Octave Cat SRM - Factory Modifications

photo courtesy of G. Cox

This is the first in what will be a series of posts about modifications for the Octave Cat SRM synthesizer.  The series will cover some of the original factory mods as well as some that I've done myself.  I hope to include enough detailed information to enable anyone else to perform the mods on their Cat SRM.  




the factory mods

Here are links to the documents in which Octave Electronics described the mods that they offered:




-  Cover page
-  Mod list
-  Package price list
-  Page 1 ( Mods 1 - 6 )
-  Page 2 ( Mods 7 - 10 )
-  Page 3 ( Mods 11 - 12 )
-  Page 4 ( Mods 13 - 17 )
-  Page 5 ( Mods 18 - 19 )
-  Page 6 ( Mods 20 - 21 )
-  Page 7 (Mods 22 - 23 )











I bought my SRM from a seller on eBay sometime in 2006-2008.  It came to me having had several modifications performed at the factory over 20 years prior.  This is the only modified Cat SRM I have personally seen, and I have seen several SRMs over the years.  It came with these documents shown above, along with calibration instructions and the addendum to the user manual (five pages titled "Instruction Manual Modifiactions").  I will include these other documents in a later post.  Some of the factory mods to my SRM are NOT on the list and appear to have been specifically requested by the original owner.


my modified Cat SRM

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Shown here are several factory mods:

Output Mute, Pitchbend Range and Normal switch, Keyboard/External Control select switch, Glide On/Off switch, LFO DC-Offset switch, Keyboard or LFO to Sample + Hold Trigger In switch.

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Some more factory mods and some of my mods: 

12/24dB slope switch, HP/LP mode switch, Resonance Send/Return Loop, Noise Send/Return Loop and Routing switch

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Another factory mod and more of my own:

VCO1 Audio to VCF Mod1, VCO1 and 2 Send/Return Loops and VCF/VCA routing switches

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I added these Doepfer-style sockets at the edge of the case.  These sockets have "normal" connections, allowing a signal to run through the socket until a cable is plugged in.  The cable interrupts the normal signal and inserts the external signal. The sockets shown here are Audio and CV inputs to the VCF and VCA.  The VCF Out signal can be interrupted via the VCA In socket.  The External In sockets mix with the VCF Out instead.

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 These sockets, switches and pots are part of the factory mods, however they don't all match those listed in the documents above.  I haven't yet determined what function the Gate socket provides.  The LFO, VCO2, Interface, and Pedal inputs are all CV inputs for use with a pedal or other CV source. 

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The Pedal Input is connected to the VCF, VCO1, and Pitch switches and pots, allowing you set a mod depth for those three parameters with the pots and to have on/off control with the switches.