TM 11-6625-3025-14/ET426-AA-MMA-010/E154 VII210/T.O. 33A1-8-902-1-1
5. Turn the VIDEO LEVEL control to 0. The video portion should no longer be visible except for a slight residual
level of the window signal, not exceeding 0.05v. The blanking and sync levels should not have changed.
Black level for other functions must be checked to match the black step of the gray scale.
Select the GRAY SCALE function. While observing the waveform at horizontal rate, VIDEO LEVEL at 10, place
the black step (first on the left, to the right of the horizontal blanking interval) at the center of the oscilloscope graticul e by
use of the oscilloscope horizontal and vertical position control. Make sure oscilloscope input is direct (dc) coupled for this
check. Select the V STRIPE function. The black and white levels of video should not shift. If any shift is observed, black
level must be adjusted.
6. If adjustment of any signal parameter is indicated, use control R11 of the video card (figure 6-8) to adjust the
sync portion of the signal to 0, and using control R46 of the video card, adjust the blanking pedestal to 0. The only signal
visible would be the video portion of the gray scale signal.
7
Adjustment of the black level clipper is necessary to match the black level of digitally derived signals to the
black level of the gray scale, which is not adjustable. The optimum setting is a compromise at best, and misadjustment
can cause signal distortion. Common problems to watch for include:
(a) Black level clipper too low - pedestal will appear higher on bar, dot, window and related signals as
compared to the gray scale signal, and the pedestal level will appear to vary as VIDEO LEVEL is turned
up and down. The signal will not go completely to zero level as the VIDEO LEVEL control is turned fully
counterclockwise to 0.
(b) Black level clipper too high - bottom of gray scale will "push down" into the sync portion of the signal, and
it may be necessary to turn the VIDEO LEVEL control clockwise from 0 some amount before the bar,
dot, window and related signals will appear from black level. Extreme misadjustment may cause lower
steps of the gray scale to be clipped or compressed.
If adjustment is necessary, select gray scale. Turn VIDEO LEVEL to 10 and recheck the vertical position of the
black level step on the oscilloscope graticule. Select V STRIPE and observe waveform while adjusting the black level
control R18 on the video card. The V stripe black level will shift up and down as the control is turned clockwise and
counterclockwise. Carefully adjust it to the point where the black levels are the same, as observed while switching back
and forth
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