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the filament voltmeter to read 10 volts. Then place
the OFF-ON switch in the OFF position and
connect a link between the two studs just selected.
(5).
Set the radio transmitter signal switch on
VOICE and the filament voltmeter switch on
MOD. FIL. Repeat the operation of (4) for the
28 V and MOD. FILAMENT rows of studs.
(6).
Again place the OFF-ON switch in the ON
position. Set the filament voltmeter switch on
CW FIL. (signal switch on VOICE). The CW
filament voltage will now be less than 10 volts.
In the same manner, connect between the adjacent
studs of the COMP and CW FILAMENT rows
until the filament voltmeter again reads slightly
over 10 volts. The OFF-ON switch may then be
placed in the OFF position and this link fastened
in the proper place.
(7).
Cut off the charging generator, place the
24 V/28 V switch in the 24-volt position and in a
similar manner to the preceding select the proper
studs on the 24 V and CW FILAMENT rows and
the 24 V and MOD. FILAMENT rows so that the
filament voltmeter will indicate 10 volts in either
position. As now adjusted the filament voltages
will not require further adjustment unless the
cable lengths or charging rates are changed. Ordi-
narily, in aircraft installations, a battery-charging
generator is used at all times. The 24 V/28 V switch
should accordingly be placed in the 28-volt position.
If it is desired to check the operation of the radio
equipment when the charging generator is below
operating speed, this switch should be changed to
the 24-volt position since operation at low voltage
will reduce the life of the tubes.
d.
Replace Plug PL-59 in the dynamotor unit or
power unit.
NOTE : It will be noted that the filament volt-
meter will show from 0.3 volt to 0.5 volt rise when the
transmitter key is closed and plate current is being
drawn.
This is a normal condition of the radio transmitter
circuits and should be ignored, providing the above
filament adjustment has been made correctly.
12.
OPERATION AT HIGH ALTITUDE
Radio Transmitter BC-375-E and associated equip-
ment may be expected to give satisfactory service on
CW at all altitudes up to 27,000 feet. On TONE and
VOICE, however, insulation breakdown may be
experienced with Transmitter Tuning Unit TU-8-B
(6200-7700 KC) above 25,000 feet and with Tuning
Unit TU-9-B (7700-10,000 KC) above 19,000 feet.
These altitude limitations may be exceeded slightly
by care in tuning and by carefully guarding against
accumulation of dust and other foreign matter in the
equipment. Complete assurance of effective operation
between 6200 and 10,000 kilocycles at altitudes
between 19,000 and 27,000 feet may be had on CW
alone. Transmitter Tuning Unit TU-26-B may be
expected to give satisfactory service at all altitudes
up to 15,000 feet.
13.
DYNAMOTOR UNIT PE-73-C
Before being used for the first time, it is recom-
mended that the end bells of the dynamotor unit be
removed to make certain that the armature rotates
freely in its bearings, and a check should be made to
determine how much time has elapsed since the
dynamotor unit was last serviced. If this time exceeds
five months, follow lubrication instructions appearing
in this book. Do not lubricate more often or add more
lubricant than is specified in those instructions.
The voltage at the input terminals of the dyna-
motor unit must be 28 volts in order to obtain rated
power output from the radio transmitting equipment.
SECTION IV
OPERATION
14. RADIO TRANSMITTER ADJUSTMENT
WARNING: Operation of this equipment in-
volves the use of high voltages which are dangerous
to life. Operating personnel must at all times observe
all safety regulations. Do not change tubes or make
adjustments inside equipment with high voltage
supply on. Do not complete or maintain any con-
nection between radio transmitter and dynamotor
unit unless all shields on the transmitter are in
place.
Assuming that the installation and preliminary
adjustments have been made in accordance with
SECTIONS II and III. the following operating pro-
cedure is recommended. Some of these adjustments
need be made only at the time of installation.
Under no conditions should any of the switches on
the radio transmitter or tuning unit be changed with
the key or microphone button depressed. Insure that
all switches position correctly; do not attempt to leave
switches between points. Failure to observe this pro-
cedure causes undue arcing of the switch and severe
strain on the vacuum tubes.
4
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