The Moog Sonic Six came out the same year as the Minimoog Model D and was actually developed as the Sonic V by a company called Musonics which purchased the financially struggling R. A. Moog Company in 1970. After the merger, the Moog team did some tweaks on the synth and released it as the Sonic VI or Sonic Six. It’s different from a “real Moog” in a lot of ways, many of which happen to be ways that make it more difficult to work on (of course), and it also lacks the helpfully detailed service documentation that the Moog engineers typically released. It was designed to be portable, used for educational lectures and demonstrations, and folds up into a suitcase-like case and has a built in speaker. It’s got some weird features, like two LFOs that you have to mix together before you get to apply them to anything, and the ability to switch to a microtonal keyboard scaling on the fly.
It used a lot of the same parts as a Moog which meant it got a lot of the same repair treatment. Cleaning controls, replacing electroyltics and cracking Mullard “tropical fish” film caps, reflowing cold and dull solder joints here and there, replacing trimmers in anticipation of it making the tuning/scaling process easier. The transistor-generated noise wasn’t working because the 10uf capacitor that was meant to remove the DC offset before it passed through the white/pink integrator was failing to do so. It’s unusual to see a low-ish value electrolytic like that fail in a Moog but there you go, it happens. The 3080 OTA in the separate VCA for the output stage was whistling and had to be replaced. It uses TO-99 metal can ICs but they are easy to replace with DIP-8 ones with the help of these little adapter PCBs I got made (available in our shop here).
When I was scaling and tuning it up I accidentally turned a trimmer you’re not supposed to turn, the temperature adjustment (Chip Temp) trimmer for the UA726 self-heating transistor pair IC used in the exponential converter for the oscillators. The tuning/scaling routine for this synth is really weird with each oscillator affected by 5 different trimmers, and my shortcuts for scaling Moog oscillators didn’t quite apply to the way it was responding in the first place, but after I accidentally tweaked this trimmer an unknown amount the scaling went nuts. The SM mentions that this can be used as a coarse scaling control but suggests you probably just shouldn’t touch it and fails to explain how it should be adjusted. I was finding myself with different octaves of the keyboard showing different scales in ways that didn’t seem to make sense (first octave too small, second octave too wide, third octave too small, fourth octave too wide ?!) I centered all the trimmers including the Chip Temp one and started trying to retune it over and over with the Chip Temp trimmer set in random positions while asking around for advice in a Facebook synth tech community and started hearing horror stories about people’s experiences tuning this synth.
Eventually I got caught in this loop where the bottom and top notes were Cs but the middle C was playing a C#, or else it was B, C, B and I would just cycle through the same 6 positions over and over. Once I figured out that I was stuck in this loop, had reached a sort of equilibrium and had therefore essentially eliminated those trimmers as variables, every time I got to middle C as C# I would turn the Chip Temp trim until it was a C and repeat… I did this a bunch of times and gradually the middle C# came down all the way until it was a C. Meanwhile, Darian was reading a thread from the synth-diy.org listserve out loud to me… apparently “synth tech hero” Kevin Lightner had the same question/problem 13 years ago and the conclusion was just that… tuning this thing sucks.
I got it so all octaves of Cs were +/-5 cents of where they should be on both oscillators and then just had to stop. It was then that I began to contemplate the significance of the fact that the service manual does not say to repeat the tuning procedure “until octaves are in tune” the way most do… it just says to repeat it until “no further improvement is attainable.” That seems like they’re admitting that it’s never going to be very good and you are just going to have to give up at some point.
I can get tunnel vision and obsess about the one part of a repair or restoration job that didn’t go as well as I’d hoped. I have to make myself think back to what the synth was like when it arrived here and realize how much better it is than it was before. When it arrived, everything about it was finicky, noisy, various functions of it intermittently crapping out. No wonder it didn’t make a good impression.
It’s also interesting to try to make myself see and hear a synth from the point of view of someone whose opinion of it is not colored by any knowledge of shortcomings in its design and construction. That’s when I realize that it actually sounds good. This synth actually sounds like a Moog now!