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Theory of Operation
3–96
1780R-Series Service Manual
from the vector intensity current source, U383C. Vector intensity is blanked if
any one of the inputs to U289A is low. U486D steers current from the readout
intensity current source, U486C. The display is blanked when no current is
diverted into the output, which drives the Vectorscope Z-Axis Amplifier. Q380 is
saturated to shunt the video signal to ground, unless a video signal is summed
into the vector brightness circuit in the picture monitor mode of operation.
Waveform Z-Axis Amp. The junction of R396 and R296 is the summing junction
for this amplifier. It is at +5 volts DC. R397 and R498 are a voltage level shifter
to bias the base of Q499 at 0 volts, when the summing junction is at +5 volts.
R296 is the feedback resistor, which sets the overall amplifier gain at 36
volts/mA of input current. Q499 is an emitter follower that drives Q498, a
common emitter amplifier. Q298 is a common base stage driven from Q498. The
collector of Q298 is the output of the amplifier. Q199 is a constant current source
that is the collector load for Q298. C299 is a speed-up capacitor that modulates
the constant current source to increase amplifier rise time.