Theory of Operation— 492/492P Service Vol. 1 (SN B030000 & up)
U2038A output is low, emitter current is furnished to Q2044,
which in turn will furnish current through R2051 (the input
resistance of the Z-Axis Driver Amplifier) to Q3047. U2034
is a single-shot that produces a 3 ^s pulse to blank the crt
beam during trace return between readout and signal
display.
The other source of input current to the Z-Axis Drive
Amplifier is Q2042. This transistor is turned on by U2038B
when R/O UNBLANK is high and the R/O OFF is low.
Q1028 serves as a current source for the divider (R1030-
R1025) that sets the operating point for Q2042 and Q2044
which sets the intensity level. Diodes CR1045 and CR1043,
connected from base to base of Q2042 and Q2044, limit the
display intensity by preventing the bases from going more
positive than about 0.6 V above the emitter voltage of
Q2022. This circuit, which includes the adjustment (R1027),
sets the maximum current for both Q2042 and Q2044.
The Z-Axis Drive Amplifier is an operational amplifier
consisting of transistors Q3047, Q4058, Q4059, and related
components. The input resistance for the amplifier is R2051,
and the feedback resistor is R3052. The output is clamped
by diodes CR3059 and CR3066, to protect the amplifier
from transient surges in case of crt arcing.
Transistors Q1017 and Q1015 provide current for the
trace rotation coil. The adjustment (R1021) sets the current
so the displayed trace is aligned with the graticule.
Transistors Q3045, Q4063, Q4065, and related circuitry
are for use in future applications.
HIGH VOLTAGE SUPPLY < ^>
Refer to the block diagram adjacent to Diagram 28. The
High-Voltage Supply furnishes the -3 8 6 0 V to the crt cath
ode, the filament voltage for the crt, and provides dc resto
ration for the Z-AXIS DRIVE signal. The circuit consists of
the following:
1
) the high-voltage oscillator, which produces the crt fila
ment voltage and the 200 Vac that is stepped up and ap
plied to the voltage doubler circuit;
2
) the voltage doubler, which rectifies and filters the high
voltage for application to the crt cathode;
3) the high-voltage regulator, which samples the high volt
age and regulates the operation of the high-voltage
oscillator;
4) the Z-Axis clipper and rectifier circuits, which couple the
Z-AXIS DRIVE signal to the crt control grid.
High Voltage Oscillator. This circuit consists of transis
tor Q1073, transformer T2065, and associated compo
nents. The output of the oscillator, approximately 200 Vac,
is coupled across T2065, where it is stepped up for applica
tion to the Voltage Doubler, and stepped down for applica
tion to the crt filament.
Voltage Doubler. The voltage doubler consists of
CR4041, CR4035, C4027, C5021, C4024, R3038, and
R1039. The output of the doubler is taken off the anode of
CR4035 and applied to the crt cathode. Reference voltage
for the regulator is taken off the end of R1039.
High-Voltage Regulator. This circuit consists of amplifier
U4083 and surrounding components. The high voltage is
applied through a voltage divider consisting of R1017B and
R1017C, which is connected through R1042 to +15 V. The
sample of the high voltage at pin "U ” is applied through
R4075 to the input of comparator U4083. The correction
signal, in the form of dc drive, is applied to the base circuit of
Q1073 to set the oscillator current.
CR4078 and CR4077 at the input to U4083, protect the
input against excessive voltage excursions. The circuit con
sisting of CR4071, R3079, and R4074 protect the oscillator
if the +100 V supply should fail. Normally, CR4071 is back-
biased. If the +100 V is not present, CR4071 conducts and
clamps the input negative; the output of U4083 swings neg
ative and Q1073 remains cut off. This circuit ensures that
Q1073 will begin oscillating only after U4083 switches.
CR3077 (in the output circuit of the regulator) prevents the
base circuit of Q1073 from going negative.
Z-Axis Clipper. This circuit consists of diodes CR1056
and CR1046, plus associated components. 225 Vac from
pin
8
of T2065 is coupled through C1058 and R1048 to the
junction of CR1046 and CR1056. The regulator circuit con
sisting of VR1041 and R1051, hold the cathode of CR1046
at +110 Vdc. Thus, if the Z-AXIS DRIVE signal is
+ 110 Vdc, the two diodes clip the incoming 225 Vac to a
total excursion of 1.2 V. If the Z-Axis peak-to-peak voltage
j is at ground potential, the ac voltage at the junction of the
two diodes swings from ground to +110 V. The voltage
that passes the clipper circuit is coupled through 01031 to
the Z Axis rectifier. The Z-AXIS DRIVE signal is also coup
led directly to this circuit, where it is coupled through C1041
to the crt grid circuit.
The clipped Z-Axis drive signal is rectified by CR2044
and CR2046, which are the principle components of the sec
ond section of the Z-Axis circuit. The rectified voltage is
then fed to the grid of the crt. C1041 couples the fast
changes of drive voltage to the crt grid to speed up the
response of the grid circuit. Neons DS2052, DS2054, and
DS2057 protect the crt from high-voltage arcs from external
REV FEB 1983
5-47
