In some computers to get vertical sync they simply
invert horizontal sync. This very nonstandard process
will cause tearing at the top of the page. The phase
lock loop in the horizontal section will sense that the
sync edge had moved over and will try to compensate.
Normally composite sync looks inverted but actually
the horizontal pulse is moved forward so the raising
edges are in place.
With C188 in place the output
of the exclusive or gate will have an 200 nano second
pulse on both edges of the horizontal sync pulse.
The 74LS22l(IC106) looks at the raIsmg edge of
sync. Normally the raising edge is the first edge but
during vertical sync it is the second edge.
case of "inverted H sync" for vertical sync the
74LS22l(ICl06) must look at the first edge all of the
time whether the edge is raising or falling.
gate produces a 200 nano second pulse
on both edges.
The 74LS22l(IC106) will trigger on
the first pulse and ignore the second.
5.7. DATA CENTERING
The half of the 74LS22l(ICl06) on the video board
is used for data centering. The one shot triggers with
the leading edge of sync and time out with the back
edge of the flyback pulse.
The data centering
potentiometer should be set to center the picture
within the raster.
The LM 120 1 is a wide band video amplifier
intended for use in high resolution monitors.
LM120l contains a 100 MHz amplifier, driver, a black
level clamp, a background level control and an
40 dB attenuator circuit for contrast.
A video preamplifier is AC coupled and biased
through a resistor from a 2.4 volt reference.
preamplifier drives an attenuator whose gain is
controlled by pin 4 (IC1Ol, ICl02, ICI03). The bias
level is maintained by comparing the output of the
attenuator with the voltage on pin 2 ( ICIOl, ICl02,
In the center of the LM120l is a one
transistor amplifier where the gain of each channel is
matched to set white level.
buffered by an emitter follower, brings the video out
on pin 8 (IC10l, IC102, IC103).
The clamp input pin
(IC10l, IC102, IC103)
watches the power amplifier's output during the back
Any difference in pin
and pin 6 (IClOl,
IC102, IC103) during clamping will be amplified and
used to drive a current source which modifies the
voltage on pin 2 (IC10l, IC102, IC103). This clamp
voltage will be used to set bias level.
6.3. POWER VIDEO AMPLIFIER
This two transistor cascode amplifier has both
emitter and collector peaking to get the best
The resistor on the base
reduces the chance of oscillation and also helps to
protect the amplifier from tube arcs. The amplifier
is buffered by an PNP/NPN emitter follower. The
output of the amplifier is isolated from tube arcing
by a carbon composition resistor and an arc arrestor.