|| PHILCO SERVICE HINTS & TIPS |
|PHILCO SERVICE HINTS & TIPS - NUMBER 33
Aligning the Magnetic Tuning System
In the 1937 and 1938 model years, Philco introduced a line of high-end radio models that had the new Automatic Tuning Dial and the Cone-Centric Dial mechanisms. The models in 1937 were;
Due to minor mechanical inaccuracies in repeatedly tuning to the same dial presets, automatic frequency control [AFC] was employed which Philco called Magnetic Tuning. Here is a brief description of this unique circuit and how to align it re-printed from a John F. Rider publication of 1937.
The Magnetic Tuning System employed in some Philco receivers is a system of automatic frequency control designed to make possible quick and accurate tuning and to compensate for the effects of oscillator drift. It is composed of two principal units; (1) the discriminator, which determines whether the set is mistuned and which provides control voltages whose magnitude and polarity indicate the extent and direction of the mistuning, and (2) the control circuit, which, in response to the control voltages, changes or shifts the oscillator frequency accordingly.
The control circuit does not present any problem from the point of view of alignment since there are no adjustments required. However, it is extremely important that the discriminator transformer be accurately aligned at the intermediate frequency. As you will observe by glancing through the alignment instructions for any one of the receivers equipped with a Magnetic Tuning system, the receiver is first aligned in the usual manner with the Magnetic Tuning switch in the off position. This includes the complete I-F., oscillator, and R-F. adjustments and also the primary winding of the discriminator transformer, since this winding feeds the second detector. We repeat: these adjustments must be made with the Magnetic Tuning switch in the "off" position.
With these adjustments completed, it remains to adjust the secondary trimmer of the discriminator transformer. This is accomplished in the following manner: Couple the signal generator to the antenna of the receiver and set the frequency to 1000 K.C. Tune in the signal very carefully for maximum output. A strong value of input signal must be used. Without disturbing any of the settings, the Magnetic Tuning switch should be turned to the "on" position and the secondary trimmer of the discriminator transformer adjusted for maximum output.
When the Magnetic Tuning system is functioning properly, there will be no hiss or change in tone as the automatic tuning switch is shifted on and off. If there is a change in tone or a hiss, it indicates that a frequency shift has taken place, and the adjustment of the secondary discriminator trimmer should be repeated.
From Aligning Philco Receivers - by John F. Rider, 1937, p.XXXI