indication and when
necessary, the R.F.
and
first detector
tuning
condenser
end-plates are
adjusted. The
high frequency
adjustments
should then
be
repeated to assure best
results.
Bear
in
mind that the signal
generator output
should be re-
duced when
the output
meter indications increase
with align-
ment, to
prevent
slamming
of
the meter
pointer.
When
the receiver is
of the
multi-band type, the
next ad-
justments are
made on the other
bands, in
accordance with
the procedure
outlined
above.
The
I.F. circuits
are no longer
touched. The
R.F. stage,
detector and
oscillator
trimmers and the low
frequency oscil-
lator
padders (if any)
are the only
units
to be
adjusted for
each
individual band. First select a
frequency at the
high
end
of the band
and
then
one at
the low portion
of
the
dial.
At times, with
receivers
using pentagrid converters
(com-
bination
of first detector and
oscillator), there may be
some
interaction
between oscillator and first detector
trimmer ad-
justments at
the high
frequency end of the bands. In cases
such as
this, the "rocking"
process previously described
should be
applied even at the_high_end of the
band to find
that
combination of
oscillator and first detector parallel
trimmer
adjustments which offer best
operating
efficiency.
The
foregoing
generalized alignment procedures are not
meant to supplant more
specific manufacturers' service notes
which should ALWAYS be
consulted before attempting
to
ad-
just the
tuned circuits of
a
receiver;
especially
those re-
ceivers
incorporating automatic frequency control. The use
of the
A.V.C. Substitution method does not
interfere
with
manufaturers' instructions, but
merely serves
to
simplify
the
alignment
technique.
The
foregoing information is, how-
ever,
sufficiently complete
to
allow the
operator
to
proceed
with receiver
alignment
in the absence of specific
data.
AUTOMATIC
FREQUENCY
CONTROL CIRCUITS
The
adjustment of A.F.C. circuits differs
to some extent
in various
receivers, and hence no
specific
details are given
here,
but rather just a
general idea
of the points behind
A.F.C.
circuit
alignment. At the same time,
it would be well
to state that
automatic
frequency
control is not popularly
employed in
broadcast
receivers and
it is therefore not very
probable
that many
technicians ever encounter
such
a
re-
ceiver
circuit. Broadcast receiver
manufacturers
have gener-
ally
chosen
a
more simple and
economical solution
to
the
problem of tuned ciruit
stability
in
the form of highly
stable
air-tuned
trimmers,
silver-mica
condensers, as well
as tem-
perature
compensating
condensers.
For
A.F.C.
adjustment,
the signal
generator is
set to the
exact
I.F. specified in the manfacturers' manual
so that the
frequency control
network
may
be
caused to function
uni-
formly
on
both sides of the resonance curve. A high
resist-
ance
voltmeter or V.T.V.M. is connected
across the
two
cathodes of
the discriminator
tube, such as a
6H6, and the
necessary
trimmer adjustments
are made (in
accordance
with
manufacturers'
instructions)
to
give
zero voltage read-
ing.
When this
is obtained, the
discriminator
has been
balanced
for proper
operation; assuming
the I.F. amplifier
is of course also aligned
for optimum
performance,
at the
SAME frequency
at which the
discriminator
tube
has been
balanced.
It
is
of utmost
importance
that manufacturers'
service
notes
be closely followed
for
proper A.F.C.
circuit alignment
and that
the I.F.
stages and
discriminator
definitely
be
aligned
at the same
intermediate
frequency.
This does
not mean that
you must have
crystal
accuracy
in the setting
of the frequency
chosen. All
that this
means is
that the Signal
Generator
dial
should not
be moved
when
making
the
discriminator
tube
and I.F. stage
alignment. In
other
words,
as long as
the discriminator
and I.F.
stages
are
aligned
at the same
frequency
(regardless
of
whether this
frequency
may
be a little
above or
below the
specified
fre-
quency), no trouble
will
be experienced.
In short,
a circuit
specified as having
an I.F.
of 460
Kc can
be aligned
just
as accurately at
465
or
455,
so long
as the
entire series
of
inter-related
circuits
are also aligned
at
that
frequency.
THE ADJUSTMENT
OF
FREQUENCY
MODULATION
RECEIVERS
The
long-familiar system
of amplitude
modulation
oper-
ates on
the principle
of constant
carrier frequency
and
the amplitude
of the
carrier
is varried
in direct
proportion
to the intensity
of the
sound picked
up
by the microphone.
The frequency
of
the sound
delivered
from the
microphone
to the transmitting
equipment
basically
merely determines
the
rate at which
the amplitude
of the
carrier
is varied.
In
a
frequency
modulated
signal
the
amplitude
of the
carrier
remains essentially
constant,
and with
modulation,
the carrier frequency
shifts
symmetrically
about the mean
carrier frequency.
In other words,
the intensity of
the
sound
picked
up by the microphone
directly determines
how much
the original frequency
of the carrier
will
be
shifted.
This
carrier
shift (which
is equal
on both
sides of
the unmodu-
lated
carrier
frequency),
determines
what is
referred
to
as
the BAND
WIDTH.
Therefore,
let us
assume
that the
carrier
frequency
is 90 megacycles and
the
transmitter
is
so de-
signed
that
100%
modulation
corresponds
to
a symmetrical
frequency
deviation
of 75
Kc.
The
total
band width
of
the
transmitted signal
is
then
fundamentally
75 Kc above
and
below the average frequency
of
90
megacycles,
or
a band
width of 150
Kc.
It is not important to burden
the
reader with
the
technical
details of why
a
band-width
of approximately
100 to
200
Kc was chosen. Suffice it
to
say that by
selecting
sufficient
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