Most of the time you can get them for very small money or even for free. These old organs from the seventies. They are big and bulky and nobody realy likes them. But they are cool because they are of analog nature. So it is very easy to make modifications in the circuits. I always go for the drum machine. Most of the times the drum machine has its own pcb board an can be seperated very easy.

Then I try the resistors and the capacitors. Connect a potentiometer in parallel to a resistor and see if the changing of the value of this resistor has a good influence on the sound. If so you can replace this resistor by a potentiometer. Same way with the capacitors. Try out, if a hihger or smaller value makes a musical sense. On this Bontempi drumbox I now have acces to the volume and the pitch of the five drumvoices.

Intensity and distortion for the kickdrum, tom and claves.

Amount of noise for the snare.

Hihgpassfilter for the hihat.

The decay of the noise amount for the snare and hihat is controlled by different values of capacitors. These are chosen by a rotaryswitch.

I also made triggerinputs for each drumvoice so I can compose my own patterns and are not dependant on the internal patterns. Each triggerinput has a switch to aktivate the trigger. If the internal patterns are running these switches can be used to mute the drumvoice.

At least there is a clockinput to sync the internal patterns to an external clock.