John,
Your perception is on track. ?Considering a passive LCR equivalent circuit model of a Quartz crystal mounted in a holder with wire connection leads, the third overtone, the next highest possible resonance after the fundamental, has a lower impedance, purely resistive at resonance, and much, much lower than the nearby parallel resonance, w.r.t. frequency. ?The series resonance occurs first, then as frequency increases slightly beyond series resonance, the crystal becomes inductive with a very high impedance at parallel resonance (again purely resistive), occurring at the fundamental and each valid overtone. ?With increasing overtones, the series resonant impedance (resistance) drops somewhat linearly. ?With the voltage across the crystal being proportional to the crystal impedance, the third overtone will (normally) result in a lower voltage.
Apparently when this radio was designed, these ¡°higher¡± crystal frequencies were generally only available for third overtone. ?Hence, the Colpitts oscillator, using parallel inductance, was used for greater output. ?The third overtone series resonance crystal creates a low impedance in the cathode circuit, only at third overtone resonance resonance to lower negative feedback and increase the gain. ?With the tuned tank circuit in the feedback path, the Colpitts oscillator configuration is realized. ?The 2-B manual couldn¡¯t accurately call this a Colpitts oscillator, because of the use of the low impedance path created by the crystal in the cathode circuit, and apparently didn¡¯t what to take the manual-space to explain it thoroughly (alright with me). ?Even though fundamental frequency crystals are available at these higher frequencies these days, you what to use what the circuit was designed for, the third overtone series resonance (using AT-cut Quartz). ?I will be interested to know the difference in price between an HC-48/U holder and an HC-49/U holder, with 10 ppm crystal frequency accuracy, depending on how WD8DAS quotes your crystal.
Let¡¯s see what the L10 adjustment will do, after checking the circuit wiring. ?Make sure someone before you did not modify the circuit.
73,
Michael Smith, N4KZO
