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 with germanium transistors was found in documentation from the 50's. The transistors are...
The circuit presented was found in a documentation from the 1950s, generating a sinusoidal signal...
This circuit was originally indicated as a signal reinforcement step for the previous receiver...