Filter
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Theory of Operation
1780R-Series Service Manual
3–9
hum is not attenuated. The DC Restorer is turned off by the absence of the
sample pulses at the sample input (pin 5) of U1217. Hold capacitors C1216 and
C1313 discharge to 0 volts through R1314, and the emitter of Q1215 stays at
0 volts.
Operational amplifier U1211 (along with C1212, C1211, R1112, and R1216) is
an active low-pass filter that stabilizes the characteristics of the DC Restorer
loop. Q1215 provides a low output impedance for the filter at higher frequencies.