geojunkie ) writes:

For AM reception you must listen to either the upper or lower
sideband, unlike my SX-71. I notice that no matter where the
selectivity is set (2,3 or 5 khz) that I must retune when switching
between sidebands. It is like the spread of the side band oscillator
crystals is a bit wider that the selectivity. I must tune lower up a
bit or upper down. Is this normal? If not, how can I close this gap a
bit?

I don't know about that particular receiver, but keep in mind that
except for one or two ham receivers (and some expensive professional
receivers), the BFO always moves when changing sidebands. There is
only one filter so the BFO has to move from one side of that filter
to the other.
-------
| |
| |
| |
BFO BFO
goes goes
here for here for
lower SB upper SB

The BFO is places so the suppressed carrier lies on the slope of
the filter, and the whole of the sideband fits into the filter with
the unwanted sideband being out of the picture.

The exceptions I mention have two filters, one for upper sideband and the
other for lower sideband, and the BFO is the common point between them:

------- -------
| LSB | | USB |
| | | |
| | | |
|
|
BFO
goes
here.

In this case, it works the same, but since there are two filters
the BFO stays on the same frequency while the filters are switched.
Not too common, since it's costly to have that extra filter.

Anyway, when you shift the BFO in the first case, you need to
retune to get the "suppressed carrier" of the incoming signal to
that spot. I'm too lazy at the moment to come up with some examples,
but think about the incoming signal. You get a certain beat note
on that signal. But if you shift the BFO, it and the incoming signal
can no longer cause the same beat note. You are retuning so the incoming
signal is once again on that curve of the filter, on the same side as
the BFO is now at.

That was the way of the early SSB receivers. With time, tricks came
into design play to compensate. One involves an additional conversion,
and I'm too lazy to try to explain it here. Besides, the most common
scheme is simply to add a bit of shift to the local oscillator so
the retuning is done automatically. So with any recent receiver, say
the past thirty years or so (at least decent receivers) one will not
notice the effect you mention. Even with older receivers this gimmick
was in place, but I'm not sure at what point one could say it was common.

Michael VE2BVW