Theory of
Operation—465B
Service
C4248
Figure
3-8.
Detailed block diagram of the Horizontal Amplifier circuit.
X-Axis
Amplifier
In all positions of the TIME/DIV
switches
except
X-Y,
the input signal to the base
of U4269A will
be
sawtooth
waveforms from the sweep generators. In the X-Y position,
however, the sweeps are disabled, and the signal
applied
to Q4269A comes from the
Channel
1
Preamplifier
via the
X-Axis Amplifier. This stage includes Q4284, Q4285 and
associated
circuitry.
Transistor Q4284 is connected as a feedback
amplifier,
with R4285 as
the feedback element. The input
resistance
is
made
up
of R4384 and the gain-setting
adjustment of
R4381. When not operating in the
X-Y Horizontal Display,
the base
of Q4284 rises toward the
+15
volt supply, but is
clamped at
approximately +5.7 volts
by
CR4287 and
R4287. This reverse
biases the base-emitter junction of
Q4284.
The base of Q4285 also rises to
approximately
+5.7 volts and
with the junction of R4281-R4280 at
approximately
0
volt, Q4285 is biased off.
When the A TIME/DIV switch is set to
X-Y position (fully
counterclockwise),
-8
volts is applied to the
junction
of
R4382 and
R4383.
In
addition, +5 volts is applied to the
emitter
circuit
of
Q4285 through CR4285.
This
biases the
X-Axis Amplifier into conduction. The +5 volts is also
applied to pin 18
of U7375
in the Sweep and Z-Axis Logic
circuit to disable sweep
generation.
Input
Paraphase Amplifier
The Input Paraphase
Amplifier
is
comprised of U4269A
and U4269B (part of a
transistor
array).
This is an emitter-
coupled amplifier stage that converts the
single-ended
input signal to
a
push-pull output
signal. The signal at the
collector of U4269A
is
opposite in phase
to the input
signal, while the signal
at
the collector of U4269B is in
phase with
the input signal. Thermistor
RT4373 reduces
its value with increases in
ambient temperature to in-
crease the gain of the
stage. This compensates
for slight
changes in
amplifier gain that occur as
operating
temperatures
vary.
The Horizontal POSITION potentiometers, R14288A
(Coarse) and
R14288B (Fine),
are
mounted on the same
shaft in
a
mechanical arrangement that allows R14288Bto
rotate about
one-eighth
turn in either
direction before
R14288A moves. The Fine Potentiometer has
approxi-
mately one-tenth the range of the Coarse potentiometer.
Gain Setting Cascode Amplifier
A cascode
push-pull amplifier stage is made up of
Q4273, Q4274, U4269C
and U4269D. The gain of the
Horizontal Amplifier
is
controlled by
adjusting the
resistance
connected between the emitters of Q4273 and
Q4274. The XI Gain adjustment,
R4274,
is
used to adjust
the
unmagnified horizontal
gain
and the X10 Gain adjust-
ment, R4373,
is
used to adjust the magnified
horizontal
gain. Magnifier Registration adjustment, R4275. is used to
balance the quiescent dc
current
in Q4273 and Q4274 so
that a
center screen display does not change positions
when switching between magnified
and
unmagnified
display.
REV
B MAR 1980
3-19
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