This circuit generates the busy tone of ordinary telephones. The transistors can be the BC548 and the components of the T filter and the multivibrator can be changed to obtain the exact tone.
This circuit was found in an English magazine from 1978, but it is still current, just change the...
This is the circuit of a small commercial amplifier from the 1970s from Philips Argentina. The...
This circuit is from a 1965 General Electric hobby manual. The circuit can be assembled with...