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condenser C8
to
the oscillator grid, where t~y arc mixed with the signals
•ppearing
in
the valve from the s,,gnal grid. The base of the stcondary w,nd,ng
or the deta:lor lr&nsformer T2
is
conna:tcd
to
the voltage divider in
the
filament circuit of the
IA7GT/G convc,ter
valve. This divider consilts of
rcsistou
R2
and R3 and supplies
a
slight negative bias, approximately 0.8 volts,
to rhe signal grid. Condenser Cl I serves to complete the detector tuned
circuit.
.
Anoth<r
1P,GT/G valve
IS
used
as the intermediate frequency amplifier.
from the
signals appearing
at the
plate
of the
converter
valve, those ,t 455
kc
ate
selected by the transtormer ·14, and
impce$$ed
aero$$ the grid of the i-f
valve.
Here
they aie amplified
and
passed through the sa:ond
i-f transformer
T6 to the second detector circuits.
Ir
is in the
i-f
amplifier
that the
major per·
tion of
the
amplificatioo and selectivity of the complete receiver
is
obtained.
A IH5GT
/G valve is wed for the detector, automatic volume control, and
1st
audio
ampli.ficr
circuits. The
detector
circuit consists of the diode section
of
v,
and
the
diode
load
R!2 and RI~. R12 in conjunction v.•ith <;:23, acts
as an r-f filter
and prevents
r-f voltage from going
into
the audio
system.
The
audio
component
is
passed
through
the audio coupling condenser C22
to
the top of
the
audio volume control RI 7. On C-W the grid of the 1H,G'f/G
is
connected
directly to C22 while on PHONE Rl 7
is
connected into circuit
to
provide
control ov<r the audio gain of the aml;'lifier. The automatic volume
control circuit consists of RU, RI, R6, Cl and <.;39
and
functions as follows:
.
In
the "diode
detector
circuit
there
is developed
across
the
load
resistor,
RI 2 plus Rl4, a voltage
proportional
to
the
carrier strength at
the
diode.
The diode end of this load is
negative
with res~t to ground
and
a
negative
A
VC voltage is applied to
the
r-f and i-f amplifier grids. The A
VC
resist9rs
Rt
and R6 are intended to provide decoupling between the r•f and
i-f
circuits.
Valve V4, a !G4GT/G triode, is used for
the
beat-frequency-oscillator.
The circuit
is
set
into
operation
by
the
PHONE-C.W.
switch St which, wl,en
set at C.W., supplies voltage
lo
the plate of V4
through
the dropping resistor
RIO. The tank circuit of transformer T5
is
adjusted to appcoximately
4,6
kilocycles and
oscillations
set
up in this
transform« >re coupled
through
the small capacity existing across pins #4
:-
#,
of
v,
to the
diode
plate,
where
they are
mixed with the i-f signals,
resulting
in
a
IO00 cyde beat note.
In addition to applying high tension voltage
10
V4, SJ also
performs
three
othtr
functioos
whtn
turntd from
PHONE to C.W. The
A
VC line to Vl
and V 3
is
shorted
10
ground and
connection
to the A VC
resistor
Ill,
broken,
thus
removing
the automatic control of volume. The audio gain is fixed by
transfeuing
lhe
audio
grid
of
V5 from the
slider of
R 17
to
the junction
of
C22
with R l
7.
Also
the
scmn grids
of
VI and V 3, which on PHONE
receive
a
fixed supply from
the
junction of
Rn
,nd
RI
I,
are connected to the
slider of Rl
1
where .scrc:cn voltage is
varied by
its
manipulation;
this provides
an
r-f
sensitivity control to replace the audio gain
control
now fixed. In brief
the
change1 ,ccomplished by changing from PHONE to C.W. are:- (t)
Supplying
HT to
BFO circuit (2) Removing lhe AVC (3) fixing
the
audio
gain (4) Providing r-f gain
control through
the screen supply
to
VI
and
V3.
4
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