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Theory of Operation
3–82
1780R-Series Service Manual
present on the sync backporch. This effectively locks out black and white signals
or the vertical interval, where there is backporch but no burst.
U383B is one half of a dual comparator that samples the output of the Mixer
stage low-pass filter. Its output is low for phase lock and pulses high for an
unlocked condition.
U475 is a triple, three-input NAND gate used as a pulse gating circuit. U475A
generates the lock/unlock levels, that after peak detection control the Pulse
Generator. U475B provides an output that phases up the PAL signal+V/–V
flip-flop (U472). U475C generates an enable signal that allows the Pulse
Generator to operate only on +V bursts for PAL operation and is inoperative for
NTSC.
In PAL operation the Pulse Generator provides the controlling current pulse to
the Sampling Transconductance Error Amplifier, and is only enabled during the
lines containing +V. To accomplish this, and to provide proper phase + and –V
pulses to the field selection circuits (for PAL operation), jumper W480 connects
the /Q output from U472A to inputs for U475A and C in order to provide a high
for +V lines. When W480 is not in place (NTSC operation) the same inputs are
held high by pull-up resistor R480. U475B generates a clear signal for U472A,
to ensure the correct phase of the + and –V pulses.
When an unlocked condition occurs, U383B output goes high, which ANDs
U475A to pull its output low. A low on the output of U475A discharges C483,
which is normally pulled high by R379. When the level on the + input of U383A
goes below the comparator threshold level set by the – input, the output goes low
turning off Q370. DS184 will come on only if there is a condition where lock
should occur, but does not.
For PAL instruments the Pulse Generator does not sample on the lines with –V
burst. On a +V line U475C is ANDed to pull CR273 low and turn on Q270. On
–V lines U475C is not ANDed holding its output high to turn off Q270, which
turns off the Pulse Generator, making the phase-lock loop sample only on +V
lines.
Pulse Generator. The Pulse Generator is made up of Q269 and Q270. Current
amplitude to the Sampling Transconductance Sampling Input is controlled by
Q370. When Q370 is off the current in the collector circuit of Q269 will be high
and the Sampling Transconductance Amplifier gain will also be high which
increases the loop gain (bandwidth). When lock occurs, Q370 saturates and
collector current in Q269 is reduced, which lowers the current to the sampling
input of U179 and reduces loop gain.
EXT CW Input. Q594 and Q493 form a Difference Input Amplifier. It can common
mode out ground currents from the rear-panel EXT CW INPUT bridging
loop-through. C596, L592, L491, and C394 are a bandpass filter. The value of