Here is a link that Roger sent me a while ago. Its a website by Frans Samshuijzen with detailed descriptions of some mods that he did to a set of Casio SK-1 keyboards appropriately titled: “Casio Twins“. The mods include a full pitch mod with the use of a TI 74LS624 voltage controlled oscillator. It is also worth noting that a 555 timer circuit would theoretically do the same thing, except that the max frequency of a 555 is about 2Mhz and with the LS624 chip the schematics indicate a range of between 1.5 and 10Mhz with a max of 20Mhz. The downside however is the fact that the LS624 is far more expensive, $2.55 a pop. Still worth every penny considering that the Casio will provide for a full 3 Octave mind melting sound experience, along with longer and grainier recording times. Other mods include a nice line level adapter schematic for the microphone input and a remote trigger option. Audio of the improvisations on the Casio Twin set can be heard here and there is more information about what all the controls are here.