3-83
TRIGGER BACK-PORCHCLAMP. The Trigger Back-
Porch Clamp circuit formed by US04,U410A, and associ-
ated components, is enabledfor a short time during each
horizontal-sync pulse immediately following the sync tip
(during the back-porch time). The output of the Trigger
Back-Porch Clamp is used to hold the back-porch level of
the composite-video signal to a predetermined dc level.
This, in combination with the action of the Sync-Tip
Clamp, produces sync pulses that are approximately 4 V
in amplitude.
DiodeCR502 acts to reduceAGC Amplifier gain quickly
if the negative-sync-tipamplitudeat the collector of U610B
drops below-0.8 V. If the diode becomesforward biased,
as it might should the signal amplitudego suddenly nega-
tive, OSlO will be turned on for a longer time until the sig-
nal amplitude returns to a lower level. Amplifier US10can
then increasethe channelresistanceof OS14more quickly
to reduce gain of the AGC Amplifier and return the sync-
tip amplitudeto the correct level.
The Sync-Tip Clamp holds the negative-sync tips at
about + O.SV, so the resulting sync pulses are approxi-
mately 4 V in amplitude.Anytime the negative-synctips at
the collectorof U610Bgo below about+O.SV, input pin 3
of US10 will go below the ground reference at the other
input. This causes the output of US10 to go low when
enabled, and CS12 begins discharging slowly toward
- 1SV. This decreasing voltage is applied to the gate of
FET OS14 to increase the channel resistance and
decrease the gain ofthe AGC Amplifier. Since US10is a
transconductance amplifier, it can change the voltage
across CS14only a small amount during each sync pulse,
and a few horizontal-linecycles are neededto reduce the
gain of the AGC Amplifier to the new operating level.
Between sync tips, when amplifier US10is disabled, the
long time constant of R610 and CS12 holds the bias for
OS14 (and thus gain of the AGC Amplifier) nearly
constant.
SYNC-TIP CLAMP AND AUTOMATIC GAIN CON-
TROL. Transconductance Amplifier U510, in conjunction
with the AGC Amplifier,is usedto clamp the sync-tip level.
Amplifier U510 is enabled by the bias current supplied by
OS12when sync tips turn that transistor on. This amplifier
acts as a weak operational amplifier to set the sync-tip
level constant when OS12 is conducting to supply bias
current to pin 5 of US10.
threshold level.The switching threshold is set by the bias-
ing resistors of OSlO, R408 and R409, to about SO%of
the sync level to eliminateany video information.The out-
put signal from the collector of OSlO is the composite of
all detected sync pulses, and the output of OS04 is an
invertedreplicaof that signal.
Theory of Operation-2430 Service
SYNC PICKOFF COMPARATOR. The Sync-Pickoff
Comparator,composedof OS04and OSlO,switches when
the amplitude of a sync pulse crosses the comparator
Transistor
0502
and its associated circuitry act as a
start-upcircuit that monitors the dc output level of U610B
and applies an offset voltage to pin 10 of U710C should
that level go below zero volts. This occurs when going
from a "no-signal" or low-signalcondition to a strong sig-
nal. If the dc output levelgoes belowground.diode CR612
will becomeforward biased,shutting off 0502. With OS02
off, the -lS V supply applied via resistor RS06 will for-
ward bias CR606 to chargeC713 negatively.This pullsthe
output voltage of U710C negative and decreases base
drive to U610B. Reducingbase drive reducesthe collector
current so that thecollector voltage of U610B returns
positive until the above zero-volt output level is restored
and CR612 becomesbiasedoff.
Emitter-followerU610Ddrives one input of a differential
amplifier made up of U610A and U610B, while the other
input is driven by the output signal of U710C. Transistor
U610C and its associated components form the current
source for the amplifier. The collector output of U610B
drives the input of the Sync PickoffComparator.
FIXED GAIN AMPLIFIER.The Fixed Gain Amplifier cir-
cuit, formed by U610A, B, and C,
0502,
and U710C,pro-
vides additional gain to the video signAl from the AGC
Amplifier. The Trigger Back-Porch Clamp circuit monitors
the back-porch level of the resulting signal and injects an
offsetting dc level into the Fixed Gain Amplifier via U710C
to shift that levelto approximately+4.5 V.
The base-emitterbias of the differential output transis-
tors within U612 are controlled by the NEG-SYNCsignal
from Video Option Control Register U750 (diagram20).
When the NEG-SYNCbin is set HI, the transistors con-
nectedto pins 2 and 9 will be biasedon, with those at pins
6 and 13 biasedoff. When NEG-SYNCis set LO, the con-
ducting transistors are switched, and the polarity of the
output signal driving transistor
0612
is inverted.
Common-basetransistor
0612
level shifts the output sig-
nal from the AGC Amplifier and provides voltage gain to
drive U610D.
The composite-video input signal is applied to oneinput
of the differential AGC Amplifier at pin 3 of U612 and to
Dc-Offset Amplifier U710B via a low-pass filter composed
of R714 and C714. The low-pass filter averages the signal
at the input of U710B so that only the average (dc) signal
level appears at the output of U710B and on pin 11 of
U612. Since the input signal swings about this average
level, the AGC Amplifier output signal will
be
centered in
its linearamplificationregion.
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