External sample-input interface for switching between microphone and line signal.

The trimpot of 10 K is optional.

The matrix of all buttons and keys :

So any X-Y contact can be made with an external switch plus the proper diode. To keep the A1 key pressed down for example. Or trigger autoplay.

Replacement of the clock signal for the processor, plus indications of the lines to cut in the circuit :

The actual added VCO clock-circuit, running on local Casio-power.

The practical situation on the PCB.

Green : C15 removed - Blue : pcb tracks to be cut - Red dot : Clock input - Yellow : power points

External trigger circuit for the Casio SK-1 Twin set :

The TR bypass leads to the option to connect one of the trigger-signals with KB 2 of the twins also. The IC MCT 1 is a standard optocoupler. Of course other trigger-presets can be chosen, by using other KO an KI lines.