and D1016, is configured as a voltage doubler. The circuit
When the voltage difference between the plates is significant,
provides approximately -3700 volts for the cathode of
few electrons strike the phosphor and the CRT is blanked,
The voltage on one defection blanking plate (pin 9) is
D1020, which develops an additional -150 volts (approxi-
fixed at approximately +80 volts by the voltage divider
mately) for the CRT grid to cathode bias.
R1040-R1042. The voltage on the other plate (pin 7) is de-
pendent on the output level of the operational amplifier
A sample of the high voltage is taken from the voltage
Q1080 and Q1081.
divider circuit and applied to an error sensing and amplifier
circuit, Q1000, Q1001 and Q1002. This circuit controls the
The quiescent level (no-trace period) of Q1080 collector
current through the high voltage oscillator to regulate the
is about +10 V and the electron stream from the cathode of
high voltage output.
the CRT is deflected to the side because of the voltage dif-
ference between the plates, No beam or trace is visible on
Q 1 0 0 3 , with the primary windings of T1010 plus the dis-
the screen. During sweep time, a negative unblinking pulse
tributed circuit capacitance, comprise the high voltage oscil-
is applied to the base of Q1081 This raises the operational
lator. Q1002, in shunt with the emitter-base winding of the
amplifier DC output level to the voltage level on the other
oscillator, regulates the oscillator current.
blanking plate so the electron stream can now poss through
Q1000 compares a sample of the high voltage with the
to the CRT screen, The beam or trace is now visible.
-10 volt regulated supply. The error voltage is amplified
I n t e n s i f i e r C i r c u i t . Signal intensification or hose line
through the complementary amplifier and regulator Q1001
suppression provides increased contrast between spectrum
and Q1002.
signals and the baseline of the display.
The emitter of Q1002 or output of the regulator is con-
The video signal is applied through diodes D1050 and
nected through R1009 to the +10 volt supply, This provides
D1051 to one side of a comparator amplifier Q1050 and
the initial forward bias to the base of Q1003. Positive feed-
Q1051. The input video signal is compared against a DC
back from the collector winding to the base-emitter wind-
level set by the INTENSIFIER control R1013 and the resultant
ing causes the circuit to oscillate. Frequency of oscillation
differential output is applied across the base-emitter junction
is dependent on transformer winding capacitance, including
of Q1070.
reflected reactance of the secondary windings, Frequency
of oscillation is approximately 50 kHz.
The positive-going input video from the detector circuit
produces an output voltage from the comparator amplifier
The voltages supplied by the secondary windings of T1010
which will decrease the forward bias of Q1070. This produces
are: +175 V for the +150 volt regulator circuit in the low
a negative-going voltage at the collector of Q1070. This
voltage power supply; -3700 V for the CRT cathode; 6.3
voltage is applied through the CONTRAST control R1075 to
VAC for the CRT heater, and grid bias voltage for the
the input of the operational amplifier Q1081 ond Q1080. A
CRT. All of these voltages are regulated by the regulator
negative-going signal from the intensifier circuit adds to the
circuit. The amplitude of the oscillator output signal, or
unblinking voltage signal and modulates the CRT blanking
the transformer primary voltage, is dependent on drive
plates so that the CRT beam is intensity modulated (Z axis
voltage to the base of Q1003. The DC base voltage of
modulation),
Q1003 is set by the base voltage of Q1002.
Diode D1073 provides a reference voltage to ground for
the collector of Q1070. With no signal input, the collector
The HV Adjust R1001 sets the forward bias for the am-
potential of Q1070 is approximately +10 volts, The IN-
plifier Q1000. This sets the current through Q1001 and
TENSIFIER control sets the quiescent current through the
Q1002. Far example: A decrease in the high voltage load
intensifier circuit, which sets the DC level at which intensifica-
(current demand decreases) will decrease the forward bias
tion begins, CONTRAST control R1075 adjusts the amplitude
of Q1000 and a positive-going signal is applied to the base
of the moduating signal to the unblinking circuit. This deter-
of Q1001. This decreases the DC voltage at the base of
mines the contrast between the suppressed baseline and the
Q1003. The feedback therefore decreases the oscillator out-
video signal.
put and the output high voltage will remain constant, Ripple
reduction is a factor of the amplifier gain.
Low Voltage Power Supply
R1032 (Intensity Range) and R1033 (INTENSITY level con-
trol) provide a range from 0 to approximately 100 volts
The low voltage power supply in the Type 491 (see Power
bias for the CRT to vary the CRT beam current. FOCUS
Supply schematic diagram) consists of three inter-related sup-
R1028 and ASTIGMATISM R1038 controls provide a variable
plies that operate together as a system. This system delivers
positive (with respect to the cathode) control voltage to the
regulated and filtered voltages of -10, +10 and +150
focusing anode and astigmatism grid. These two controls
volts, A common power transformer T900 supplies the input
are normally adjusted in sequence for optimum beam focus
power to each of the supplies. The input circuit to the power
over the CRT graticule area.
transformer primary can be altered so the Type 491 will
operate through a voltage range from 90 VAC to 136 VAC.
Trace Rotation.
The Trace Rotation control provides
A second plug-in connector switches the transformer primary
means to align the horizontal trace on the CRT with the
winding from 115 V nominal to 230 V nominal line voltage
graticule lines. The Trace Rotation adjustment R1035 varies
by connecting the windings in series for 230 VAC operation
the magnetic field about the coil around the CRT. It will
or in parallel for 115 VAC operation, The Operating section
rotate the horizontal beam approximately +3.
of the manual describes connector switch positions for each
Blanking Circuit. Blanking in this CRT is dependent on
voltage range. Unless otherwise specified, the Type 491 is
shipped with T900 connected for 115 VAC input.
the voltage difference between the deflection blanking plates.
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