East-West Pincushion; Crt Cutoff; Crt Gun Waveforms - Zenith ZCM-1490 Service Manual

East-West Pincushion
The east-west (E-W) pincushion adjustments control
the symmetry of the left and right halves of the dis-
play. To adjust the E-W PIN:
1. Display a crosshatch pattern in the 480-line
mode.
2. Adjust the E-W level control to reduce PIN dis-
tortion in the vertical lines. Use the edges of the
monitor bezel as references to obtain the most
symmetrical display possible.
3. Adjust the E-W trap control (R7012) to eliminate
trapezoidal distortion in the display. Use the
edges of the monitor bezel as references to ob-"
tain the most symmetrical display possible.
4. Locate the vertical lines nearest the left and
right edges of the display.
5. Adjust the E-W phase control (R7039) to obtain
. the straightest possible vertical lines along the
left and right sides of the display.
6. Use both the crosshatch pattern and the fill
screen test and verify that the E-W adjustments
are correct. Repeat steps as needed to obtain
the best possible display.
CRT Cutoff
The cutoff controls (R5316, R5317, and R5318) set
the DC level at which the CRT is cut off during blank-
ing and retrace times. The cutoff controls shift the en-
tire waveform viewed at each of the three CRT guns
up or down, thereby setting the level of the cutoff
pulses (the rectangular pulses in the waveforms of
Figure 4-6). Perform this adjustment in a dimly lit
area. To adjust the cutoff controls:
1. Use the fill screen test to
fill
the display area
with the capital Z.
2. Set the external contrast control to maximum.
3. Set the external brightness control to minimum .
GREEN CRT WAVEFORM RED CRT WAVEFORM
BLUE CRT WAVEFORM
Adjustments
FOIL SID!; OF VIDEO
OUTPUT BOARD
Figure 4-6. CRT Gun Waveforms
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