Sequential Circuits Pro One – CPU upgrade and MIDI retrofit

sequential circuits pro one synthesizer
Sequential Circuits Pro One

When we got this Pro One it was acting insane: keyboard playing only one note and/or pitch sliding upwards, loud bursts of noise, weird envelope/triggering behaviors. This synth is unusual for its vintage (early 1980s) in that much of the control voltage for the analog circuitry is generated by a digital CPU, which was hot enough that it burned Darian’s finger when he happened to touch it, badly enough that he still has a blister a week later! So clearly the CPU was bad– and further testing confirmed it.

The best and possibly only option at this point was to replace the entire CPU with the Turbo CPU upgrade from Music Technologies Group, and I took this opportunity to retrofit it for MIDI control with the MIDI version. So far I very much recommend this upgrade. It was reasonably priced and reasonably easy to install, and after ordering the upgrade kit it arrived in only three days.

Work Done: Replaced broken keyboard ribbon cable, power supply capacitors and failed regulator, identified failed CPU as main source of problems. Replaced with MTG Turbo CPU + MIDI (wire up CPU daughterboard and MIDI board, drill/cut holes and wire/install MIDI jacks). Cleaned keyboard contacts, pots and switches. Tuned and scaled oscillators.

Pictures on Sequential Circuits Pro One circuit boardDave Smith liked to put little messages and pictures on the PCBs (circuit boards) of Sequential synths. Here’s what I found inside this one.

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