Crystal variable oscillator (CB9506E)

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

Reset Pulse Generator (CB6615E)
This circuit is from Dutch documentation from 1967. The circuit works with a component of the...
Logic Probe 7811 (CB19217E)
The integrated circuit used in this application is not very common. The circuit is intended for...
LM3900 Voltage Regulator (CB4031E)
The output voltage depends on the zener and the current determines the type of transistor used....