This two-transistor circuit is from a December 1964 Radio Electronics. Modern transistors can be tested. The headphone must be high impedance or crystal with a 2k2 resistor in parallel.
This oscillator for telegraphy practice was published in Radio Electronics magazine in March 1958....
Found in documentation two years 80, this toy instrument uses an integrated circuit little...
We found this circuit in a Radio Electronics from July 1968. The circuit generates rectangular pulses...