Here's a brief run through of how I modified my Yamaha RX-120 Rhythm Machine. By using a patchbay connected to the sound ROM, there are a lot of effects that you can achieve, including synth type tones, flanging, ring mods, bit crushing, and others. Using more multiple patch cords lets you stack these effects. The preset only RX-120 shares the same sound set (and chip) as the programmable RX-17. By using MIDI to trigger the RX-120, you have a ton of flexibility for programming.
Additionally, by probing on the output opamps, you can come up with some crazy feedback filters too!