Note:
Inasmuch as
we
are
now
going
through the
usual
method
of
receiver
alignment,
the
operator
must
bear
in
mind to
always
employ
the
smallest
R.F.
signal
intensity
necessary
for
the
alignment
procedure.
The
reason
for
this is
that in
this
manner,
the
automatic
volume
control
system, if
any,
will
not
function,
and
we thereby
avoid
the
undesirable
leveling
action
of auto-
matic
volume
control.
For
those
not
fully
familiar
with this
action
of A.V.C.
circuits,
it
should be
understood
that
the
purpose
of
automatic
volume
control
is to
maintain
constant
R.F.
signal
input to
the
detector
over a
wide
range
of
signal
intensity
applied
to the
receiver
antenna
terminals.
In
brief,
the
greater
the
input
signal
strength
to
the antenna
post,
the
greater
the
A.V.C.
voltage
developed,
which,
applied to
the
control grids
of the
R.F. and
I.F.
amplifier
stages,
reduces
their
gain
proportionately.
In
the
other
direction,
a
reduction
in
antenna
signal
carries
with it
the
development
of
smaller
A.V.C.
potentials,
and
accordingly,
the
stages
controlled
by
the
A.V.C.
network
operate
under con-
ditions
of
reduced
bias
or
increased
gain.
Hence,
unless
the
test
signal is
kept
below
the
point at
which
A.V.C.
takes
hold,
trimmer
adjustments
will be
auto-
matically
followed by
changes
in
A.V.C.
voltage,
and in
this
manner,
the
receiver
will
attempt
to
maintain
constant
output
in
spite
of
signal
variations
due
to
trimmer
adjustments.
Of
course,
there
are
methods
of
aligning a
receiver
with
automatic
volume
control
working,
but
the
resonance
indicators,
in
such
cases,
cannot
be
just a
simple
output
meter, but
must be
associated
in
some
way
or
other
with
the
tuned
circuits or
A.V.C.
controlled
stages.
Let us
now
continue
with
the
alignment
procedure
from
the
point
where we
briefly
departed.
The
R.F.
output
attenu-
ators
are
operated
as
follows.
The
system
for
controlling
the
magnitude
of the
R.F.
out-
put
voltage
consists
of
a
coarse
adjustment
potentiometer,
"R.F.
Control-1",
which
feeds
the
vernier
(smooth)
output
"R.F.
Control-2".
This
latter
potentiometer
directly
con-
trols
the
R.F.
output
voltage
appearing
at
either
the
"low"
or
"high"
coaxial
cable
output
connectors.
In
general,
the
"low"
output
terminal
will
suffice
unless
the
receiver
is
badly
misaligned,
or
the
test
requires
unusually
high
output;
at
such
time
the
"high"
output
terminal
is
employed.
The
screw-cap
allows
shielding
of
the
unused
terminal
to
mini-
mize
leakage
possibilities
from
this
source.
The
setting
of
"R.F.
Control-1"
determines
the
maximum
voltage
available
with
"R.F.
Control-2"
turned
fully
on.
This
latter
control
is
self-calibrating
for
comparative
gain
measurements,
and
allows
division
of
whatever
maximum
voltage
is
made
available
from
"R.F.
Control-1",
into
100
parts
proportional
to
the
numerical
setting
chosen.
For
example,
with
"R.F.
Control-2"
at No.
7,
only
7/10
of
the
full
voltage
is
being
supplied to
the
output
system;
at
No. 5
only 5/10
and at
No. 1
only
1/10,
etc.
With
the
foregoing
in
mind, set
"R.F.
Control-1"
about
mid-way
between
the zero
and No.
1
mark, and
then
rotate
"R.F.
Control-2"
until an
appreciable
indication
appears
oh
the
resonance
indicator or
output
meter. If
insufficient
or
no
indication
appears,
bring
"R.F.
Control-2"
back to
its
zero
position
and
advance
"R.F.
Control-1" a
bit
further
(in
other
words,
increasing
the
maximum
available
signal).
Now
again
rotate
"R.F.
Control-2"
to the
right,
watching
the
resonance
indicator for deflection. If
still little
or
no
indication is obtained, return
"R.F.
Control-2" back to its
zero
position, further increase setting of "R.F.
Control-1",
and repeat the previous process until
adequate deflection of
resonance or output
indicator
is
obtained.
When a discernible output
indication is
finally obtained,
then
proceed
to
make
the
necessary capacitative or inductive
adjustments
to
the circuit under alignment. As
the output
indication increases with alignment, reduce the R.F.
signal
input
accordingly,
to
protect the output meter, and
at the
same time to
prevent overloading of the stages
being
ad-
justed. In
addition this latter measure will insure
operation
below the
A.V.C. threshold
(the
point at which the A.V.C.
system takes hold).
The circuit is
at
resonance when
ad-
justed to give peak indication on the
indicating
device
em-
ployed.
We
then
proceed with alignment of the next stage,
in accord with the
preceding paragraphs, making new fre-
quency settings when necessary, and circuit adjustments in
accordance with the
set
manufacturers' service notes.
In
passing,
it may
be
well
to restate, at
this time, that it
is
not
always
necessary to use a
modulated
R.F.
signal
to
align
a radio receiver. As an example,
one may use
an unmodu-
lated
signal and obtain resonance indications from
the A.V.C.
circuit
by using the receiver's own Tuning Eye,
a
V.T.V.M.
or else
a 20,000 ohms per volt sensitivity D.C.
voltmeter,
across the A.V.C. load resistor. These and other methods are
covered
in the section dealing specifically with resonance
indicators, page
(8)
.
THE AUDIO TEST SIGNAL
This instrument employs
a
separate audio
oscillator-
modulator.
The two pin
jacks
located
in
the
"Audio Signal"
selector box, at the lower right hand corner
of the
panel,
provide the external audio test signal. The high impedance
(5Q0,00^^hms) of this circuit
is
suitable for
direct connec-
tion to the grid of amplifier stages or to high
impedance in-
put circuits
of
audio
amplifiers.
Audio test
connections
should
be
made in accordance with the
pin jack markings.
The "high" pin jack is for
connection
to
the
amplifier tube
grid
or the
high side of
an
amplifier's input
terminals and
of course, the
"gnd" pin jack to the
amplifier's low or
ground side.
*ToV
*P
<
-p «^
/ CvfrA
The peak
open-circuit voltage available at these
two pin
jacks is
about
75 to
100 volts at a frequency
approximating
400 cycles.
The signal intensity is jf^y controllable
from
zero to
maximum by clockwise rotation
of the
"Modulation
Control".
The
Audio Signal is
available when the
"Audio
Signal"
selector is set to the
"400
Cycle Audio"
position.
A .01 mfd.
400 W.V. blocking^
condenser is,„ajready _in-
cluded in series
with
the
"high" terminal to
prevent D.C.
short-circuiting in the
event that
it
is
necessary to make con-
5
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