The 3235 is an envelope follower, comparator with a reference level, and a trigger delay which will generate a fixed-width trigger pulse with a variable delay from the rising edge of the input.
This description is from the Series 3000 Modular Synthesizer Instructions Manual.
The two LEDs are mounted back to back and soldered between the wiper pad and ground. There is no series limiting resistor so I cut the trace and added a 1K resistor. Not shown on this photo is pin 13 of the center IC is floating. It is the Reset-0 input to the one-shot and should be tied to Vcc.
This reference diagram shows the values of the parts.
This image shows the comparator generating a pulse wave from a triangle.
This image shows the envelope follower.
The trigger delay took a bit of detective work. It is simply two one-shots, one variable delay from the trigger input and the second a fixed width trigger pulse. The reference diagram clearly shows a 4528 IC and a 22 nF capacitor. Even other photos of the 3235 show these parts, but a 4528B just doesn't function. Even the data sheet says it cannot be used for timing greater than 10 ÁS.
The arrow pointing to the 22 nF capacitor indicates 2 mS which would be a reasonable trigger width. However, 22 nF and 1M will not generate a 2 mS pulse. The schematic shows a 82 nF capacitor which isn't a standard value and that won't generate a 2 mS pulse either.
The description says the delay is variable from 0.25 mS to 250 mS which is a range of 1000:1 but the control is 2.2M with a 4K7 in series which is only a 468:1 ratio.
I used a MC14538B with 1M control to generate a delay of 5 mS to 1 S and a 2n2 capacitor for a 2.2 mS trigger. After some investigation I found that originally different vendor 4528 parts would not function and later the design was changed to a 4538. The original 4528 parts could have been non-B revisions and vendor parts were later standardized when the 4538B was available.
This image shows the minimum delay of 5.27 mS.
This image shows the trigger width of 2.76 mS.
This image shows the maximum delay of 1.02 S.