Crystal variable oscillator (CB9506E)

Found in Radio Electronics from the 90s, this circuit can vary the frequency in small values around the crystal frequency.

Triangular Oscillator (CB15684E)
The integrated circuit 4136 consists of operational amplifiers of type 741 in the same enclosure and...
100 Hz High Pass Filter (CB14132E)
In the figure we have an active high pass filter of 100 Hz, suggested by National Semiconductor in its 1994...
Zener Coupling (CB4139E)
According to the consulted American publication of the 1980s, a zener diode can be used for DC...