Vertical Deflection; Vertical Centering - Zenith ZCM-1490 Service Manual

Vertical Deflection
The vertical deflection circuitry is responsible for gen-
erating the scanning current needed in the vertical
deflection coils to fill the raster from top to bottom.
The vertical sawtooth waveform is sensed by R2127
and applied to IC2101 pin 6. A sample of the vertical
output is also fed back to this IC. Diodes CR2114
and CR2115 limit the swing of the Signal input ap-
plied to IC21 01. The output of this IC is applied to
driver transistor 02106.
The output at the collector of 02106 is applied to the
input of the complementary-symmetry amplifier
formed by 02107 and 021 OS. Note that 02107 is a
PNP-type transistor, while 0210S is an NPN-type.
The complementary-symmetry, or push-pull action of
these two transistors occurs as follows. For a
positive-going sawtooth at the base of 02106, a
negative-going drive is applied at the base of 02107,
increasing its collector current. The same negative-
going drive applied at the base of 021 OS reduces the
forward voltage at the base, resulting in less collector
current for this NPN transistor.
Similarly, when a negative-going sawtooth is applied
to the base of driver 02106, a positive-going drive is
applied at the base of 0210S, increasing its collector
current. In a like manner, less collector current re-
sults in 02107.
Diodes CR2107 and CR210S set the crossover bias
and determine the time that both 02107 and 021 OS
will be active during the middle portion of the saw-
tooth.
To quicken the vertical retrace action, a "boosted"
voltage is generated for retrace. As the yoke voltage
rises, 02110 senses the rise in voltage at its base
and activates. This in turn activates retrace switch
transistor 02109. During the scan time prior to
Circuit DeSCriptions
retrace, C2119 in the emitter circuit of 02109
charges to about +72 V through RX2139, RX213S,
and CR2112. When 02109 is activated at the start of
retrace, the stored voltage at C2119 is applied to the
collector of 021 OS. At this point, 021 OS is conducting
and the voltage is transferred to the yoke. The
"boosted" voltage causes retrace to occur much
faster than it otherwise WOUld.
Diode CR2109 couples +16 VDC to the collector of
0210S during scan time; during retrace time, it is
reverse-biased as the boosted voltage is applied.
CR2113 protects the base-emitter junction of 02109
against reverse-bias damage. CR2114 and CR2115
protect IC2101 from static discharge at the inputs.
Capacitor C2112 performs two functions. First, it in-
creases the load impedance in the collector circuit of
02106. Because the output circuit may produce
crossover distortion with large Signals, a large
amount of feedback is incorporated to compensate
for this. To accomodate this feedback, more gain is
required. C2112 supplies positive feedback, increas-
ing the load impedance presented to driver Q2106,
and thereby increasing the gain. Second, C2112
maintains 0210S in a conducting state at all times.
ThisJs necessary because large positive peaks in the
signal can cut 0210S off by placing the base and
emitter at the power supply potential. The voltage
stored across C2112 while the circuit is idling keeps
021 OS conducting at all times.
Vertical Centering
The vertical centering control (R2148) electrically
centers the display from top to bottom within the
raster area. R2148, 02111, and 02112 form a volt-
age divider. Electrical centering is accomplished by
supplying direct current through the vertical deflec-
tion coils.
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