This QST circuit generates 4 frequency signals according to the crystals used. Transistors admit equivalents although the indicated types can still be found with some ease.
This midrange filter for sound systems was found in the May 1988 issue of Radio Electronics...
The figure shows how to couple two TTL circuits using an optical coupler to obtain isolation. The...
This circuit can handle 25 A unde a 32 V voltage using a hight power old transistor. This...