GE BC-375-E Preliminary Instructions page 15

voltage is present in the radio transmitter even
though there is no plate ammeter reading.
In checking low voltage and filament circuits, the
single conductor cord with the Plug PL-59 should be
disconnected from the transmitter, thus removing
the high-voltage supply. It will be noted from
observation of the schematic diagram (Fig. 1) that
two tube filaments are connected in series. Thus, if one
fails, two filaments will go out. The defective tube can
be determined by use of a continuity meter.
The following information is supplied to aid in
servicing the equipment :
a. Voltage Readings
(A high-resistance voltmeter should be used—at
least 1000 ohms per volt.)
(1). Low-voltage input of 24 to 28volts, depending
on the power-supply voltage, should be obtained from
terminal 45 of Socket SO-41 to ground.
(2). Speech-amplifier plate voltage of approxi-
mately 425 volts should be obtained at the plate
connection of the speech-amplifier tube when the
transmitter is on VOICE. It is recommended that a
tube socket adapter be used which will allow not only
readings of voltages but also all currents for the Tube
VT-25.
(3). Modulator bias voltage of 72 to 75 should be
obtained across capacitor 1160, the positive side
being at ground potential. Selector switch should be
on VOICE.
(4). Speech-amplifier bias voltage of 35 to 40
should be obtained across capacitor 1144, the positive
side being at ground potential. Radio transmitter
switch should be on VOICE.
(5). Plate voltage of 1000 to 1100 volts should be
obtained between terminal 61 of Socket SO-39 and
ground.
(6). Keying voltage of approximately 200 volts
will be obtained across resistor 1115 when keying
relay 1189 is open.
(7). Microphone-supply voltage of 4.5 to 5.3 volts
d-c should be obtained across resistor 1145.
For this test, the microphone should be in the
circuit.
b. Current Readings
(1). Speech-amplifier plate current should be from
19 to 21 milliamperes. Adjustment of its bias voltage
to obtain this value is accomplished by potentiom-
eter 1114 which is accessible in the tube compart-
ment. The normal setting of this potentiometer will
usually be between dial positions 6 and 7.5.
(2). Modulator plate current may be determined
by observing the increase in total plate current reading
when changing from CW to VOICE and impressing
nbrmal modulation. The modulator plate current
should average 100 to 160 milliamperes for sustained
tones. A greater or smaller value than this indicates
that a readjustment of the input level should be made.
(3).
Master-oscillator plate current can be de-
termined on the total-plate current-meter by remov-
ing the p-a and speech-amplifier tubes and placing the
transmitter on CW. The current indicated should
be from 30 to 75 milliamperes, depending on the
transmitter tuning unit in use.
(4).
Power-amplifier plate current may be deter-
mined by subtracting from the total plate current on
CW the currents drawn by the master oscillator and
speech amplifier. Its value should be from 100 to 150
milliamperes at full load.
(5). Master-oscillator grid current can be deter-
mined by connecting a d-c milliammeter in series
with resistor 1109. Correct value should be from 30 to
50 milliamperes.
(6). Power-amplifier grid current can be deter-
mined by inserting a d-c milliammeter in series with
resistor 1112. Correct value is from 15 to 25
milliamperes.
Various other circuits may be checked by referring
to the schematic diagram. Then by referring to the
various unit connection diagrams, the location of the
various circuit elements may be ascertained.
c. Neutralization of Power Amplifier
Neutralization of the power amplifier is not ordi-
narily necessary, since this is done when the units are
tested at the manufacturer's plant. However, if the
setting is disturbed for any reason, the following pro-
cedure may be used to restore the adjustment.
The radio transmitter should first be set up and
tuned to see that all power supplies are correct and
the transmitter is functioning properly. Then remove
the power supply cords and the back of the radio
transmitter. From Fig. 2 locate wire 43 (red) on
filter capacitor 1163. Remove this wire with a solder-
ing iron and tape clear of terminal on capacitor
1197A. This removes the power-amplifier d-c plate
voltage and leaves the r-f circuits intact. Connect
the vertical plates of a cathode-ray oscillograph to
terminals LOAD A and GND on the radio trans-
mitter. Place the ANT. IND. TUNING, M at dial
zero and the ANT. CIRCUIT SWITCH, N on
position 3. Tune the p.a. to resonance as shown by
a maximum amplitude of r-f carrier on the oscillo-
graph. Proceed by turning the neutralizing control,
behind the tuning chart, for a minimum amplitude
on the oscillograph, at the same time maintaining
the p.a. in tune. When the best minimum is reached,
with the p.a. in tune, the tuning unit is neutralized.
In the lower frequency tuning units, the oscillograph
pickup will be practically zero at neutralization, while
on the higher frequencies, considerable amplitude
will be noticed from stray ground currents. A check
on the neutralization may be made by noticing the
low reaction on the total plate meter, at neutraliza-
tion, as the p.a. is tuned through resonance. The
operator must remove all power plugs, while the
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