The 4740 Envelope Generator has a manual trigger and a fixed and variable output. LEDs indicate the Attack and the Decay/Sustain phases. This kit was $25.95 in the 1976 catalog.
4740 PAiA Envelope Generator Manual
4740 PAiA Envelope Generator Schematic
These are the PCB traces. The added capacitor is to better debounce the manual trigger switch. The panel is floating so I added a ground lug under a mounting screw and wired it to ground.
I upgraded the socket and the LM3900, replaced the electrolytic capacitor and upgraded the timing capacitor to film. With the original CA3401 there was an issue with Attack set to minimum where the voltage would rise to less than 5V and then slowly ramp up. Replacing the CA3401 fixed most of the issue but replacing R13 with a 2K2 corrected the entire issue. The power cable terminates in a 4 pin MTA connector.
The 4740 outputs a clean standard ADSR waveform.
I restored a second unit and had a more significant issue with the voltage rising during the sustain phase. I ended up changing R16 from 3K3 to 4K7 to improve setting the sustain level. At minimum, the sustain level would only decrease to 0.7V. I added a diode in series with R14 which decreased this from 0.7V to 0.2V. I also changed the Attack, Decay, and Release potentiometers to log. The Attack time is less than the Decay and Release, but close enough I left it.