Two Simple Crystal Test by BC548
In the first circuit, above the BC548 is wired as a colpitts oscillator, the frequency tuned by insertion of a crystal. A good crystal will create high frequency oscillations, the output at the collector is rectified by the germanium OA91 diode and a deflection will appear on the meter. Thw more active the crystal, the higher the output deflection which may be adjusted with the preset.
