Section II
Paragraph
2
RESTRICTED
AN
08-40SCR274-2
c.
TUNING THE TRANSMITTERS. -Transmitters
must be tuned up with the "TONE-CW-VOICE" switch
on Radio Control Box BC-451-A in "CW" position and
must not �e readjusted in any way after switching to
"TONE" or "VOICE". To tune-up a transmitter pro
ceed as follows:
( 1) Set the "FREQ UEN CY" dial to the desired
transmitting frequency.
(2) Set the "ANT. COUPLING" control to about
"3" on its scale.
(3) Turn the "LOCAL-REMOTE" switch on An·
tenna Relay Unit BC-442-A or BC-442-AM to "LOCAL"
(4) Set "T ONE-CW -VOICE" switch on Radio
Control Box BC-451-A to "CW".
(5) Set "TRANSMITTER SELECTION" switch
on Radio Control Box BC-451-
A
to the number cor
responding to the rack position of the transmitter
which is being tuned.
(6)
Make sure that neither the microphone button
nor the key is closed. Turn "TRANS. POWER" switch
to "ON", Dynamotor DM-33-A should 'itart.
(7) Allow a minimum of 15 seconds for tubes to
heat up.
(8) Lock the "built-in" telegraph key on top of
Radio Control Box BC-451-A by rotating it clockwise.
(9) Adjust "ANT. INDUCTANCE" for maximum
antenna current reading on Antenna Relay Unit BC-
442-A or BC-442-AM. Make this adjustment with
"ANT. COUPLING" at a lower setting than that which
gives highest antenna current.
(10) Vary "ANT. COUPLING" until the maxi
mum rw antenna current reading is obtained on An
tenna Relay Unit BC-442-A or BC-442-AM. This setting
must be carefully made.
(11) Adjust "ANT. INDUCTANCE" for max
imum cw antenna current.
(12) Observe the antenna current on "VOICE"
and "TONE". Antenna current readings will vary widely
with'the antenna and the choice of frequency. For a
short "built-on" fore and aft antenna, the reading on
"CW" probabl y will be greater than half s(ale. On
"VOICE" it will be considerably less than for "CW",
and for "TONE" it will be between the values for
"CW" and "VOICE".
(13) Tune-up the other transmitters in the rack,
following the same routine as for the first. It is good
practice to return to the first transmitter and retrim the
"ANT. INDUCTANCE" control on "CW".
(14) Lock the three controls of each transmitter
by rotating the "LOCK" knobs one-half turn clockwise
to a stop, in which position the engraving, "LOCK", on
the knob will read right side up.
(15) Release the built-in telephone key by rotating
it in a counterclockwise direction.
(16)
Mark the frequency, to which each transmit
ter has been tuned, in soft pencil in the appropriate
blank space on the plate above the "TRANSMITTER
SELECTION" switch. Record tIle transmitter data on
the "write-in" plate on the front of each transmitter.
d.
CALIBRATING THE TRANSMITTERS.
(1) Each transmitter is supplied with a special fre
quency checking circuit which is used for checking the
frequency at one point on the dial; it does not, how
ever, control the frequency.
(2) The frequency of the crystals supplied with the
different transmitters are as follows:
Radio Transmitter
Crystal Frequency
BC-696-A (3-4 me). - ............................... 3.5 me
BC-457-A (4-5.3 me) ............................... 4.6 me
Be-458-A (5-7.3 me) ..............................
.
6.2
me
BC-459-A (7-9.1 me) ............ , .................. 8.0 me
Note
When a transmitter is operated at or near the
frequency of the crystal in that transmitter,
a dark three-cornered shadow appears in the
round spot of green light on the screen of tube
V-53 which is part of the cjrcuit.
(3) Always recheck the frequency calibration in
the following manner after any tube is replaced in the
transmitter:
(
a)
Open the hinged cover (at the top back of'
transmitter) to such an angle that the reflection of the
entire tube V -53 may be seen.
( b)
Tune the transmitter to the lowest frequency
which will open the shadow in the tube. (Most easily
done by starting from the lower frequency end of the
dial.) The indicated dial frequency should now cor
respond with that of the crystal, if the dial and crystal
frequency do not correspond, manipulate the "FRE
QUENCY" knob so that the dial is set exactly to the
crystal frequency.
(
c
)
Adjust (with a small screw driver) the
capacitor, (covered by a snapslide between the two
covers, on the top of the transmitter) to a point where
maximum shadow angle is noted on the screen of tube
V-53. Clockwise rotation of the screw on the capacitor
lowers the transmitter frequency.
(
d)
Turn the "FREQUENCY" control to make
certain that no
opening
of the tube is
observed
for any
indicated dial frequency below the one corresponding
to the value shown on the crystal holder.
2-2
RESTRICTED