Michael Black wrote:

I thought it was the norm for car radios to be permeability tuned, right
up to the time synthesizers came along. No manually tuned car radio
that I've looked at had a variable capacitor, with the exception of
a relatively recent one that had a frequency counter for the readout.

Car radios would be the best source of the mechanics, at least for
front end tuning and not oscillator use. Of course, the issue isn't
changing the permeability of a coil, that's easy with a tuning slug,
but making it easy to do that from a front panel control, ie the knob
doesn't move in and out. The pre-synthesized car radios would proved
the mechanism. But you did see the occasional article in the old
days about making up such a mechanism, though those tended to be
for oscillators where there was more fuss because they wanted/hoped
for linear tuning.

Michael VE2BVW


Some of the cheap imported car radios used the same plastic cased
variable capacitor with mylar film between the plates that were used in
a six transistor pocket radio. They didn't have a tuned front end and
amp like the US made car radios. They were ok if you only wanted to
listen to a local station but they were very poor quality radios.

I think I would use a variable capacitor and electronically switch
the inductor for different ranges, like the broadband L-C Based VCO in
the telemetry receivers I worked on. Diodes were used to short segments
of the inductor, at the grounded end and the band segment was controlled
by simple logic. Bias on hard to short a tap to ground, and reverse
bias it to prevent it from clipping the RF when its off. They used
discrete +/- 12 or 15 volt switching , but a decent buffer amp would
work and could be driven by CMOS or TTL compatible logic. If the
receiver has any switched DC signals to show which band its on, you
could make it change bands without touching it. You could even sample
the L.O. with a counter circuit and use the output to control the
tuning. If you want to g to extremes, you could add a microprocessor
with lookup tables to drive a stepper motor that can adjust the variable
capacitor(s) for you as you tune the radio.

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Michael A. Terrell
Central Florida