Oh memories, this was probably the first synth mod I ever did so I could use that chorus on other stuff. The easiest way is just to pick an IR3109 and run an input there on pin2. I did a basic opamp inverting buffer with a TL071 and ran it into C5. Worked well, but to use it you need to put the sustain all the way up and turn off the VCO/noise then push a key until audio gets through and weight down the key so the envelope stays engaged. It has six note polyphony and that is only adding an input on one voice, which is why you need to repeatedly hit the key until you hear sound from your external input, you need to cycle the Juno through its 6 notes until you get to the one with the input. You could run the input into all 6 notes and avoid that, or just start up in the Unison/Test mode. You also could lock the VCA open with a switch/pot to put a fixed control voltage on the VCA so you do not need to weight a key down to get sound.
The 106 is quite abit different, completely different VCF/VCA. Instead of using the IR3109 for the VCF and a BA662 for the VCA Roland switched to using a all one one chip sort of deal for the VCF/VCA. The procedure is pretty much the same though, just doing it on the VCF/VCA chip instead. The Schematic for the 106 is floating on the net, its labeled well and you will have no problems finding the VCF/VCA chip on it. The part number escapes me at the moment.
I have been trying to find my Juno schematics for weeks now, starting to think they are gone for good. Thanks for posting these. You do not happen to have the rest of the schematics for this do you? Mainly the chorus section?
adam
