Video Input Processing; Video Amplifiers - Zenith Z-100 PC series Service Manual

The monitor deflection module contains the sync
processing, high voltage, horizontal and vertical
deflection circuitry, and associated feedback paths.
Integrated circuits condition the incoming horizontal
and vertical sync signals for use by the deflection cir­
cuitry. Horizontal deflection amplifiers provide the
current required to move the electron beam in the
CRT from left to right. Similarly, vertical deflection cir­
cuitry provides the current required to move the elec­
tron beam from top to bottom. The high voHage
needed for the CRT anode is also generated here.
Associated feedback-type circuitry includes the an­
ode voltage regulator, pincushion correction, beam
current limiting, and blanking pulses.
The FTM/PIN focus board generates the proper
waveforms needed to produce a symmetrical display
on the CRT. Because this monitor incorporates a flat
technology CRT, a much more complex pincushion
(PIN) correction circuit is required. Waveforms are
generated to correct the display from east to west
(E-W) and from north to south (N-S). These correc­
tion waveforms are then superimposed upon the
horizontal and vertical scanning waveforms to form a
symmetrical display.
In most CRTs, the shadow mask is curved to follow
the contours of the screen and suspended by
springs. This shadow mask may distort with changes
in temperature, sacrificing some image quality even
under ideal conditions. In the flat technology CRT,
the shadow mask is stretched across a frame under
extreme tension, resulting in a flat shadow mask that
remains flat even with changes in temperature. In ad­
dition, the screen itself is perfectly flat. The result is
higher resolution and a smaller overall tradeoff be­
tween contrast, brightness, and resolution.
Video Input Processing
The red (R), green (G), and blue (B) analog color sig­
nals enter the video output module at connector SR9.
These signals are DC-terminated by 7SQ resistors
R51 01, R51 02, and RS1 03. The color signals are
then AC-coupled to the video inputs of ICS101 by
capacitors CS101, CS102, and C5103.
ICS101 is a three-channel, variable gain video
amplifier. A variable DC voltage applied to pin 2 of
ICS101 controls the gain of the three channels. The
external contrast control acts as a voltage divider to
supply this variable voltage at pin 4 of connector
SA1. The integrator formed by RS112 and C5111
helps to smooth the action of the contrast control.
The gains of the three channels will track to within
about 3% over the range of the contrast control
(+
8
VDC at maximum contrast to 0 VDC at minimum con­
trast). The outputs of this gain stage are emitter fol­
lowers. The characteristic low impedance output of
the emitter follower allows for more efficient signal
transfer to the following amplifier stages. The gain­
controlled video signals appear at pins 12 (R), 1S
(G), and 18 (B) of IC5101.
ICS101 also contains an automatic brightness limiter
(ABL) circuit. A voHage sample proportional to the
average CRT anode current from the deflection
board is applied to pin 1 of IC5101. As the average
anode current increases, the voltage at pin 1
decreases. The ABL circuit is designed so that as the
average anode current exceeds 750 ~, the gain of
the video signal is decreased. The resulting negative
feedback loop limits the maximum average anode
current to 750 ~. This circuit is also referred to as a
beam current limiter.
Video Amplifiers
After the analog video input signals are processed by
ICS1 01, they are further amplified before being ap­
plied to the color guns of the CRT. This amplification
occurs in three stages, each of which is discussed
individually in the following sections. In addition, the
DC component of the color signals is restored here.
Cutoff, retrace supression, and black level circuit de­
scriptions are also included here.
Circuit Descriptions
Page 5-3