This is a very simple mod to make your Casio SK-1 even more functional. This add on allows you to transmit and receive audio from your phone or device to your SK-1. In this setup the board assembly I used fit in easily and came with a battery source. You will need to make a small cutout on the back and remove 2 back panel plastic mounts. I soldered wires to the momentary switch for on/off/pair control and mounted it to the side. Alternatively you can solder wires to the in & out of the Bluetooth module and attach them directly to the terminals of the SK-1 in/out jacks thus eliminating the hole in the back.