Raytheon DIDS-400 Series Technical Manual page 198

DIDS-402-2AM13
and decrease in value each time the minor vertical sine wave approaches its
maximum positive or negative value. The constantly fluctuating Y-axis voltages
cause the monoscope beam to sweep up and down across the character symbol.
This painting effect is phase-shifted by 180· on alternate CRT scan cycles so that,
effectively, the character symbol is scanned 24 times (12 times per frame). This
painting effect, when combined with the ramp voltage, assures that all parts of
the character symbol are thoroughly scanned by the monoscope beam.
Like the X-axis deflection amplifier, the Y-axis push-pull output stage
employs a skew correction circuit. The outputs of Q4 and Q5 are balanced by
adjusting R21, and the re sulting X-axis skew correction voltage is coupled to the
X-axis deflection amplifier.
4-26.4 Unblanking Pulse Generation
The purpose of the unblanking pulse generator (figure 4-70) is to develop
spiked outputs in coincidence with the bottom-most peak of the minor vertical
sweep signal. This is accomplished as follows.
The minor vertical sine wave developed by the diddle coil is returned to
the monoscope deflection amplifier at input terminals AJ and AK. The signal at
each terminal is 180 degrees out of phase with the opposite terminal, since'they
represent opposite ends, of the diddle coil. This 180-degree phase difference is
split to 90 degrees by an adjustable phase network (R37 and C13). Thus a minor
vertical sine wave shifted 90 degrees in phase is applied to pin 9 of the unblanking
pulse generator (A6): Since this signal is shifted by 90 degrees, the point at
which the sine wave crosses its zero axis is equivalent to the negative peak of
the minor vertical signal.
The phase-shifted signal applied to pin 9 is coupled to an emitter-follower
(A6A) which, in turn, drives a Schmidt trigger formed by A6B and A6C. This
circuit develops a square wave output (present at A6, pin 4) which is integrated
and applied to a second,emitter-follower formed by A6D. This spiked unblank
pulse output appears in coincidence with the negative peak of each minor vertical
sine wave. As stated previously, 12 of these pulses occur each character time.
On Timing and Discrete Board A14, two of these pulses are used to form the
lower portion of the cursor symbol ( ). The unblanking output is also used for
blanking the video amplifier during CTS+4 and CTS+5 when an Idle code is decoded
in the CE register. This blanking Umits the time the
I
short underline
I ( )
is
displayed on the CRT to five bit times instead of the normal seven bit times.
4-26.5 Monoscope VI
The monoscope (figure 4-71) is a vacuum tube device that generate s a video
output for displaying alphanumeric and special symbols on the CRT screen.
An electron beam originating at a cathode element at one end of the tube is
accelerated toward a target anode at the other end of the tube. Beam accelera-
tion results from the difference in potential between the cathode and target anode
voltages. The cathode is at a
-1.
2 kv level developed by the high voltage power
supply, and the target is at 0 volt.
4-156