Filter
Top Products
Theory of Operation
3–72
1780R-Series Service Manual
deflected away from the center, one or two of the comparators will drive a
summing point, at the emitter of Q323 (a common base amplifier) and turn it on.
When Q323 turns on, its collector goes low to disable U327C. Whenever there is
no deflection and the display is centered, Q323 is turned off and a high is applied
to one input of U327C. If no center dot blanking is desired, P321 (Center Dot
Blanking Disable) can be moved to pins 2 and 3 of J321.
Horizontal Sync Pulse Shaper. U695A is a monostable one-shot that regenerates a
horizontal sync pulse from the approximate 1/2 line input H sync. C695 and
R693 are the timing elements for the one-shot, which is triggered at the leading
edge of input H sync.
Center Dot Blanking Logic. The gates in U327 and U333 make the blanking
decision. When the output of U333C is low, which occurs when either input goes
high, the CRT is blanked. U333D is used as an inverter for the Horizontal Sync
Pulse Shaper-regenerated sync pulse.