Octave-Plateau CAT SRM II Monophonic Synthesizer Overview Video

Published on Sep 10, 2013 FarewellOldFriends·21 videos

"Old Friend No. 13 has left home.

A dual-oscillator, monophonic, analogue synthesizer.

This unit is getting old, and its internal connections could do with a bit of cleaning, but it still works. There is a bit more audio leakage from the oscillators when all levels are set to zero than I would like, but in general, it performs as it did when I bought it second-hand in the late '80s.

Please note that although I put on a caption stating that CAT schematics were widely available on the Internet, in practice these are only for the CAT SRM; there seems to be no source for CAT SRM II schematics.

As can be seen from the internal shots, the circuit boards are as originally built, all at Revision B. They have never been removed: the locking varnish on all the board retaining screws is still intact. A couple of bridging wires and associated track modifications on Board B are, I assume, a mod implemented during production.

Not all of the multi-pin inter-board jumper connectors are of the locking variety, and in particular the one on Board B, which slides onto stake pins vertically, has a habit of working a little loose, leaving the oscillators running continuously. I could have cured that with some hot-melt glue but do not want to disturb the original internal condition of the unit.

Mercifully, I have never had to repair the unit demonstrated here. However, any electronic engineer worth his or her salt faced with such a task should be able to trace the circuits where necessary to determine the differences between this and the original CAT SRM. I get the impression that the VCO and VCF designs in the SRM II use purpose-designed CEM chips, rather than the discrete circuits of earlier models.

Additional note: Just in case someone notices that I've missed out a large chunk of this unit's potential sound palette, I should point out that VCO1 has an enormous frequency range, so if its audio output is switched off (see later in the video) and its coarse frequency control is brought right down, it can act as an alternative or additional LFO with a wide range of waveforms that can be brought to bear on VCO2 or the filter.

(Text updated 27 Sept 2013)"