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11-14
RADIO
RADIO AND OTHER ACCESSORIES
(410)
are most commonly observed as a loss of sensi-
tivity either over a portion or over entire band;
loss of sensitivity, often characterized by the
selectivity being noticeably unequal on the two
sides of the point of best reception; change in
fidelity; and inaccurate dial readings. Loss of
fidelity will be apparent as a loss of high or
low audio frequencies. If the I-F amplifier is
not tuned to the specified frequency, t
he
oscil-
lator and other tuned circuits will not track.
The dial readings will then be incorrect and a
portion of the band will have low sensitivity.
d. Test
Oscillator
Connections-
Dummy
Antenna
Use.
The chassis or frame of the radio receiver
is considered as being at ground
potential
and
the "0" or "GND" terminal of the test oscilla-
tor should be
connected
to the chassis wherever
good contact can be est a blished. The "ANT"
or "HIGH" terminal of
the
Test Oscillator out-
put must be
connected
to the antenna con-
nector or other points in
the radio
receiver as
specified
in "Tabulated Alignment Specifica-
tions" (subpar. g or h). The use of a fixed con-
denser in
series
with
the
test oscillator lead is
specified in some instances. A .1
mfd.
condenser
is used in aligning the I-F stages and a .00006
(1948) or .000068 (1949) condenser is used in
series with the antenna connector. This con-
denser, sometimes
called
"Dummy
Antenna"
,
provides a proper input loading to the re-
ceiver.
It is
important
that this condenser be
connected
at the point where the Test Oscilla-
tor lead joins
t
h e radio set, and should not be
connected at t
he
test oscillator. Shielded leads
should
be used.
cient sensitivity be used to avoid the possibil-
ity of using too m
uch
Test Oscillator output
to get a readable indication on the output
meter. Sometimes it is desirable to connect the
output meter from .plate to plate of output
tubes; when this connection is employed be
sure that a .1 mfd. condenser is connected in
series with the meter to afford protection from
the
d-e
potential.
f.
Align ment of
t
he
Tuned Circuits
Tuning adjustment with trimmers or adjust-
able iron cores is accomplished while applying
a modulated signal, of the specified frequency
to the input of the stage being
adjusted.
Maxi-
mum Output Meter indication, of the ampli-
tude of Audio-Frequency output, of the radio
receiver, shows when tuning is correct. The
various tuned circuits are aligned by a
djust-
ing each in this manner. During all alignment
adjustment, the output of the Test Oscillator
must be kept as low as possible, consistent with
a reasonable output meter indication, to pre-
vent A-V
-
C action from taking place and
making all adjustments seem very broad.
The tuning tool used must have a m
inimum
of metal so it will cause little or no tuning re-
action. If removing the tool, after making an
adjustment, reduces the output appreciably, a
slight compensating mistuning will correct the
error and produce maximum output when the
tool is removed.
g.
Tabulaied
Alignment Specifications-
1948
Mode ls
These alignment specifications apply to 1948
model radios bearing the following numbers:
Rec eiver
(only)
Part No.
7257814
7257804
Ra
di o
Mo d e l
Nu m be r
Car Series
40
980797
50-70
980798
Volume Control Maximum
Signal generator output
minimum
for satis-
factory
output indication.
See figure 11-15 for
location
of adjustment
e
. Output Meter
Co
n nect io ns
Any standard type of output meter can be
employed
during
alignment. The meter
should
be connected across the
secondary
of the out-
put transformer. It is best to lea ve the voice
coil connected while using the output
meter.
It is essential that an output meter
with su
ffi-
TABLE OF ALIGN
MENT SPECJFICATIONS-1948 MO
DELS
Series Condenser
Signal
Adjust
o
r
Connect to
Tllne
Receiver
Generator
Scre
wl
Dum
m y An
tenna
To
Frequency
In O
rd e r
0
.1
Mfd.
6SA7 Pin #8
No Broadcast
Signal
262KC
ABCD
'.000060
Mfd.
Antenna
Extreme Hi
.
Freq.
1615KC
E F G
Co
nnector
End of Dial
".000060 Mfd.
Antenna
Signal
Generator
1430KC
.J
& K
Connector
"
' Se e be
low
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