Found in a 90's EDN this circuit can be used as a shield. The transistor is a general-purpose type and the frequency of the emitted sound is determined by R1 / C1.
Indicated for scanning circuit applications, we found this generator in a Radio Electronics from the 80s....
This circuit tests crystals up to a few megahertz with ease. The circuit is from a 1977...
Found in documentation from the 70s, this circuit uses an operational one from that time. It goes for...