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Theory of Operation
1780R-Series Service Manual
3–69
for a DC level of 1 volt, with no chroma, to accommodate the Recursive Filter
(Diagram 35).
Demodulator Switching. U386A and B control the selection of the demodulators.
For either dG or dP the Preset and Clear for U386B controls the selection of the
demodulators. When dP is selected it is high and dG is low, which presets
U386B and sets /Q low and turns on the Differential Phase Demodulator (U267).
When dG is selected dP goes low and clears U386B which sets Q low and turns
on the Differential Gain Demodulator.
When alternate dP and dG is selected, both the dP and dG lines are pulled high,
and U386A controls the selection of which demodulator is on and dictates the
correct sequence (dP on the first half of sweep and dG on the second half). The
default sweep for alternate dP and dG is a 2-line ramp. Near the end of ramp
time the Ramp Generator (Diagram 10) outputs a TRIG EN pulse. The positive-
going pulse is shortened by a half-shot, Q677, and presets the Q output of
U386A high. On the next 2-line sweep, the high is clocked into U386B, setting
/Q low and turning on the Differential Phase Demodulator. At the same time
U386A has toggled so that its Q output is low. Thus the following H sync,
occurring in the middle of the 2-line sweep, clocks this low into U386B, causing
the outputs to reverse state and turn on the Differential Gain Demodulator.