from plate to ground, helps to eliminate the
trouble. Examine the contacts for dirt, corrosion,
1,800-cycle ripple in the amplifier pulse.
or pits. The contact pressure is increased by
d. Stage V16. As the strength of the pulse
bending the springs with long-nosed pliers.
e. Alignment. With proper voltage on grid 6
increases, a positive potential is applied to the
grid of thyratron tube V16 (884), causing current
of tube V4, the direct current in the synchronous
to flow in the plate circuit. The positive voltage
motor should be between 75 and 85 ma in Facsimile
develops at the beginning of the phasing pulse
Sets AN/TXC-1A; and approximately 90 ma in
when transmitting for negative reception. At
Facsimile Set AN/TXC-1B. In Facsimile Set
this time, the signal level increases, scanning from
AN/TXC-1C, motor current should be 40 to 60
black to white. A positive pulse is generated im-
ma on 30-rpm operation and 55 to 80 ma on 60-
mediately after scanning the phasing pulse when
rpm operation. If the input voltage is not cor-
transmitting for positive reception. At this point,
rect, adjust it by changing R34 in Facsimile
the signal increases when scanning from white to
Transceivers TT-1/TXC-1 and TT-1A/TXC-1 to
black. The exact phasing point for negative re-
a higher or lower value, or adjusting R93 in
cording is displaced from the exact phasing point
Facsimile Transceivers TT-1B/TXC-1 through
for a positive recording by the length of the phas-
TT-1F/TXC-1. If the voltage is not correct as
ing pulse. The dog trip arm (fig. 19) must be posi-
indicated by the signal-tracing chart, try changing
tioned so that it will split the two pulses (positive
the tubes. Make current measurements with the
and negative) evenly on each side of center. The
motor running.
plate of V16 is connected directly to RB+. Plate
current flow must paas through the cathode cir-
cuit. The low impedance path to the cathode is
The fork oscillator unit is designed to supply an
through the phase magnet and capacitor C50.
1,800-cycle signal of extreme accuracy. The regu-
Before phasing, one side of C50 is at cathode po-
lated B+ voltage must not vary more than 5
volts. The oscillator unit is calibrated at the
tential and the other side at ground potential.
After the capacitor is charged, the impedance of
factory; however, small adjustments in frequency
the cathode circuit is very high, and the plate cur-
may be made in the field by varying cathode re-
rent ceases to flow. If C50 fails, current flows in
sistor R25 of the unsealed unit or R304 of the
the plate circuit at all times. The failure can be
sealed unit. Enough adjustment is available to
detected by watching for the glow in the tube.
compensate for minor changes caused by aging
The normal bias voltage is controlled by voltage
of tubes, new tubes, and small differences in the
bias resistors R78 and R17. If the phase magnet
RB+ voltage. Since the frequency of the fork
fails to operate and no discharge glow is noted in
oscillator unit is upset easily, it is not practical,
V16, try another tube before tracing trouble in
in most cases, to make repairs at the installation.
the phase magnet.
a. General. The phasing circuit is inoperative
a. Resistance and Voltage Measurements. The
except during the phasing operations. During
balancing anode of the photocell should have be-
phasing operations, the input signal is picked up
across cathode resistor R72 of V14. The signal
tween .3 and .6 volt dc, depending on the value of
R49 in Facsimile Transceivers TT-1/TXC-1
is rectified by the recorder lamp in the plate
through TT-1B/TXC-1. With the power turned
circuit.
off, measure all resistances. Check potentiometer
b. Pulse Filter. The phasing pulse, which is
R45 for good contact between the contact arm
represented by a dip or an increase in the steady
and resistor. A poor contact in equipment that
1,800-cycle signal, is smoothed out into a dc pulse
has been standing idle for some time may be
by the action of 50-microfarad smoothing capaci-
cleared by rapidly rotating the CONTRAST con-
tor C45.
trol back and forth 15 or 20 times. Check for a
c. Stage V15. The pulse-amplifier stage, V15,
good contact between resistor R42 and the sliding
is similar to a normal voltage amplifier. The
contact. This can be done by connecting the
signal is applied to the cathode and the grid is
meter on one side of R42 to the contact and apply-
at ground potential. This must be taken into
ing a light pressure to the contact ring. Capac-
account in tracing trouble, since it is frequently a
itor C58 usually does not develop trouble because
cause of confusion. Capacitor C47, connected
