alignment of
the
I.F.
amplifier. If
the receiver to be
aligned
incorporates a
selectivity control (variable I.F. band
width)
in
the form
of
a
"Narrow,
Normal, Wide" switch or some
similar type, it is necessary that
this control
be
rotated to
the
maximum
selectivity (Narrow) position
before any ad-
justments are made.
Next, and this is
very important, to
prevent
interference
(unwanted
beat notes)
from the
local
oscillator of the
receiver, it is
imperative ^at
{he ope
rator
short circuit
the
oscillator
section
of the
condenser
gang.
This
mav
be
done merely by using a short
double clip lead
from rotor to stator or
stator to
ground.
Use
a
.QQ2jto—flL
mfd.
fixed condenser
in
series with
the
jumper lead just
mentioned.
As in
the case of
the T.R.F. receiver, the
actual
basic
alignment method
and means
of resonance indication are
optional
with the
operator; though
with
the
presence of
A.V.C.
(moreso in
a
superheterodyne than
the T.R.F. re-
ceiver),
it is
desirable that the service
technician choose
alignment
method either #2 or #3
previously
outlined
on
page
(9).
For the
alignment
of
the I.F. system,
the receiver band
switch
(if
any)
should
be
set to the broadcast
band position,
so as not to
place a low
impedance short-wave coil between
the first detector grid
and ground. The
BLACK lead of the
coaxial
output cable should be connected to
the
receiver
chassis and
the receiver chassis tied to a good
external
ground.
If
the
superheterodyne
does not
incorporate
A.V.C.
it is
not always necessary to
employ
a
blocking condenser in
series
with the RED output
cable
lead,
unless
otherwise
directed in the
manufacturers' service notes.
However, if
A.V.C. is
incorporated,
it is quite desirable to use a blocking
condenser in series
with the RED
lead
so
.that there is
no
D.C. return to
the test
oscillator's
attenuation
circuit which
would disturb the
operating
characteristics
of the
receiver
by
shorting
out
the A.V.C.
voltage or fixed minimum A.V.C.
bias. The RED lead
(through its blocking condenser), is
then connected
to the grid of the first detector.
The Signal
Generator dial should be set to the specified
I.F. frequency,
using the smallest R.F. signal
and modulation
level in
keeping with good
output
meter
deflection. If align-
ing below the A.V.C.
threshold, the receiver
volume
control
should
be
at
maximum position, because of the small
test
signal employed.
However, if you are operating with
A.V.C.
Substitution or
measuring A.V.C.
voltage
as the means of
resonance
indication, the
receiver volume control
can be set
to any
convenient position.
Begin the I.F.
trimmer adjustments
at
the
second detector
and work
back towards the first detector, repeating adjust-
ments to
make sure that
interaction effects between
the pri-
maries and
secondaries
of
the I.F.
transformers
have been
mutually
compensated.
There
is always
the
possibility
that the I.F. amplifier
may
be so badly out of line that an output meter deflection cannot
be obtained even with maximum settings of
all signal gen-
erator
and receiver
controls.
In such event
(though not very
common), it would then be necessary
to
inject the I.F. signal
into the grid of the last I.F. stage
and
work back stage
by
stage. The first
detector is then
the last tube to receive the
I.F.
signal.
The
choice of
a
resonance
indicator
will determine
whether
it is
necessary to use a
modulated or
unmodulated signal.
With
some types of indicators, namely, those
wherein the
reading
is dependent on
the
A.V.C. system,
it is not required
to work below
the A.V.C. threshold and any strength
of R.F.
signal
that does
not overload the receiver, may
be employed.
This is the very
purpose
of the A.V.C.
Substitution
System,
thereby insuring
easier alignment
as
well
as better normal
local reception.
Having
completed
the
I.F.
amplifier alignment,
the RED
and
BLACK
output leads
of the
signal generator
are con-
nected
to the antenna
and ground posts of the receiver
and
we
then prepare for Oscillator, R.F. stage and
first detector
adjustments. An appropriate DUMMY
ANTENNA
should
be inserted between the receiver antenna
post and the signal
generator RED lead, whenever
manufacturers'
service in-
structions
suggest same,
remove
the shorting
lead
pre-
viously
PLACED ACROSS THE OSCILLATOR
TUNING CONDENSER.
Keep
the
receiver selectivity
control
at maximum selectivity
position and the gain control
in the same position
as it
was
in the previous operation.
Set both receiver dial and the
signal
generator to the high
end
of
the broadcast band,
generally about 1500 Kc. If
the
tuning condenser uses
a special oscillator
tracking
section,
there may
possibly
be no adjustment
that can
be
made
to
the oscillator circuit other
than (only if absolutely
re-
quired)
bending
of
the
slotted
end-plates of the tracking
section. In general (except in small-sized, very low-priced
receivers), this is not the
case and the oscillator
parallel
trimmer
screw
is
slowly
turned until
the signal is heard.
Then carefully
adjust
it for maximum output-meter
deflec-
tion.
The first detector (and R.F.
stage if any) parallel
trim-
mers are then
also tuned for
maximum indication.
This
tem-
porarily concludes the adjustments
for
the high
frequency
end of the band.
Next,
the
receiver
and Signal Generator
are
set to the
low
end
of the
broadcast band, generally
about 600
Kc,
and the oscillator
series
padding
condenser
is adjusted
by
"rocking"*
(see note below) for maximum
output meter
*Note: The oscillator
alignment procedure,
referred to as
"rocking,"
merely consists
of
rinding that combination
of
oscillator
low
frequency padder
adjustment
and
receiver
tuning dial position
which gives maximum output regardless
of whether the receiver
tuning
dials falls exactly
on
the
600 Kc spot or not.
In this man-
ner, the receiver is
adjusted
for
maximum
operating efficiency.
12
To Next Page
Loading...
Need help?
General