Theory of Operation
1720/1721
4–5
Circuit Descriptions
The following descriptions are divided by diagram number and then further
subdivided by logical circuit blocks. The descriptions follow the order of the
diagrams in Section 9. Individual diagrams can be folded out and consulted
while studying these descriptions.
DIAGRAM 1 INPUT AND DEFLECTION AMPLIFIERS
EXT REF
CH-B
CH-A
CH A
CH B
Gain
cell
GAIN CAL
EXT REF
VIDEO
VARIABLE
CHROMA
Video signal input to the 1720/1721 Vectorscope is through high-impedance
bridging loop-through inputs. Gain is normalized by the Input Amplifiers prior
to being input to the Gain Cell. The amount of amplification provided by the
Gain Cell is controlled by the Gain Cal, Variable, and the choice of 75% or
100% amplitude color bars.
The External Reference input is also a high impedance bridging loop-through,
which is dc coupled to a unity gain operational amplifier.
The Channel A and Channel B input buffers are ac coupled (C199 and C497),
grounded base amplifiers with Q297 and Q493 as the active elements. Q391 is a
saturating switch that shunts current away from the Channel B input when
Channel A is being displayed. Q296 serves the same purpose when Channel B is
being displayed. R594 and C198 are adjustments that match the phases and
gains of Channel A and Channel B. They modify the input resistance of
Channel B and the input capacitance of Channel A. Signal current flows through
either CR297 or CR496 (depending on channel selection) into the summing
Video Input Amplifiers
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