Nick,
I'm not familar with the set you're restoring, but expect many divergent views on my desertation. Reactance or slug tuning is accomplished in one of several ways. Movable powdered iron, ferrite or brass cores(slugs). All of which have one or more drawbacks. Powdered iron slugs are used infrequently in the VHF (FM) band because of the loss of Q (quality factor) of the inductor, although increasing the apparent inductance. I've noted many in older AM car radios though. Ferrite of various mixtures (ferrites and binders) is more common at these frequencies, and in addition can actually raise the Q factor as well as inductance. Slug material mixture is selected based on required Q factor of the design. Wire size and turn spacing also affect the Q factor. Ferrites of various mixtures, shapes and dimensions are actually used as inductors in numerous microwave (not ovens) applications. Brass on the other hand decreases the inductance, being drawn into the core, as if turns were removed, but also reduces the Q factor. My preference is fixed air core inductors with air or low loss dielectric variable tuning capacitors for the best compromise of Q factor of the capacitor as well as the inductor. Fine tuning of the tank circuit is accomplished by compressing or spreading the inductor turns. The designs of today use variable capacitance diodes, no bigger than #2 pencil lead to replace the behemoth ganged tuning components of yesteryear. The design engineer's ultimate goal is to provide a tuning circuit of the the optimum bandwidth (Q factor) for its application, circuit impedance and tuning coverage within the band of interest. Once manufacturing and the bean counters become involved, the engineer's goal becomes secondary, with plan "B" or "C" being the outcome. It was a long time ago, so any temporary "senior moments" should be excused.
marv
:I am working on a Stromberg Carlson SR405 Hi-Fi AM/FM receiver which has an interesting tuning arrangement. The AM uses a standard variable tuning capacitor. This is connected to a gear and rod that move slugs in a transformer coil for the FM tuning.
:The tuning works well for both AM and FM. I believe this is called 'reactance tuning'?
:As I am not an engineer, I would be interested in the pros and cons of 'reactance tuning' Stromberg Carlson seems to be one of the few companies to use this although as I recall it used to be common in auto radios. Would appreciate any comments. Nick
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