magnet de-energized until the next phasing pulse
phasing pulse (a sudden decrease or increase in
arrives. Capacitor C49 and resistor R77 con-
the 1,800-cycle signal) is smoothed into a dc
stitute a differentiating circuit, serving to sharpen
pulse by the filter action of 50-microfarad capaci-
tor C45 across R72, C46 between cathode and grid,
the peak of the useful positive pulse.
and C47 between plate and ground. Resistor
(1) Phasing error. The phasing pulses are
transmitted at slightly different times
R74 serves as the grid resistor. Screen current is
when transmitting for negative and posi-
fed through screen dropping resistor R75, which
is bypassed by C48. The dc pulse developed
tive reception. When transmitting for
across plate load resistor R76 is coupled to the
positive reception (maximum signal on
grid of trigger tube V16 through coupling capaci-
black), a positive voltage large enough
tor C49. Both plate and screen are supplied
to make V16 arc is developed immedi-
from the regulated +250-volt supply. Refer to
ately after scanning of the phasing spot.
This is at the instant when the scanning
changes from the white of the phasing
tube is a gas-filled thyratron, type 884 connected
spot (minimum signal) to the black of
so that each phasing pulse applied to ita grid
the phasing ring (maximum signal).
causes the tube to arc and energize the phasing
However, when transmitting for negative
magnet. When the PHASE switch on the
reception (maximum signal on white),
transceiver is operated, the normal holding cur-
the necessary positive voltage pulse is
rent from the cathode circuit of tube V14 is
developed at the very beginning of the
removed, permitting the magnet armature to en-
scanning of the phasing spot. This is
at the instant when the scanning changes
gage the stop arm on the primary clutch. Tube
from the black of the phasing ring (mini-
V16 has a positive potential of 250 volts applied
mum signal) to the white of the phasing
directly to ita plate from the regulated +250-volt
supply. In the absence of a phasing pulse, the
spot (maximum signal). Therefore, the
cathode is approximately 57 volts positive with
exact phasing point of positive record-
respect to ground, being tapped onto the voltage
ing is displaced from the exact phasing
divider composed of R17 and R78 across the
point for negative recording by the
regulated +250-volt supply. Since the tube grid
length of the phasing pulse (length of
is at ground potential, there being no voltage
the phasing spot on the phasing ring).
drop across grid resistor R77, the grid is effectively
(2) Correction of error. To divide this error
negative with respect to the positive cathode, and
evenly, the dog trip arm is so positioned
that it splits the two pulses (positive
the tube is nonconductive. Capacitor C50 is
charged 57 volts positive because, in series with
and negative) evenly on each side of the
the phasing magnet coil, it is across cathode re-
center. The error is then so small on
sistor R78. When a phasing pulse on the grid of
both positive and negative transmissions
V16 causes the tube to arc, the voltage drop
that no appreciable portion of the picture
across the tube and across R17 is suddenly
is obscured by the image of the clamp
bar.
reduced to 16 volts, the characteristic voltage
drop across this tube when it arcs; then the
cathode potential suddenly becomes 234 volts,
the PHASE switch is operated, and before a phas-
increasing approximately 177 volts in a positive
ing pulse arrives, the holding current normally
direction. This sudden change in potential across
applied to the phasing magnet from the cathode
circuit of V4 is removed. The phasing magnet
C50 causes sufficient electron flow through the
phasing magnet to attract the armature and release
armature is released and stops the travel of the
the stop arm on the clutch; but as soon as the
stop arm (fig. 18), which is friction driven by the
charge on capacitor C50 is equalized by the cur-
motor through the primary clutch. The stop arm,
rent flow through the phasing magnet, the im-
on which the phasing operation actually is per-
pedance of the cathode circuit again becomes
formed, normally drives the dog trip arm which
very high (with R78 serving as a current-limiting
causes the drum to rotate. However, once the
resistor) and the tube arc is extinguished. The
PHASE switch is operated and the stop arm is
negative grid of the tube then resumes control,
held stationary by the phasing magnet armature,
keeping the tube nonconductive and the phasing
the drum is driven at a slower rate by action of the
