Class Ab Output Stage; Cutoff; Retrace Line Prevention; Black Level - Zenith ZCM-1490 Service Manual

Class AS Output Stage
The amplified video signal at the collector of output
transistor OS206 is applied to a pair of emitter follow-
ers (OS207 and OS208). Diodes CRS20S and
CRS206 force the complementary pair to operate in a
class AB mode, thereby reducing crossover distortion
as compared to class B operation. This stage iso-
lates the cathode capacitance from the collector of
output transistor OS206 and provides a low-
impedance drive for the cathode clamp. (Isolating the
cathode capacitance is necessary to mitigate the ef-
fect of excess capacitive reactance. Because capaci-
tive reactance is inversely proportional to frequency,
it can reduce the load impedance seen by the output
amplifier, thereby reduCing the gain of the amplifier.)
The output of this stage is AC-coupled to the CRT
cathode (the green gun) by CS302.
Cutoff
After the output is AC-coupled to the CRT cathode,
the DC component is restored by the gated clamp
circuit formed by transistors OS304, OS305, and their
associated circuitry. A positive TTL-level compoSite
blanking pulse is applied to ICS102, where it is buf-
fered and appears. at pin 3. This signal is applied to
the base of OS304. With a positive-going pulse at its
base, OS304 produces a negative-going pulse at its
collector. This pulse appears at the base Of OS30S
after passing through the green cutoff adjustment
(RS317). The OS30S emitter is coupled to the CRT
cathode through CRS30S and RS30S. Thus, the cath-
ode is forced to the peak level of the gate pulse at
the anode of CRS30S during blanking time. The
cutoff control determines the DC operating point of
the cathode by setting the clamping level.
Retrace Line Prevention
In addition to the video blanking circuitry, a retrace
supressor
~ircuit
is included so that the retrace lines
produced during flyback do not appear on the
screen. A buffered composite blanking pulse from pin
11 of ICS102 is applied to the base of OS1 01. The
output of this emitter-follower stage. is applied to
diodes CRS203, CRS207, and CRS211. In the pres-
Circuit Descriptions
ence of a pulse, these diodes conduct, thereby steer-
ing current away from the cascode output amplifier
transistors and shutting them off. This causes a posi-
tive blanking pulse to be superimposed onto the
cathode video signal during retrace times, thereby
preventing a retrace line from being displayed. In ad-
dition, this action provides a pedestal for the cutoff
circuit clamp.
Black Level
A black level control establishes the difference be-
tween black picture information and blanking pulses
in the video Signal. A buffered blanking pulse from
pin 8 of ICS102 is coupled to the variable pedestal
generator circuit formed by OS102, OS104, and as-
sociated circuitry. The output of this generator ap-
pears at the emitter of 05104. During blanking time,
a fixed voltage of 7.3 VDC appears at the emitter.
During trace time, this voltage is variable, based on
the DC voltage applied to the base of 05102. The
external brightness control is a voltage divider sup-
plying this voltage during trace time. The range of the
supplied voltage is 7.3 VDC to S.3 VDC. The result-
ing pedestal voltage at the emitter of 05104 is
summed to the three video outputs of ICS101.
Sync Input Processing
The horizontal and vertical sync input processing cir-
cuitry includes sync input buffering, mode (horizontal
scan line) selection, and sync waveform conditioning
Circuits. In addition, horizontal and vertical size,
phase, and hold controls are located..here.
Mode Selection
The ZCM-1490 can operate in one of three video
. modes. In mode 1 (3S0 scan lines), the monitor can
produce an EGA-type display. In mode 2 (400 scan
lines), the monitor can produce a CGA-type display
(double-scanned). In mode 3 (480 scan lines), the
monitor can produce a VGA-type display. In all three
modes, the video source supplying the monitor must
be a 31.49 kHz analog RGB Signal.
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