I just finished this mod on the Roland CR-68 drum machine. I bought it several years ago, and always liked the sound of it, but never really used it, since it only features preprogrammed rythms.
But now it has external trigger in for nine of it's eleven analog voices.
The mod is based on Andrew Kilpatrick's cool MIDI-solution for the machine, and he has been very helpful with this much simpler mod.
The trigger signal for each voice is simply a 5 v pulse of 15-20ms. Andrew Kilpatrick suggests connecting each trigger through a resistor of 47k. So far I haven't done that, since I need to test the implementation with different setups. I tested it with an Arduino board (which outputs 5v on it's digital pins), but I also need to test if I can trig the sounds with my MOTU audio interface (with DC-coupled outputs for CV control) and my PAIA MIDI-to-CV converter.
Anyway, here's pics! Here's the row of the nine extra trigger inputs, underneath the five existing in/outputs.
From the left:
(there are two more voices, but for now I couldn't fit the trigger input plugs properly. It's Cow Bell and Hi Bongo)