This is a little embarassing, but I don’t think that there was ever anything actually wrong with my JH Living VCO module. I think it was all my error.

I exchanged a few messages with Herr Haible on the electro-music.com forum. He was able to rule out my theory that the substitution of the transistor array was the source of the problem. Once I learned that I went back to basics and checked the value of every component and every solder connectionon the PC board. I completely disassembled the PCB from the panel and tested each oscillator individually, adding connections as I went. I also tested the master section separately. In the end I was able to properly set the 1V/Oct scaling for all three VCOs, with no changes to my original assembly. read more

I found the cause of the 1V/octave scaling problem I mentioned in my previous post. The problem was that I substituted a CA3086 transistor array for the CA3046. This is listed as a substitution on Dave Brown’s Mouser BOM. I probably chose the CA3086 because that’s what Bridechamber had in stock at the time, or something.

At first glance the two parts are viable substitutes for audio applications. Even looking at the Intersil web pages for both parts does not reveal the difference. The difference is in the HFE current gain parameter; the CA3086 has a gain of 100, while the CA3046 has a gain of 110. The Intersil web pages actually have a typo; HFE is stated as 100 for both. It wasn’t until I did a parameter-by-parameter comparison of the PDF data sheets for the parts that I found the difference. read more

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. read more