Practice code oscillator (CB21090E)

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

Sample and Hold LM3900 (CB14218E)
The circuit shown is suggested by National Semiconductor's Linear Applications Handbook. The base...
Telephone amplifier (CB8630E)
This circuit uses a pickup coil to take the signal from a telephone line without contacts. The circuit...
Opto and Strobe Interface (CB16037E)
In this circuit, the transistors are NPN for general use and the phototransistors can be replaced by...