Saturday, April 6, 2013

Octave Cat SRM - Fixed Schematics and Layout

Original documents vs actual PCBs...

original schematic
original component  layout

The Octave Cat SRM documents you can find on the web are all the same, at least all the ones I have ever seen.  They probably all came from one source, likely Kevin Lightner's Synthfool siteThe problem though is that some of these documents (the schematics and the component layout) don't actually match the circuit boards in the SRM synthesizers that I have seen.  Nor does the schematics image match the layout image. The units I have looked at all have traces which have been cut, jumper wires intstalled, missing components, and/or extra components on the back side (solder side) of the PCB.  Not only do these models I've seen not match the documents, but they don't match one another either! 

Before I post any instructions or information on the Factory Mods that were offered by Octave Electronics, Inc., I thought it best to attempt to correct the original documents to match the actual circuit boards we have in our synthesizers.  Perhaps the corrections I show on these documents will match your own unit and save some confusion when performing the modifications, general troubleshooting/repair, or tuning.  I chose not to relabel every component on the schematic, but I got the ones that were hardest to read. 

Note:  All Cat SRMs I have looked at have some different resistor values (and in some cases, trimmer values) from the schematic and from one another.  Don't worry if your values are again different than those shown here.  

Below you will find two versions of the SRM "Board B", which is the VCF/VCA section of the synthesizer.   I haven't done this work for Board A yet.   Thank you to Tony from OakleySound for confirming that the two CV Scale circuits I drew are both viable circuit options (see pages 2 of SN:998 and SN:3866 Schematics, below).

My own SRM's Board B:

My SRM is Serial Number 998.  It has some cut traces around the Volume control and jumpers to route the control to other spots on the board.  Neither the way the board was made nor the routing change matched the documents.  Also, the CV Scale part of the VCF control circuit is different from the original documents.  The component side of SN:998 is fully populated, with the exception of there being no Temp Co resistor (R193) and that C77 (not shown on the original schematics, but does have a place on the PCB) is mounted on the solder side of the board.

The SRM SN:998 schematics, page 1:

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The SRM SN:998 schematics, page 2:

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The SRM SN:998 component layout:

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Another Cat SRM's Board B, owned by Gregory Cox. 

Thanks to Gregory Cox for providing some excellent photos of his SRM SN:3866.  This unit has the same cut traces and Volume control jumpers as my board.  However, this model has no C77 installed, and is missing another unlabeled resistor.  The CV Scale circuit is again different from the schematics and different from my own unit, with resistors which are shown in the schematics mounted on the solder side of the board (as they have no place on the component side).  This SN:3866 Board B does have the Temp Co resistor included in the circuit.

The SRM SN:3866 schematics, page 1:

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The SRM SN:3866 schematics, page 2:

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The SRM SN:3866 component layout:

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Do these match your SRM?

If you have an SRM that doesn't match either of the examples shown above, and can take some good photos, please contact me via the comment section below.  Using your photos, I can alter these files to match your Board B and add them to this page and/or email them to you.