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6.3 Range Switch:—A switch may operate defec-
tively on certain positions after long periods of
inoperation. Usually rotating the switch back
and forth several times will clean the contacts
and operation will become normal.
A bad range-switch contact is likely to cause a
change in the sensitivity of the receiver, or the
frequency of a received signal, as the switch is
moved back and forth slightly in a certain fre-
quency band position. A further check is to turn
the switch off and on at one particular fre-
quency band several times and note the apparent
sensitivity of the receiver each time the switch
comes into position. The sensitivity should be
the same each time and may be adequaely judged
for this test by listening to the receiver back-
ground noise.
6.4 Circuit Alignment:
WARNING:—The Circuit Alignment
of this receiver has been done very
carefully at the factory. and re-
alignment should not be attempted
unless it has been definitely ascer-
tained that the Circuits are out of
alignment. It should be attempted
only by skilled personnel. It must be
performed in a test laboratory where
proper test equipment is available.
All adjustments must be done very
carefully and the exact procedure as
outlined below must be followed.
6.4-1 Alignment Tools:—Special tools for align-
ment of R-F and I-F circuits are provided.
They are mounted in fuse clips on either
side of the gang condenser cover, and are
available after removing the large R-F unit
cover. The shorter one of the two is for
adjustment of all R-F and I-F coils, and
the longer one is for adjustment of the
plunger type trimmers. One end of this
tool is for turning the lock nut on the
trimmers and the other end has a hook for
engaging in the hole in the end of the
plungers. After adjustment, the lock nut
should be securely tightened.
6.4-2 I-F Alignment:—The intermediate fre-
quency is 735 kc. The only satisfactory
method of I-F alignment is by means of a
sweep oscillator and cathode ray oscillo-
graph. The I.F. circuits must be aligned at
exactly 735 kc, and the frequency of the
sweep oscillator carrier must be checked
against a
73.5
kc. crystal oscillator. Make
connections as in Table III (a), and follow
the sequence as in Table III (b).
TABLE No. III (a)
I.F. ALIGNMENT CONNECTIONS
Vertical "HI" to Terminal C on last I-F Transformer
(L-47, L-48), Vertical "LO" to chassis.
Insert in series with generator output, 0.01 mfd.
See Chart below.
To Chassis.
Band No. 1
Position 3 (Rec. Mod.)
Fully clockwise.
Position 2.
Position 4 (AVC).
Oscillograph Connections
Dummy Antenna
Connection of Generator Output Lead
Connection of Generator Ground Lead
Position of Range Switch
Position of Power-Transmit-Receive Switch
Position of R-F Gain Control
Position of Selectivity Switch
Position of Noise Limiter and AVC Switch
TABLE No. III (b)
I.F. ALIGNMENT ADJUSTMENTS
(Alignment Frequency is 735 Kc)
Steps
Generator Connections
Slug Adjustments (See Fig. 4)
Trimmer Function
1
6SG7 • 3rd I-F Grid
L-47, L-48
4th I-F Transformer
2
6SG7 • 2nd I-F Grid
L-41, L-42, L-43, L-44
3rd I-F Transformer
3
6SG7 • 1st I-F Grid
L-35, L-36, L-37, L-38
2nd I-F Transformer
4
6SA7 - 1st Det. Grid
L-32, L-33
1st I-F Transformer
6.4-3 Align each transformer to give a symme-
trical flat-topped curve on the oscillograph.
Before performing step 4 above, set crystal
phasing control C-75 at approximately one-
half of its maximum capacity. This is ap-
proximately its final setting and changing
it appreciably will slightly detune the first
I-F transformer.
With Selectivity Switch in Position 2 the
I-F band width is normal without over-
[10]
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