So there was a Moog Satellite, and a Moogerfooger Freqbox.  These things clearly need to be able to talk to each other!!  =D

So it was decided that pitch CV and gate input would be added to the Moog Satellite to enable it to be MIDI triggered, and also pitch CV and gate outputs to enable it to drive the 'Frequency input' jack on the Freqbox.

I acquired the schematics for the Satellite from a surprising source - the patent office!
www.till.com/articles/moog/patents.html#US04050343
There are lots of Moog patents at that link, most of the patent documents linked have very good descriptions of how each circuit operates.  If you like electronics and music, there is some good nerdy reading there  :P

As it turns out, the Satellite already has CV and gate inputs on it's external accessory socket, so that bit was easy, just a case of connecting the socket to some jacks.  The connections are:

A-23 = external pitch CV in
A-3 = external gate in

As Moogs use 'S-trigger', where the gate line is connected to ground to indicate trigger, you can actually use the A-3 signal as gate in and out at the same time!  Pretty neat.

Also lucky, there is even a spare unused pin on the external accessory socket, perfect for this plan of adding CV out.  Actually when I opened the synth up, the 'Ext Filter' socket was also wired to this spare pin, but since it already has a jack input I disconnected it.

Now I needed to rig up CV out.  Here is the VCO circuit:

At top left are the pitch CV inputs - the Tune control, the 1 OCT and 2 OCT switches, the LFO input (MOD) and the external CV in from the accessory socket as well as the actual keyboard voltage.  These are all added through the top opamp, after which we can see the 'SCALE' trimmer and then the signal goes into the VCO.  So this is the point we need to tap the voltage from.  I found the easiest place to solder to was the leg of C18.

Unfortunately, this isn't anything resembling the normal Moog 1V/oct CV!  Although it is still the same exponential curve, it is inverted (more negative voltages play higher) and much smaller.  If you look at the opamp adder, you can see that it's an inverter with a gain of considerably less than one.  In fact the typical voltages at this point are from -40mV to +40mV! 

So, after tapping the voltage from this point, I built a simple opamp inverter with gain, with a trimmer to adjust the scale.  After some careful trimming we now have 1V/oct coming out of the VCO. 

Usefully the Satellite has +/-9V power on the Accessory jack, so I built the opamp circuit right into the breakout box.

Here's the finished article - a breakout box for the Moog Satellite, that gives it CV and Gate ins and outs.  There is still loads of room in the box for a MIDI interface, which will probably get added soon-ish.

This is a really cool mod; now instead of a one-oscillator synth and an effects box, we have a two-oscillator synth with FM and oscillator sync!!  It sounds *mean*.