• Barry

    Hello… Have been building the Minisonic 1 then 2, for nearly 50 years. Pick it up every so often but usually get stuck on the VCO’s as you say they drift terriblely. In lock down 2021 picked it up again and built the VCO’s a breadboard a played around with the values, improved but still not brilliant. It is my memesis to get it working as per the design and not to mess with it to much, however I’m not a electronics expert and yet again have had to put it away as we are moving. In the mean time I built a Spectrum synth which works well, but alas i still can not play it. Great to see that people are still working on these pieces of history.

    February 10, 2022
    • David

      I replaced R63, the resistor between the base and emitter of TR1b, with a 20-turn 1k preset. This enabled me to gradually increase its resistance until the VCO passed the ‘warm finger’ test.
      I initially set the preset half way. The set the VCO to 800z and allowed the synth an hour to fully warm up. Turning the preset clockwise increased the voltage across TR1b’s b-e junction and reduces the VCO frequency, which should then be reset to around 800Hz. The VCO frequency will initially rise when doing the finger test, showing that there is insufficient feedback. However, once TR1b starts conducting, the rise will become less and eventually will stop. or at least become very small. Fine adjustments can then be made using VR11.
      I found my VCOs required a Vbe of 6.15V and the other 6.30V. No idea why they were different.

      August 27, 2022
  • David

    Look forward to reading Part 2 once written. My brother and I built a Minisonic 2 in our youth. Still have it somewhere in the loft!

    We used MD8001 dual transistors for the VCOs. I think superior matched transistors are now available, which would give less drift.

    I’m interested to know how you did the artwork for the panel, and how/where you get it made.

    March 30, 2022

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