Eddystone Short Wave Manual page 8

.6
EDDYSTONE SHORT WAVE MANUAL
ALL WORLD
OPERATION.
Unscrew aerial trimmer to nearly mini-
mum capacity and put tuning dial
at
0°
and the tank condenser at position 0.
Switch
on by turt*ig the switch to the right, and
adjust reaction trimmer until a faint rushing.
sound is hard in the
'phones when the
reaction control is turned three-quarters of
the way clockwise. It will generally be found
that the reaction trimmer has to be screwed
fully clockWise for correct oscillation. Stations
may now be tuned in. Leaving the tank
condenser at 0, turn the tuning dial from
0 to 100 degrees, keeping the set just
off
the verge of oscillation for telephony and
weakly oscillating for 'GMT. signals. Stations
will now4e heard.
Turn tank condenser
to No. lliositio'n and again tune from 0
to 100: deg*ees; and so on until the whole
10 positions of the tank condenser have been
explorftd; -The receiver should not be calibrated
until the optitnium setting
of
the aerial
trimmer has been found.
Proceed likewise
with other. coil.
TWO-continued
For the best results with any aerial, the
aerial trimmer screw should be turned half
a turn clockwise at a time until a blind spot
is found on one of the two coils. Then reduce
the setting of this condenser until the blind
spot disappears. This position will give maxi-
mum selectivity ,and sensitivity.
In
short,
use as much capacity as possible for the aerial
trimmer, consistent with freedom from blind
spots.
The set may now be calibrated, but
the aerial trimmer should not be re -adjusted
again since this would alter calibrations.
.
;
To achieve maximum efficiency however,
a
larger value condenser
(our Catalogue
No. 1013 being quite suitable) may be used
in place of the aerial trimmer.
Blind spots
may be prestnt but can usually be moved to
a part of the band which does not matter.
If
this alteration is made the wave range charts
on page 7 will not apply.
If no signals are heard, check the receiver
with a milliammeter and a voltmeter. The
following voltages and currents should obtain
with 115 volt H.T. supply :
SP2-
Anode Current (approx.) 0.25 m.a.
knode Voltage 90 volts.
P220-
Anode Current 3 m.a.
Anode Voltage 115 volts.
Potentiometer Current 1.25 m.a.
Total Current 4.5 m.a. at 115 volts.
These figures are for an average kit and
will vary slightly for individual receivers.
CONTINUOUS BANDSPREADING.
Tuning is accomplished by means of two
parallel condensers.
The band required is
selected by
the large condenser which
is
variable in ten equal steps only and is called
the tank condenser. A small vernier condenser
slightly larger in capacity than the capacity
difference between the steps on the tank
condenser, is used for final tuning.
Suppose the coils were tuned in the normal
way with a .00016 mfd. variable condenser.
Then the 6LB coil would tune from approxi-
mately 19,350 kc/s.
at
0° on the
dial to
10,100 kc/s.
at
100°.
Thus,
a frequency
bandwidth of
9,250 kc/s. is
obtained by
turning the condenser through 100 degrees on
the dial. With the bandspreading 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 855 kc/s. approximately, 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 in-
corporated
in
its movement, it
will be
appreciated that tuning difficulties are stig
further
decreased.
To give
the
constructor
an idea
of
how the various bands are
" spread " the
curves on page 7 were taken on a specimen
receiver. These show the effect of bandspread-
ing,
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.
It will be seen that the 14 megacycle
amateur band is " spread " over 40 degrees
on the 6LB coils, while the 6Y coil brings in
the 7 megacycle band over SO degrees on the
dial.
These figures c!early show why this
receiver
is
suitable
for amateur reception.
The overlaps between the various settings of
the tank condenser have been designed so
that no stations will be missed.