Table of Contents
Advertisement
DIDS-402-2AM13
Current flowing through the load resistor develops approximately 1 volt of
video which is amplified by Al and applied to the base of Q1.
Transistors Q1 and
Q2 form a cascode video amplifier arrangement that develops approximately
50 volts of video for application to the CRT cathode element.
Cursor video is developed on the timing board by the circuit described in
paragraph 4-23.10.1.
This video is applied through resistors R18 and R19 and
is amplified in the same manner as monoscope video.
4-28.2 Blanking
The CRT video is inhibited each time a blanking pulse is developed on
timing and discrete board A14.
The pulses occur during
(1)
CTS+1 of each
character (intercharacter blanking), (2) at the end of each line during horizontal
retrace, (3) at the end of each frame during vertical retrace, and (4) when the
monoscope scan is shifted to the 'underline' character at the edge of the target.
When either of the above conditions occur, a blanking pulse is applied to
Video Amplifier Al through resistor R3.
A low input to pin 3 is the same as a
high at pin 1; consequently Al is cut off for as long as the blanking pulse is
present.
This, in turn, inhibits video to the CRT during the specified times and
thereby provides some measure of protection for the CRT phosphor material.
4-28.3 CRT Protect
The CRT protect circuit prevents burning an intense vertical or horizontal
line on the CRT phosphor when either CRT deflection amplifier fails.
The CRT
grid is normally held at a positive potential, the value of which depends upon
the setting of the brightness control.
When either deflection amplifier fails, a
positive level is developed on the CRT Protect input.
This level turns Q3 on
and essentially grounds the CRT grid through the heavily conducting transistor
switch.
4-28.4 Cathode Ray Tube V2
The cathode ray tube (CRT) is used to display up to 1040 high-resolution
characters for viewing by display terminal operators.
The CRT is shown
schematically in figure 4-72.
The CRT anode is held at a potential of +12 kvdc
by a voltage developed by the high voltage power supply.
A second voltage of
500 volts is applied to pin 10 as an acceleration potential.
Electrons are
attracted from the cathode element at a rate controlled by the setting of the
brightne ss control.
When video signals are coupled from Video Amplifier AI, conduction through
the CRT increases and a video character appears on the CRT screen.
The position
where this character appears is determined by the vertical and horizontal
deflection voltage s applied to deflection coil Ll.
In addition to the vertical and
horizontal windings of the deflection coils, a minor vertical (diddle) coil is con-
tained in Ll.
This coil is activated by the minor vertical signal coupled from
monoscope deflection amplifier A3 and increases the height of each horizontal
line (and therefore the character height).
4-160
Advertisement
Chapters
Table of Contents
