Sequential Circuits 
(The red power LED on the upper right above the Master Tune knob is a modification)
Finally I own this well known monophonic 1980 synth. Although a stick tell it is out off warranty since the 4th of October 1981 it playing fine thanks the faithful repairs and calibrations done by H.S.
Inside you find some well known analog Curtis chips:
- 3x CEM 3340 used for the 2 VCOs and the LFO
- 1x CEM 3320 as 4-pole lowpass VCF
- 2x CEM 3310 as exponential envelope generators
- 1x CA3280 transconductance op-amp as VCA
And the following digital chips are used:
- 8021 8-bit microcontroller, 30 pins, 24 ports (18 actually used), 1k ROM, massive ;-) 64 bytes of RAM
- Analog Devices AD558 8-bit DAC (only the 6 most significant bits (DB2 to DB7 pins) are used to cover a 5 octave span in 1/12th of 1 V per step. Much like on the EEH DS 500 digital sequencer)
The Pro-One features these connections on the back:
- Audio out
- Audio in
- Filter CV in
- Gate out
- CV out
- Gate / ext. Clock in
- CV in
The powercord is not detachable :-(
This is the same bad decision as on the miniMoog. Putting a synth with the non-detachable power cord into a flightcase is not really cool. You have to take care, that the jack is not scrachting over the synth while flight case is moved a bit unfriendly.
I installed the Pro-One Keyboard Replacement Kit to get a silent and more reliable keyboard action. Easy to do kit that just needs screwdrivers to install.