I got one of these lovely units on eBay for next to nothing, as it was listed in need of repair. Turns out it needed a complicated procedure involving opening up the battery compartment and filling it with double A's.
Feeling ahead of the game, I spent an afternoon poking around the circuit board.
Folks, this is one you can safely skip.
The board itself is in two layers, the most accessable of which just has caps, some resistors, a handful of jumpers, and one not so useful chip. The second board, where, I would assume, the main chip (which looks to be surface mount) is located, is firmly fastened down to the keyboard mechanism, and looking down at it you see not only the usual tops of screws, but also the bottoms of some other screws. I took all of the screws I could get to off, but was perplexed by those upside down ones, and thusly couldn't get at the second board.
Focusing on the top board I found about a dozen points that minimally affect the chord and rhythm section, either increasing or decreasing snare resonance, or muting chords or bass. I thought about hooking these up to a rotary switch, as all the bends share a common point, but there is no room inside the case for any extra hardware.
Unbent, this is a lovely keyboard. The memory function is more evolved than Casio usually builds, and the analogue percussion sounds are very very nice.