Blanking Pulses; Pincushion Correction; East-West Waveform Generator; Blanking Pulse Circuit - Zenith ZCM-1490 Service Manual

Blanking Pulses
The . composite blanking pulse signal is generated
from the vertical and horizontal flyback pulses. Tran-
sistors 02117, 03401, and associated circuitry pro-
duce these blanking pulses. Refer to Figure 5-4. The
vertical pulse is AC-coupled to the blanking circuit by
C2118. Zener diode CR2116 limits this pulse to 5.6
V. The pulse is applied to the base of 02117, activat-
ing it and, in tum, bringing the base of 03401 to
ground. With 03401 now off, +5 V appears at the col-
lector for the duration of the vertical blankirlQ interval.
Similarly, the -70 V horizontal flyback pulse is AC-
coupled by C3413 to the base of 03401. A negative
pulse shuts off 03401, resulting in a +5 V blanking
pulse at the collector for the duration of the horizontal
blanking interval.
Diode CR3404 conducts during the retrace portion of
the hClrizontal flyback pulse to prevent reverse-bias
damage to 02117 and 03401. During the trace por-
tion of the horizontal flyback pulse, CR3404 conducts
and holds the collector of 03401 low. Thus, a com-
poSite blanking pulse appears at the collector of
03401.
r-------n.~
R2176
3.'"
.....
Figure 5-4. Blanking Pulse Circuit
Pincushion Correction
As noted earlier, this monitor incorporates the new
fI?t technology CRT. Because this is a perfectly flat
display, a more complex pincushion (PIN) correction
must be performed to produce a geometrically cor-
rect display. The PIN circuitry can be broken into four
functional blocks: an east-west waveform generator,
an east-west regulator, a north-south waveform gen-
Page 5-10
erator, and a north-south output circuit. Each block is
explained individually in the following sections.
East-West Waveform Generator
The east-west (E-W) waveform generator produces
the PIN correction waveform for the left and right
sides of the display. The resulting waveform modu-
lates the B+ voltage supplied to the horizontal deflec-
tion circuitry at the vertical scan. rate. In this way, the
left and right sides of the display can be corrected as
the beam deflects vertically.
The E-W waveform generator produces three
waveforms which are then combined to form the final
E-W PIN correction waveform. A parabolic waveform
is generated to correct parabolic distortion at the left
and right sides of the raster. A ramp waveform at the
vertical scan rate is added to the parabola to correct
trapezoidal distortion at the left and right sides of the
raster. A sine wave at the vertical scan rate is gener-
ated and added to the parabola to correct phase er-
rors in this waveform. Refer to Figure 5-5.
.-
ZV25V
-=---:ji~-H
.,..
0701'
...
01033
......
C1Ol2
. .
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.
Figure 5-5. E-W Waveform Generator
A multiplier circuit (IC7001) is used to generate the
parabolic waveform. The multiplier produces an out-
put that is a product of its input terms (the X and Y
input signals) and a constant (denoted by "K'j. A ver-
tical ramp at pin 5 of connector 8U6 is buffered by
transistor 07505. This ramp is coupled to the multi-
plier inputs of IC7001 by C7001, R7002, and R7014.
Circuit
Description~