This circuit makes use of a specific 80's component and was found in an American manual of the time. A matrix keyboard allows you to select one of several music programs.
This circuit is from a 1976 Motorola CMOS manual. It can be implemented with modern Darlingtons like...
This is the basic version of a gain 20 amplifier with the LM386 providing a few hundred...
Found in American documentation from the 1980s, this circuit has a diode feedback. At the exit we...