500kHz source update

Added a polystyrene varicap for tuning. Now, setting the range, some experimentation was unavoidable. A 39pF capacitor in series to the ceramic resonators "ground pin" (capacitive divider) to ground resulted in a 500.5kHz to 503kHz tuning range. Maybe replacing the 39pF by a somewhat smaller capacitance will bring the source up by just those couple of 100Hz.