a long long amount of fruit ago, Wietermans Dockxmans presented me with a Pasco Scientific 9301A dual function generator. which has banana jacks. banana, get it?
anyway. the thing has inputs to modulate the frequency of the generators, but they work backwards. so higher CV input = lower frequency. which feels quite unnatural if you're driving it with other gear.
some previous owner added CV in jacks (wired straight to the frequency pots), but they had the same problem. i guess it's just how that Intersil 8038 IC works.
original schematic:
the internal modulation (i.e. modulating one generator with the other) can just stay as is.
and, of course, those huge dials need to function in the same direction as ever.
so i ended up with the following:
- run the original CV (from the dial) through an inverter, so that it's flipped the wrong way round.
- add the external CV and any needed offset to it.
- run all this through a second inverter, so that the original is back where it needs to be, and the external CV is flipped (which was the original objective.)
crudely drawn schematic:
for clarity's sake: i pried loose the connection between the freq pots and the pcb. this sits in between there now. i do realise it is rather crude, in that the original frequency scale of the dials is no longer valid. i had to scale the whole thing - both CV and original pot voltage - down to a useable range.
i probably could have more or less avoided that, but i'm not sure i could get it done with just a single 072. and anyway, since the amount of possible pitch shift is dependent on the chosen setting of the oscillator (one of 5 ranges), and it never even comes close to V/oct, this is basically a fancy Spacey noisemaker so i quit caring about accuracy. mainly i opted not to introduce unwanted distortion at the extremes of CV input.
in the end i added a master volume as well (bottom right in top picture.)
looking at the schematic, there's probably a lot more options for mods, external VCA inputs etc., but it's pretty versatile as is so i'm not sure if it is worth the extra trouble.
