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.
So, why did I have such trouble in the first place? I’m not entirely sure, but I know of one actual mistake I made. When I bought the trimmer potentiometers, I used the Mouser part numbers from Dave’s BOM. I didn’t look closely at the description for them and somehow got it in my head that they were 10-turn units. They’re actually 19-turn pots. I was starting my calibration by turning them to one extreme and then counting five turns to put them at the center of their range, or so I thought. Instead, obviously, I was putting them at about 25% of their range. The other difference between these pots and the Bourns pots I usually buy is that these don’t make a subtle click when you reach one end or the other of their travel. Hence I was keeping track of the number of turns I’d made while attempting to calibrate the circuit, and after around five turns in either direction I assumed I’d reached the end of their useful rotation. This was a bad assumption, obviously; instead I was using only about half of their range.
While that was a silly mistake, I will say that these VCOs seem more difficult to calibrate than the MOTM oscillators I’ve built. It took more attempts and more careful twiddling to get the JH oscillators to track at 1V/octave than I recall expending on any of the five MOTM VCOs I assembled.
It all works fine now, though. I’ll put together that page I mentioned previously but for now here’s a glamor shot of the front panel:
How does it sound, you ask? It sounds like three analog VCOs. Honestly I don’t hear much difference between it and the MOTM VCOs, but then I didn’t expect to. They’re both analog VCOs designed by very talented engineers and built with contemporary components. As such they put out textbook examples of ramp and pulse waves. There is some audible difference in how they respond to pulse-width modulation, but it’s a subtle difference.
I’m happy to finish it and finally bolt it into the cabinet. I think I’ll try to take a break from building stuff for awhile and make some music with it.
