On Fri, 9 Feb 2018 08:58:42 +0000, Ian Jackson
wrote:

In message , Archibald
Tarquin Blenkinsopp writes
On Fri, 09 Feb 2018 01:00:22 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
wrote:
On Thursday, 8 February 2018 19:03:05 UTC, Chris Green wrote:
"Dave Plowman (News)" wrote:

On a modern car you might be amazed how much is powered up all
the time.
My radio retains its station memory etc even if totally powered down.

Which doesn't actually need any power, flash memory retains its
contents with no power. Older radios used to use powered memory to
retain things but I'm not sure that modern ones do.

Older ones didn't have memories, they had valves. And at least some
proudly declared 'transistor' on the front regardless.

A decent one had a very clever push button system which did just the same
job as a station memory. But must have cost a fortune to make.

They used inductors rather than the capacitor for tuning. I always
assumed it was to prevent microphonic effects due to the vibration on
the thin vanes.

Capacitance could be varied plunger style also, as indeed it was in
405 line TV's.

A capacitor would be more liable to drift or modulation due to side
to side motion, whereas an inductor would have to move longitudinally,
not a possibility with a stiff wire/ rod coupling to the pushbutton.

It's supposition really, but I cannot think of any other logical
reason for using the inductor to tune.


The were hybrid ones which used a transistor power amp and 12v HT for the
RF valves. Getting rid of that irritating vibrator.

Yes I remember some of the write ups a few years after, "if we hadn't
discovered transistors, what wonders would valve technology have
brought to us?"

I believe (though possibly incorrectly) that the advantage of inductor
tuning makes it easier to accommodate the capacitance of the aerial coax
at HF end of MW and LW. It also avoids the inevitable capacitive
'pot-down', and increases the sensitivity at the HF end.

I don't understand. It's a long time since I played with wireless, but
surely the cable capacitance isn't a factor. Whether the final
matching is done by L or C, the coax is just a fixed impedance?

I recollect that there used to be trimmers and padders in the tuned
circuits, but these were to maintain "tracking" between the higher
frequency mixer oscillator and the aerial tank circuit.

I cannot see that this system of tracking would alter even if the
inductor became the new variable.

The sensitivity would be poor if the tracking were out, as the RF tank
circuit would no longer be at maximum output when set to a station 470
kHz away from the mixer.

The term "pot down" means nothing to me, but it is a long while.....

I tried a quick search, but couldn't find anything. Could you
enlighten?

Sadly a lot of the brain cells that accompanied me on my journey
through consumer electronics are now ossified :-(

Regards

AB