GE BC-375-E Preliminary Instructions page 8

a. CW Operation: Select the transmitter tuning
unit for the desired frequency. CW operation may
then be obtained as follows:
(1).
Place the signal switch on CW.
(2). From the calibration chart on the front of
the transmitter tuning unit, set the band change
switch A, the M.O. TUNING Control, and the
P.A. TUNING Control for the desired frequency,
and set the ANT. COUPLING SWITCH D on
Point 1. (It should be remembered that the band
change switch does not appear on Transmitter
Tuning Units TU-7-B to TU-10-B inclusive.)
(3).
Place the OFF-ON switch on the ON
position. The dynamotor should now start and the
m-o and p-a filaments will light. In case repeated
failures of Fuse FU-22 are encountered and if such
failures persist after compliance with instructions
pertaining to Dynamotor Unit PE-73-C, it is
recommended that Fuse FU-22 be removed from
the relay-fuse box and one extra Fuse Link M-168
inserted in the fuse cartridge. Fuse FU-22 will then .
have two fuse links in it and should be replaced
in the relay-fuse box of the dynamotor unit. In no
case should the use of more than two fuse links be
attempted.
(4). Press the test key or the transmitting key.
The vacuum tubes will now draw plate current as
indicated by the TOTAL PLATE CURRENT
METER. The p-a tuning should be checked
immediately for resonance by varying the control
slightly until a minimum total plate current is
indicated. When the p-a circuit
properly reso-
nated, the total plate current will be from 80 to
110 milliamperes.
(5). The antenna should next be tuned to
resonance. Because a wide variety of antennas is
possible, no specific instructions for antenna
tuning are given other than instructions to effect
antenna resonance by means of the controls pro-
vided on the transmitter, and on the antenna tuning
unit whenever the latter is in use. The subject of
ANTENNA ADJUSTMENTS (paragraph 15) is
considered more in detail elsewhere in this instruc-
tion book, and should be referred to in preparation
for operation of the equipment.
Antenna resonance is indicated by a reading of
current on the ANT. CURRENT meter and by an
increase in total plate current. The increase in
plate current is the more sensitive indication of
approaching antenna resonance. When the antenna
is tuned to resonance, the total plate current
reading will be somewhat higher than the off
resonance value. If the plate current is below 200
to 220 milliamperes, the coupling to the antenna
should be increased by placing control D on a
higher point and the antenna circuit retuned.
When finally adjusted, the total plate current
should read from 200 to 220 milliamperes. If
necessary, detune the antenna circuit slightly,
keeping the p-a dial C in resonance to keep within
the plate current limits.
(6).
The equipment is now delivering rated
power output, and the transmission may be carried
on by operating the transmitting key. In order to
shut down the equipment, it is necessary only to
place the OFF-ON switch in the OFF position.
b. Voice Operation: Assuming that the equip-
ment has been placed in operation on CW, the
following procedure is recommended for obtaining
voice operation. Note the value of total plate current
for CW operation. Then place the signal switch in
the VOICE position and, by means of the MOD.
BIAS adjustment in the tube compartment, adjust
until the total plate current with the microphone
switch depressed is approximately 20 to 35 milli-
amperes higher than for CW. The modulator tubes
are now biased nearly to cutoff for proper Class B
operation, and radiophone communication may be
carried on by speaking into the microphone. It will be
noted that the total plate current increases when the
microphone is spoken into. This increase is due to
current drawn by the modulator tubes. With sustained
normal level of speech impressed on the microphone,
the plate current should rise to an average of 300
milliamperes. If this value is not obtained, the
INPUT LEVEL control in the tube compartment
can be adjusted until the proper amount of modula-
tion, as indicated by the correct plate current, is
obtained.
c. Speech-amplifier Bias:
The correct speech-
amplifier bias will usually be found betWeen 6.0 and
7.5 on the bias adjustment dial. Settings in this range
will normally give the proper speech-amplifier plate
current and optimum side-tone frequency on all
tuning units.
d. Tone Operation:
After the equipment has
been adjusted for VOICE operation, it is necessary
only to place the signal switch on TONE for proper
tone telegraph operation. The total plate current on
TONE will usually be between 300 and 350 milli-
amperes.
e. Side Tone in Aircraft Set: If the radio receiver
circuits have been properly co-ordinated with the
radio transmitter, the receiver control' circuit will
operate each time the transmitting key or microphone
switch is closed, and the transmitter side tone will be
supplied to the headset. When the key or the switch
is opened the receiver again becomes operative, thus
allowing break-in operation. The SIDE TONE level
control located in the tube compartment, is used
to adjust side tone to a suitable value for any par-
ticular installation. It will be noted that a higher level
is desirable on voice position than on the CW and
TONE positions.
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