So, the chip is this: https://eandc.ru/pdf/mikroskhema/k118ud1.pdf (middle column).
It is apparently very hard to source in the West (it's for my Russian Polivoks). Russian websites sell it but can't send them over here for obvious reasons. I don't need it right now, I think, the VCA seems to work fine. Anyway, it's basically a 14-pin differential amplifier. The B (which is a V, actually) version is somewhat easier to source, and it looks like it should work as-is (a couple of values are slightly different but it appears that the V version is like a "better" version of the B chip). Probably it could just be cloned with discrete components after all and replaced with an adapter.
Here is another website with some curves too.
https://rudatasheet.ru/microchips/k118ud1/
If they are worth crazy money then building a discrete version isn't a bad idea.
Your second link quotes an MC1325 Motorola "equivalent" but it's not a pin for pin equivalent, just a loose functional equivalent. The MC13xx part numbers are from the late 60's - so an equally obscure part.
Using it as a gain control device is going to be limited by the 4.7k collector resistors, unless you override them by feeding the outputs into a low impedance.
This is where it's used (the VCA):
(https://i.postimg.cc/0Kq4H1D6/immagine.png) (https://postimg.cc/0Kq4H1D6)
A2 and A4 should be "regular" opamps.
The Polivoks isn't renowned to be a particularly "hi-fi" synth :D
By the way, this is what it sounds like, I'm a crap synth player and I just noodled around for a while:
https://www.youtube.com/watch?v=jS-y2OTl8a8&t=1128s
In the last patch I'm deliberately clipping the output (the LFO makes it possible to do so) and it actually sounds cool.
Quote from: fryingpan on November 12, 2024, 01:04:54 PMhttps://www.youtube.com/watch?v=jS-y2OTl8a8&t=1128s
In the last patch I'm deliberately clipping the output (the LFO makes it possible to do so) and it actually sounds cool.
It's such a distinctive synth. That Polivoks filter is only 2-pole so the thing has a brightness that it never really loses - it always seems to have "edges". And the signal path seems to overload gently the whole way through unless you specifically set out to avoid that!
It's like the opposite end of the scale from Roland!
Maybe an MC1496 with one side shut off at the lower transistor pair would do it. Or maybe a real MC1496 is closer to the intended function anyway. You just have to design the stage around it.
Erica Synths use a regular TL072 in their Polivoks VCA. But they also keep some of the original ICs for other modules. I had a look at their website and K118UD1 isn't mentioned, but you could always drop them a line. Girts may have some secret stash of rare parts.