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Theory of Operation
1780R-Series Service Manual
3–37
Diagram 13 Slow Sweep & Ext Horiz Input
The External Horizontal Input Amplifier is a non-inverting amplifier that inputs
a ramp voltage of ≈5 volts to produce a 10-division sweep ramp.
The Slow Sweep Ramp Generator produces a ramp that is triggered by an APL
change through 50%, or free-runs after about 10 seconds when no trigger has
occurred. AGC video is sent from the Reference Sync Separator to detect APL
changes. The internal reference is required to have the Sync Separator receive the
same signal displayed on the CRT.
Ext. Horizontal Input Amplifier. Whenever the /EXT HORZ EN is high (not
enabled) Q1595 is turned on and all signal current is returned to the –2.2 volt
supply. In addition, when the line is low the Output Amplifier is pulled down
enough to back bias the switching diode at the input to the Mag Amplifier on
Diagram 11.
The amplifier itself is a non-inverting amplifier with a gain of ≈4.5. Gain is
determined by the current flowing in the emitter of Q1594. CR1593 and CR1597
are input protection, setting the maximum input amplitude to ±12.5 volts.
Slow Sweep Trigger. The AGC video, from the Reference Sync Separator, is
inverted video with the chroma filtered off. R761 (Slow Sweep Trig Level) is
adjusted so that video with a 50% APL produces approximately 0 volts on C661.
C661 maintains this APL through the vertical interval.
If the Slow Sweep Trigger is set to + an increasing APL change, through 50%,
passes through a 0 volt threshold set on U458B by the SLOW SWP +/– control
line producing a low on the – input of U458B. As the APL changes from below
Overview
Circuit Theory