Eddystone Short Wave Manual page 11

EDDYSTONE SHORT WAVE MANUAL
9
"EVERYMAN" SHORT WAVER-continued
heard.
Now commence turning. the tuning
condenser, and when this has gone through
100° the first step of the tank condenser has
been covered. Advance this condenser one
step and continue to cover each step with the
tuning condenser. Probably the first signals
heard will be morse transmissions, a series of
dots and dashes of a high pitched musical
note.
With the set in the slightly oscillating
condition referred to, telephony signals will
also be first heard as a high pitched whistle
similar to a morse signal with the difference,
however, that as the tuning is varied, the note
will appear to consist of a double peak signal
with a silent point in the centre.
To receive
the telephony signal- clearly, leave the set
tuned to the silent point, slacken off the
reaction control very slightly until the set is
just out of oscillation, retune a trifle if neces-
sary, and the speech or music should be quite
clearly heard. It simplifies the operation of
the receiver when searching for stations, to
keep it in the oscillating condition, but it
should be borne in mind that clear telephony
can never be received with the receiver oscil-
lating.
HOW BANDSPREAD TUNING WORKS.
Tuning is accomplished by means of two
parallel condensers. The band required
is
selected by the larger capacity condenser, which
is variable in ten equal steps only and is
called the Tank Condenser. A small vernier
condenser slightly larger
in
capacity
(20
m.mfd.) than that of one step (14 m.mfd.) in
the tank condenser, is used for final tuning.
Suppose the coils were tuned in the normal
way with a .00016 mfd. variable condenser.
The 6LB coil would tune from approximately
19,670 kc/s. at 0° on the dial to 10,067 kc/s. at
100°. Thus, a frequency bandwidth of 9,603 kc/s
is obtained by turning the condenser through
Two
Strips
x 31e: x 94;.
Showing details of panel and baseboard
assembly.
100 degrees on .the dial.
With the bandspread-
ing system employed in this receiver the band
required
is
selected by putting the tank
condenser knob on, say, No. 5 position, and
the small tuning condenser then tunes a
frequency bandwidth of only 960 kc/s. approxi-
mately, as the dial is rotated from 0 to 100
degrees. Since we have only covered a 1/10th
of the previous waveband tuning is ten times
as easy as with a normally tuned receiver.
As
the tuning condenser has a 9 : 1 slow motion
head incorporated in its movement, it will be
appreciated that tuning difficulties are still
further decreased.
To give the constructor an idea of how_the
various bands are " spread " the curves on
page 11 were taken on a specimen receiver.
These show the effect of bandspreading, the
numbers on the curves indicating the position
of the pointer knob on the Tank Condenser,
while the figures on the horizontal line refer
to the degrees on the tuning condenser dial.
THEORETICAL 'CIRCUIT DIAGRAM.
0 H T. -I-
H T. -
LT.-
,