Practice code oscillator (CB21090E)

Found in Popular Electronics from the 90's, this circuit consists of an audio oscillator for Morse code practice.

Digital Volume Control with PIC (CB17627E)
This circuit is an excellent application for the PIC in audio, but the reader who intends to do...
10 W amplifier (CB9905E)
To a 16-0hm load, this amplifier provides a power of 10 W. We found this circuit in a Motorola...
TTL Output Magnetic Reader (CB14249E)
This reader with TTL logic compatible output uses an LM139 comparator. The circuit was found in...