Bought off ebay for £20.

Only one set of sounds, very 80s sounding. MIDI input, claims output too, but that only seems to be able to send a midi clock signal or dump out the memory to another RX21 so haven't figured out if/how you can use it to trigger midi notes. I suspect you can't as the manual is a bit vague on the details of its 'MIDI output functionality'. Still a nice little machine, lots of fun to record rhythms on.

Seems pretty good - collected links:

http://www.anoderecords.com/machines/rx21lmods.html (also applies to rx21)

http://www.anoderecords.com/machines/rx17mods.html (some applies to rx21 as well)

http://technomf.com/viewtopic.php?p=510&sid=3ce23d7f3b69764fd664eb6f98f55c21

http://www.youtube.com/watch?v=aq3gXNgJww0

http://www.youtube.com/watch?v=afUGPYFhaqI

http://www.youtube.com/watch?v=GisEmYBwTaY

see above URLs for some more info.

IC 115 - YM 2188- IC 114 - YM 2188- four pins connected up to two rails with a switch to choose each; each rail connected via a pot and a switch to switch in two caps or a straight-thru connection. extra switch to enable another capacitor on the right-hand switch. gives a variety of connection possibilities, and you can kind of bleed one bend through to another using the pot. some bends kill some drum channels (eg muting hihats) while others give lots of mad digital distortion that depends on what channels are currently playing.

IC 112 - YM2154 - think I connected pin 39 to 59(?) through an LED + switch. (warning from above urls: “Anything else on this chip can cause permanent damage.”) - hissing distortion effect, works well in conjunction with some other bends

IC 113 - YM 3012 - connected pins 12 and 4 through a switched diode+LED + pot. 'overdrive' effect

big capacitor that i forgot to note - wired up a press switch to short this cap, it kind of crashes the drum machine and gives it a weird warbling effect but the sequencer still plays.

unfortunately the 'overdrive' mod gives the output signal quite a lot of background hiss if the wires are anywhere near any of the other wires in the unit, which is a disappointment as normally this drum machine's silences are REALLY silent. not really sure how to get around this one.

http://www.doctea.co.uk/misc/mp3/Circuit%20bending/circuit%20bent%20yamaha%20rx21.mp3

http://www.doctea.co.uk/misc/mp3/Circuit%20bending/circuit%20bent%20yamaha%20rx21%202.mp3

http://www.doctea.co.uk/misc/mp3/Circuit%20bending/circuit%20bent%20yamaha%20rx21%203.mp3

http://www.doctea.co.uk/misc/mp3/Circuit%20bending/circuit%20bent%20yamaha%20rx21%204.mp3