Eddystone Short Wave Manual page 33

EDDYSTONE SHORT WAVE MANUAL
31
ULTRA SHORT WAVE
RADIO
TELEPHONE-continued
oscillation again ceases. This operation can
be repeated to a point where back -ground
noise
comes. too unpleasant
for
listening.
When such setting is reached maximum sensi-
tivity has been achieved, but it is sometimes
necessary to reduce gain slightly otherwise
background noises may be excessive.
To tune signals, turn quench and tuning
dial to
"zero." Increase reaction until
receiver just oscillates, then by gently turning
tuning control and slightly increasing reaction
so that oscillation is maintained until a station
is heard.
This will appear in the form of a
whistling note. 'The quench control can then
be increased to suppress the whistle and the
station proper will be heard.
Afterwards
by slightly advancing the quench and reaction
control, maximum sensitivity can be obtained.
THE TRANSMITTER UNIT.
The transmitter unit consists of a push-
pull oscillator with a Class B modulator, and
a driver valve.
The first two valves are
Osram, type B21, and the driver valve is a
Mazda P220.
The advantage of a Class B
modulator is that until modulated with speech
it practically takes no high-tension current,
but this rises when modulated to a maximum
of 20 m.a.
.0003
Fig. 4.
PRIM.
1,000a
of IOMA
G.B.
MIC.
Fig. 5.
The Transmitter Unit.
The Class B push-pull oscillator, and the
driver valve, need a further 17 to 18 m.a., so
that the total current consumption of
the
transmitter is 18 m.a. when quiescent,
to
38 m.a. when modulated.
A 120 volt battery supply is
suitable.
Slightly better results can be obtained using
150 volts. The capacity of the battery natur-
ally depends upon the amount of use.
A
2 volt accumulator is necessary for filament
supply.
The circuit arrangement of the transmitter
is shown in figure 4, and the component lay out
in figure 5.
V3
HT+
G.
PRIM. SEC.
HT+
P
eco00
at 15 MA.
Hr.+
Sec.
i :I
45 V. G.B.
H t+
Theoretical
Circuit
Diagram
for
Transmitter
Unit.