Filter
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Theory of Operation
3–6
1780R-Series Service Manual
inverting input, pin 2 of U418) form a low-pass filter, with a notch at the
subcarrier frequency. U418 drives C543, which holds charge during the active
portion of the video line. When Q408 is on, C512 is in parallel with C521, and
the DC Restorer response is slowed so that 60 Hz (50 Hz for 1481) hum is not
attenuated. The DC Restorer is turned off by the absence of the sample pulses at
the sample input (pin 5) of U418. Hold capacitors C521 and C512 discharge to
0 volts through R410, and the emitter of Q320 stays at 0 volts.
Operational amplifier U306B (along with C412, C411, R411, and R415) is an
active low-pass filter that stabilizes the characteristics of the DC Restorer loop.
Q320 provides a low output impedance for the filter at higher frequencies.