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Theory of Operation
1780R-Series Service Manual
3–21
Diagram 8 Reference Sync Generation
Both Internal and Reference sync inputs are filtered by active low-pass filters.
The Internal Sync Separator is a descrete clamped operational amplifier circuit
that is clamped just above sync tip level to clip off active video.
The Reference Sync Separator is monolithic providing both composite sync and
AGC video outputs. Its AGC output is used to trigger the slow sweep circuits.
The Reference Sync Separator accommodates sound-in-syncs by replacing the H
sync pulses with a fixed 5 ms pulse.
A Sync Locked Oscillator outputs a burst of subcarrier rate sine wave that can be
adjusted to start between 70 and 400 ns after the leading edge of sync. A Mixer
compares this signal to the color burst, outputting a pulse when the 50% point of
sync corresponds to a negative zero crossing of subcarrier. The sync phase output
pulse is used to drive color framing logic.
Internal Sync Separator. The internal video is input through a 1.5 MHz active
low-pass filter made up of Q1251, R1248, R1249, C1247, and C1251. The
filtered video drives the Internal Sync Separator, which is a high gain inverting
operational amplifier and a feedback clamp. The clamp adjusts the DC level of
the operational amplifier so that video just above sync tip causes it to saturate
and clip off the video. Q1152 and U1159D form the operational amplifier with
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Circuit Theory