Theory of Operation
4–6
1720/1721
junction of a differential amplifier (Q293 and Q292) that drives the Output
Amplifier (Q192). The feedback resistor that sets the gain is R293.
The External Reference Input Amplifier is nearly identical to the channel inputs
and provides a signal to the reference switch. R994 is the input resistor and
R992 is the feedback resistor, the combination of which sets the gain to 1.
The Gain Cell, whose output is chrominance, consists of U184, U383, Q385, and
Q490. A band-pass filtered (L185 – C185) signal is fed into pin 6 of U184.
U184 consists of a current source, differential amplifier pair, and a pair of
transistors connected as diodes. U383 consists of four transistors connected in a
cross-coupled gain cell arrangement with a transistor used as a heater to keep
U383 at the same temperature as U184. The diode-connected pair in U184 is a
current-to-voltage converter that drives the gain cell transistors in U383. U380 is
an operational amplifier that drives the gain port of the Gain Cell. U274 is a
Microprocessor-controlled switch that enables the Variable Gain, Gain Cal, and
the 75% or 100% color bar amplitude gain settings.
DIAGRAM 2 SUBCARRIER REGENERATOR
E
Voltage
controlled
oscillator
To phase
shifter
Ext ref
video
CHRM
L
C
Burst
clamp
Loop
phase
detector
Quad
phase
detector
Back
porch
sample
Sync
separator
Bowes
osc
H
H
Incoming signal, from the Input Amplifiers, is amplified and fed to the Lumi-
nance and Chrominance Amplifiers. Output of the Luminance Amplifier drives
the Sync Separator, which generates the H Sync used throughout the Vector-
scope, and the Back Porch Sample that enables the Phase Lock Loop.
Gain Cell
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