We found this circuit in a 1976 publication. The transistors can be BC547 or 2N2222. Crystals up to a few tens of Megahertz can be tested. The LED can be of any type. The capacitors must all be ceramic.
This plug-in control was found in General Electric documentation from the 1970s. The SCR can be...
The low-pass filter shown in the figure uses two operational amplifiers with JFET at the input to...
This circuit with transistors of the time was found in the magazine Radio Electroinics of June of...