Practice code oscillator (CB21090E)

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

Optical tachometer (CB9349E)
We found this circuit in a 1970 publication by General Electric. The components can be changed...
Symmetrical 15 V Power Supply (CB19622E)
This source is used to supply circuits with operational amplifiers, being originally created for...
Crystal divider by 10 (CB6831E)
Suggested by American documentation from 1968, this circuit divides the frequency of synchronized...