Theory of Operation—2205 Service
The preamplifier is a cascode differential pair.
The gain is set by the network connected between
pins 7 of U755C and 10 of U755D. With MAG switch
S6Q1 in X i, the XI MAG line is high, Q760 and
U745E are off. and the current sources consisting of
U745A and U755A are on for normal operation with
the gain set by R763. When MAG switch is X I0, the
Xt MAG signal is LO, Q760 and U745E saturate, and
the current sources consisting of U7558 and U745B
are on. This forward biases CR773 and CR774. a low
impedance is switched in, and the X I0 timing adjustî™–
ment is made using X I0 Mag Gain R777. Magnifier
registration is adjusted by R782 so that there is no
horizontal trace shift when switching between the
XI0 and XI positions. The XI MAG line is held at a
cN ^4 nt jligh when in X~Y mode.
The sweep signal, or the X-Axis signal (deî™–
pending on mode of operation) with the horizontal
position signal from R726, is buffered by Q750 and
Q759 and applied pin 6 of U745C. Pin 9 of U745D is
held to a fixed voltage level by Q725. The horizontal
position signal adjusts the trace horizontally in both
the sweep and X-Y modes.
Horizontal Output Amplifier
The Horizontal Output Amplifier provides final
amplification of the horizontal signal to drive the
horizontal crt deflection plates.
Signals from the Horizontal Preamplifier circuit
are used to drive two shunt feedback amplifiers. The
feedback makes the input impedance of these
amplifiers low. The base voltages of Q770 and Q780
are at nearly the same dc level due to forward-
biased diodes CR781 and CR791 between the two
emitters.
Transistors Q770, Q775, and Q779 form a casî™–
code feedback amplifier for driving the right
horizontal crt deflection plate with R784 and R785
setting amplifier gain and C783 providing high freî™–
quency compensation. For low-speed signals Q779
serves as a current source for Q775, and at high
sweep rates, the ramp is coupled through C785 to
the emitter of Q779. This provides additional pull-up
output current to drive the crt at high sweep rates.
The amplifier, consisting of Q780, Q785, and Q789,
drives the left horizontal crt deflection plate as
described above with zener diode VR792 level
shifting the collector signal of Q780.
The BEAM FIND function is turned on when S390
is closed and the Beam Find signal is connected to
the negative supply. Q776 saturates, the cathode
voltage of VR776 goes negative, and CR780 and
CR890 become forward biased. Current though
these diodes causes the output common-mode
voltage of the two shunt-feedback amplifiers to be
shifted positively to reduce the available voltage
swing at the crt plates. This prevents the trace from
being deflected off-screen horizontally.
Z-AX1S AMPLIFIER
The Z-Axis Amplifier, shown on Diagram 7, conî™–
trols the crt intensity level via several input-signal
sources. The effect of these input signals is either to
increase or decrease trace intensity or to
completely blank portions of the display. The Z-Axis
signal current, as determined by the Z-Axis
switching logic and the input current from the EXT
INPUT OR Z connector (if in use), are summed at the
emitter of common-base amplifier Q825 and
thereby sets the collector current of the stage. The
common-base amplifier provides a low-impedance
termination for the input signals and isolates the
signal sources frorh the rest of the Z-Axis Amplifier.
Common-base transistor Q829 passes a conî™–
stant current through R832. This current is divided
between Q825 and Q829, with the portion through
Q829 driving the shunt-feedback output amplifier
formed by Q835, Q840. and Q845. The bias level of
Q825 therefore controls the emitter current available
to Q829. The shunt feedback-resistors of R841 and
R843 sets the transresistance gain for changing the
input current to a proportional output voltage.
Emrtter-foHower Q835 is dc coupled to Q840 and,
for low-speed signals, Q845 acts as a current
source. Fast transitions couple through C845
providing added current gain through Q845 for fast
voltage swings at the output of the Amplifier.
External Z-Axis input voltages establish proî™–
portional input currents through R823, and Amplifier
sensitivity is determined by the transresistance gain
of the shunt-feedback amplifier.
The INTENSITY potentiometer controls the base
voltage of 0804 to set the amount of emitter current
that flows through that transistor and, therefore, the
level of the Z-Axis signal.
When the sweep is displayed, the emitter of
Q817 is LO. causing CR817 to be reverse biased.
This allows the current through R818 to flow through
CR818 and turn on the Z-Axis.
When X-Y is displayed, CR817 is forward
biased, reverse biasing CR818. Transistor Q818 is
reverse biased allowing the intensity to be set by the
current through R820 and CR820.
When CHOP VERTICAL MODE is selected, the
CHOP BLANK signal is sent to the collector of Q825
through CR824 during the display-switching time.
Oiode CR825 is reverse biased and the forward bias
3-12
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