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Theory of Operation
3–14
1780R-Series Service Manual
Chrominance Filter. The chrominance filter is a Lerner-type filter, exhibiting
arithmetic symmetry and linear phase across its bandwidth. U827 amplifies the
filtered chrominance signal to drive both the Waveform Monitor and Vectorscope
circuits.
Differentiated Step Filter. U840A and B with associated circuitry form an active
low-pass filter. U840C, D, and E form an operational amplifier. C838 and R837
are connected as a differentiator or highpass filter.
Auxiliary Input. AUX IN is switched into the signal path by saturating Q922.
When not selected, Q1022 is on to maintain the 75 W input impedance.