very high pitched. I'm guessing that this is a capacitor issue. Perhaps one is leaking or damaged. Hopefully this individual gets his issue sorted out soon so he can enjoy the oddness that is the PAiA 2720 VCO.