I started to calibrate my JH Living VCOs on Saturday and quickly discovered that I can’t complete the 1V/oct scaling adjustment. Even at the most extreme setting of the trimmer potentiometer they go up by less than an octave per 1V change in input CV. Grrr.
This is a puzzler. All three VCOs exhibit the same problem, and all three of them appear to operate correctly otherwise. The obvious answer is that I put the wrong components in somewhere, but I’ve now verified the values of all of the resistors and capacitors in the first VCO and found no errors. The only component substitution I made was to use a CA3086 transistor array instead of a LM3046. This substitution was suggested on Dave Brown’s Mouser BOM. As far as I can tell from looking at the data sheets, the two parts should operate identically in this context.
So, I’m stumped. I posted a query on the relevant electro-music.com forum in hopes that someone might have some troubleshooting suggestions. I suspect I can fix the problem by changing the resistor immediately downstream from the scaling trimmer, but I’d rather like to know why the problem is happening in the first place.
