On Sat, 24 Aug 2013, gareth wrote:

I wonder if some more experienced and senior (in the literal sense) amateur
could explain to me one difficulty that I have in understanding single
signal
reception with the crystal phasing control?

AIUI, the phasing control is adjusted so that the frequency that would give
the audio image is phased out by being in the notch.

When I had an SP-600, I don't remember using the phasing control much.
I'd leave it somewhere, I'd set the BFO and all was generally fine.

When that type of filter came along, it didn't replace anything, it added
something (so there was a wave of commercial receivers where you could get
two models, one without the filter, the other with the filter). The
single signal was because it suddenly did get rid of the audio image.
Tghe phasing control doesn't really change that, it allows some level of
control and you also get a notch at the same time.

DOn't forget the crystal gave really quite high selectivity, the phasing
was simply to balance out the capacitance of the crystal holder. Since
this added some control, you'd see the phasing control on the front panel.
Most of those filters had a selectivity switch, which would vary the
amount of loading of the output of the crystal filter, which allowed for
varying selectivity (though since it was only one crystal, the skirt
selectivity wasn't that great). The wider the filter was, the less effect
the phasing control would have on it. Which is likely why I never saw
much use in the phasing control on that SP-600, I'd generally keep it at
3KHz

MIchael VE2BVW

If that is true, how is it then possible to adjust the BFO to a
pleasant-for-you tone,
because such adjustment will alter the CIO frequency?

Surely the BFO has to be set in advance to be halfway between the serial
resonant
frequency and the notchable parallel resonant frequency?

This is a query stimulated by my current project which is to build a
boatanchor style
RX, with a flywheel-loaded slide rule dial like that of the Eddystone EA12