Theory of Operation—2213 Service
input current in a manner that complements the nonlinear
triode characteristics of the crt. This output voltage is
applied to the Z-Axis Amplifier via the Z-Axis Gating
circuit. The Intens Level signal is also applied to the Focus
circuit (Diagram 9) for use in focus tracking of the intensity
level changes.
The intensity of the display is allowed to reduce to zero
through the action of CR809, VR809, and R809. Without
this circuit, the Auto Intensity circuit would not allow the
intensity to go to zero when the AUTO INTENSITY
control is set to minimum intensity.
Z-Axis Amplifier
The Z-Axis Amplifier controls 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-Drive
signal current and the input current from the Z-AXIS
INPUT connector (if in use) are summed at the emitter of
common-base amplifier transistor Q841. The algebraic sum
of these signals determines the collector current of Q841.
Input transistor Q841 provides a low-impedance termina
tion for the input signals and isolates the signal sources from
following stages of the Z-Axis Amplifier.
Signal current from Q841 flows through CR844 and
develops a signal voltage drop across R844. Increasing
current through Q841 reduces the forward bias of Q844,
thereby reducing the current through Q844. This action
causes the collector voltage of Q844 to go more negative
(toward the —8.6 V supply) and increases the forward bias
on emitter-follower Q845. As emitter current of Q845
increases, negative-going voltage developed across R847 is
applied to the bases of complementary-pair output tran
sistors Q847 and Q850. Positive transistions of the Z-Axis
signal are coupled to the base of Q850 via C852. The fast-
rise transitions are amplified by Q850 to speed up the
response time. For negative transitions of the Z-Axis signal,
as well as for dc and low-frequency signal components,
Q847 acts as the amplifier, with Q850 supplying the
current.
Diode CR856 prevents the Z-Axis output signal from
going negative, and neon lamps DS854 and DS856 provide
protection to the Z-Axis Amplifier in the event of high-
voltage arcing in the crt.
The amplifier gain with respect to the Z-Drive current is
set to about 10 by the negative feedback supplied from the
collectors of Q847 and Q850 to the base of Q845 via
feedback resistor R846. The gain with respect to the
external Z-Axis Input signal is held to about three by R801,
R802, and R803 in series with the external input signal.
Diodes CR801 and CR802 provide protection for the
Z-Axis Amplifier in case of an accidental application of
excessive signal amplitude to the Z-AXIS INPUT connector.
When CHOP VERTICAL MODE is selected, the Chop
Blank signal is applied to the collector of Q841 during the
display switching time. Signal current is shunted away from
CR844, and the forward bias of Q844 increases to the
blanking level. When blanked, the output of the Z-Axis
Amplifier drops to about +10 V, and the crt beam current
is reduced to below viewing intensity to eliminate chop
switching transients from the display.
For an X-Y display, the Sweep Z-Drive signal current is
switched off. When the XY signal is LO, CR837 is forward
biased and Intens Level current flows through R837 to
Z-Axis Amplifier transistor Q841 to establish the display
intensity.
The last input to the Z-Axis Amplifier is the Beam Find
current. Normally, BEAM FIND switch S390 is closed, and
—8.6 V is supplied to the base bias network of Q841 and
Q844. When the BEAM FIND switch is opened, the —8.6 V
is removed, and the bias voltage becomes more positive.
Transistor Q841 becomes more forward biased while Q844
becomes much less forward biased. The current through
Q844 is reduced, and the base bias voltage of Q845 is
thereby increased. The output of Q845 then goes to a level
that produces a fixed, predetermined Z-Axis output signal
level. Thus neither the AUTO INTENSITY control nor the
Z-Drive signal have any control over the intensity level
of the crt display whenever the BEAM FIND push button
is pressed in, and a bright trace (or dot if no sweep is
present) will be displayed.
HORIZONTAL
The Horizontal Amplifier circuit, shown on Diagram 7,
provides the output signals that drive the horizontal crt
deflection plates. Signals applied to the Horizontal Pre
amplifier can come from either the Miller Sweep Generator
(for sweep deflection) or from the XY Amplifier (when
X-Y display mode is selected). See Figure 3-7 for a detailed
block diagram of the Horizontal Amplifier circuit.
The Horizontal POSITION control, X I0 magnifier
circuitry, and the horizontal portion of the beam finder
circuitry are also contained in the Horizontal Amplifier
circuit.
Horizontal Preamplifier
The sum of the sweep and positioning current is applied
to the input of one side of a differential amplifier
composed of Q730 and Q731. For all conditions other than
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