CGS Serge 1973 envelope generator

CGS76 the CGS Serge 1973 envelope generator is an adaptation of the original Serge Envelope Generator from the early "R" series PCBs. It has been produced under license from Serge. This is NOT an ADSR style generator, and can be used as a voltage controlled LFO.

PCB assembly notes

 * All transistors on the board have pin-outs that correspond to the original epoxy type transistors. This means they have the emitter in the direction shown on the 2N4248 with the symbol in its outline. PN4248 are reversed - i.e. EBC from the flat face, rather than CBE as used on the PCB. A such, you will need to install any affected transistor substitutions in backwards with reference to the PCB. You will need to refer to the data for the transistors you are using.
 * There is provision of the PCB for adding a LED to monitor both the Envelope Output and the Window Pulse Output. The connections are labeled LA (LED Anode) and LC (LED Cathode). They are positioned near the corresponding output pad.
 * Pot connections are marked with arrows indicating the appropriate end of the pot's travel and the wiper (CCW arrow, CW arrow and triangle).

Setup

 * DTRIM is used to set the initial cycle speed. Turn RISE and FALL to the fasted positions. Adjust DTRIM until you get 4.00 volts on pin 4 of the LM3900 (NOT the one marked MC3401 on the PCB). I did this with END patched to CYCLE.
 * The second trim pot is to stabilize the HOLD function. Too high, the signal drifts, too low and the signal "pulses". To adjust this, plug the ENV out into a VCO, then trigger hold during an envelope. Adjust the trimpot using the VCO tone as a guide.

Comments on usage

 * The HOLD function only works when the Window Pulse Output LED is lit.
 * The rise and fall pots are wired to control rate, not length. As such they seem backwards for envelope generator functions, but correct for LFO functions.
 * To use it as a VCLFO, patch end to cycle, and feed a CV into duration.

Parts list
This is a guide only. Parts needed will vary with individual constructor's needs.

CC-BY-NC
Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work. Serge Tcherepnin retains all rights to his work.

Suppliers

 * CGS76 1973 Envelope Generator, revision 1.2, Elby Designs
 * Small Bear Electronics, for Davies 1900H style knobs