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DIDS-402:-2AM13
4-19.1.2 Vertical Deflection Amplifier
The vertical deflection amplifier (figure 4-22) receives a vertical drive
pulse from the line counter of the timing circuitry.
This pulse, which is active
for 13 line times and inactive for 1 line time (retrace), is converted to a saw-
tooth waveform by a waveforming network in the amplifier.
During the active
scan time, the vertical sawtooth gradually increase s in value to move the
electron beam from the top to the bottom of the screen.
Simultaneously, the
horizontal deflection voltage is causing the electron beam to produce 13 visible
lines on the CRT screen.
At the end of 13 line times, the vertical sawtooth
rapidly collapse s which allo}Vs the electron beam to return to the top of the
screen.
Simultaneously, the 13th line horizontal retrace pulse is returning the
beam to the left side of the screen.
Thus, at the end of 14 line times, the
electron beam is again positioned at the top left corner of the screen (line one,
character one) and a second 13-line scan is initiated.
In
addition to the vertical drive pulse, the vertical amplifier receives a
second input from the horizontal amplifier.
A ·sam.pling of the horizontal deflection
volt'age is applied to the yertJcal amplifier and acts as a skew correction signal.
As the beam scans across the
screen~the
increasing value of skew-voltage 'bucks'
the vertical drive sawtooth and produces a constant vertical deflection current
throughout the duration of the line.
In this manner, the horizontal sweep remains
perfectly horizontal and no 'sagging' is experienced as the horizontal scan
approaches the end of the screen.
Similar to the horizontal circuitry, the CRT grid is effectively grounded
when the vertical sawtooth collapses.
This provides CRT protection during
vertical retrace by cutting off the CRT for 80 character times.
4-19.1. 3 Minor Vertical Expansion Signal
The minor vertical amplifier contained on monoscope deflection amplifier A3
produces an amplified
1.
3-MHz sine-wave signal.
This signal is applied to the
diddle coil which is located on the neck of the CRT
~
The purpose of the minor
vertical expansion signal is to 'expand ' the height of the horizontal scan lines.
The minor vertical expansion voltage is frequency-interlaced so that during
alternate CRT scans the sine wave signal will be 180 degrees out of phase.
This
interlacing produces a sharply defined character height as shown in figure 4-23.
010568-522
Scan 1 Minor Vertical
Scan 2 Minor Vertical
Combined effect after
two s cans of CRT
Figure 4-"23.
Effect of Minor Ver.ticalExpansion on CRT Scan Lines
4-50
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