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Theory of Operation
1780R-Series Service Manual
3–109
Focus Amp. The Focus Amp consists of Q380 and Q381 with R379 as the input
resistor and R382 as the feedback resistor. Coarse focus is adjusted with R182,
which is located at the Focus divider resistor string. The amplifier output varies
between 0 and –250 volts.
Line Select Focus. When the instrument is in line select, Q480 is turned on
during the selected line in a periodic manner. This causes a negative-going pulse
to appear on the collector, which is coupled to the CRT focus grid through
capacitors C480 and C280. The amplitude of the pulse, and thus the amount of
focus change, is determined by the setting of the Line Select Focus adjustment,
R384. DS480 and DS361 are spark arrestors, in case there is a high voltage arc.
Z-Axis Amp. The junction of R202 and R312 is a summing junction at +5 volts
DC. R301 and R300 provide a voltage level shift so that the base of Q401 will
be at 0 volts, when the summing junction is at +5 volts. R312 is the feedback
resistor, and sets the overall amplifier gain at 36 volts per milliampere of input
current. Q401 is an emitter follower driving a common emitter amplifier (Q400)
that drives Q210, a common base stage. The collector of Q210 is the output of
the amplifier. Q211 is a constant current source collector load for Q210. C410 is
a speedup capacitor to modulate the constant current source and increase
amplifier rise time.