Sequential Circuits Pro-One – CPU replacement + MIDI

STATUS: Done! (Nov 14, 2015).

I bought the MTG Turbo CPU replacement, the extra MIDI board, and the CV option (to control filter and stuff via midi).

Check out Music Technologies Group Turbo CPU here!
  The Turbo CPU board replaces the original Pro-One CPU and fits right in the old CPU socket. The main reason for me to get it was that there was a MIDI option for it as well. There are other benefits such as faster performance (although this may border on unnoticable) and also completely new features such as an extra LFO (which I probably won’t use).

The new CPU board

The Turbo CPU attaches to the separate MIDI board with 6 wires. 4 for MIDI, 2 for the extra CV stuff that will control pitch bend, cutoff etc over MIDI.

Step 1 – soldering the 6 wires to the CPU board
CPU module with wires

Step 2 – removing the old CPU
Removing original CPU

Step 3 – drilling holes in PCB for cable ties
Drilling holes

Step 4 – mounting new CPU board and cable ties
CPU board and cables mounted

The MIDI/CV board
The MIDI and CV board

I added molex connectors to the board and the 6 wires so that the Pro-One lid (which has the PCB with the CPU) can be detached from the bottom (which has the MIDI board and connectors)

Step 1 – Molex Connectors on MIDI board
Molex connectors on PCB

Step 2 – Molex connectors on wires from main PCB
Molex on wires 1 Molex on wires 2

Step 3 – MIDI board wiring (MIDI connectors and LEDs)
MIDI board wires

Step 4 – Mounting MIDI connectors

Holes for MIDI connectors
Drilling with increasingly bigger drills, then filing the last bit, phew…

Step 5 – Mounting MIDI board and connecting it all
MIDI card mounted

Now I did a quick test of both MIDI In and Out – all working!

The CV connections
The remaining 5 pin connector is for the wires with CV signals going from the MIDI board and back to the main PCB (for pitch bend, filter control etc over MIDI).

Step 1 – soldering 2 wires to control Filter Cutoff and Resonance
CV Filter

Step 2 – soldering 2 wires to control Osc 1&2
CV Oscs

Step 3 – soldering molex connector in the other end
CV Molex

Another test. Now I could control pitch bend over MIDI! The range defaults to 2 semitones, nicer than the Pro-One’s “about a fifth” and much more playable (Looking at you, non-spring loaded pitch bend wheel). I also had control over filter resonance and cutoff (connected to mod wheel and using the extra LFO of the new CPU.. not sure how I like this, but it’s all configurable).

Finally I pushed the MIDI LEDs into the holes drilled in the chassis.

MIDI connectors

They are ‘modern’ bright blue LEDs – not a big fan.. Maybe I’ll replace them with something more retro like green or red in the future.

Here is an overview pic of the internals (the bigger brownish board is from my keybed replacement mod):
Overview final result

There – all done!

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