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Theory of Operation
1780R-Series Service Manual
3–73
Diagram 28 Demodulators
Chrominance from the Gain Cell is clamped to 0 volts. The Vertical and
Horizontal Center Dot Clamps provide a fixed bias level to the demodulators
when the Chroma Clamp forces the signal input to 0 volts.
The Vertical Demodulator (R-Y or U) uses the QFSC signal from the Subcarrier
Regenerator as the carrier or reference for demodulation. The Horizontal
Demodulator (B-Y or V) uses the IF
SC
from the Subcarrier Regenerator as its
reference.
Demodulator outputs are low-pass filtered by active, 500-kHz, low-pass filters.
Chroma Clamp. Chrominance from the input Gain Cell (Diagram 25) is clamped
at 0 V when center dot blanking occurs. Q645 is turned off at Chroma Clamp
pulse time. When Q645 turns off Q742 turns on and clamps both demodulator +
signal inputs to ground.
Center Dot Position Clamps. The R-Y Amplifier output is fed back to the Vertical
Center Dot Clamp through R235. The clamp circuit consists of U138 and Q135.
The B-Y Amplifier output is fed back to the Horizontal Center Dot Clamp,
which consists of U337 and Q342, through R436. U138 and U337 are operation-
Overview
Circuit Theory