"gareth" wrote in message
...
I wonder if anyone can offer a picture of the variable
selectivity arrangement of
the Eddystone EA12, which I believe was also used by some
other manufacturers?

This was the scheme whereby coupling between the primary
and secondary of IF
transformers was varied nechanically by distancing the two
halves of the transformer
from each other.

I have access to photos showing the operating levers, but
what interests me for
the moment is to have a peek, or peeks, inside the IF cans
themselves.


This is the same arrangement used by Hammarlund in the
Super-Pro series. Its been too long since I had a Super-Pro
IF can open to remember exactly how the movable coils were
fastened and guided but the variation was done by a post
coming out the bottom of the can and operated by a cam on a
shaft turned by the front panel "selectivity" knob. Very
simple mechanism. This type of variable selectivity is the
only one with completely symmetrical expansion of the
passband. Those using tapped inductances coupled to the IF
coils, such as used in the SP-600-JX and RCA AR-88, are
nearly as good. Capacitance coupling usually results in
some asymmetry as seen in some Hallicrafters receivers
although the variable pass band in the SX-28 works quite
well.
Later in this thread someone asks about variable
crystal filters. There are essentially two forms: the
original Lamb filter and the later one patented by
Hammarlund in 1938 and first used in the HQ-120-X. The
Hammarlund filter has the advantage that the center
frequency does not vary with the bandwidth adjustment or
when the phasing null is moved around. The Hammarlund
circuit was used in the SP-200, 400 and 600 Super Pro
receivers as well as the HQ series and, in slightly modified
form, by TMC in the GPR-90 and by Collins in the 75A-1,2,3
and 51J series. National and Hallicrafters mostly used the
earlier Lamb circuit. as did Hammarlund in the SP-100
Super-Pro.


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Richard Knoppow
Los Angeles
WB6KBL