Theory
of
Operation-2225 Service
The DC Restorer is referenced to the -2-kV crt
cathode voltage through R858 and
CR854. Initially,
both C855 and C854 charge up to a level
determined by the difference between the Z-Axis
output voltage and the crt cathode voltage.
Capacitor C855 charges from the Z-Axis output
through
R858, CR854, and CR855 to the crt
cathode. Capacitor C854 charges through
R858,
CR854, R854, and CR853 to the crt cathode.
During the positive transitions of the ac drive, from
the lower clamped level toward the higher clamped
level, the charge on C854 increases due to the rising
voltage. The voltage increase across C854 is equal
to the amplitude of the positive transition. The
negative transition is coupled through C854 to
reverse bias
CR854 and to forward bias CR855. The
increased charge of C854 is then transferred to
C855 as C854 discharges toward the Z-Axis output
level. Successive cycles of the ac input to the DC
Restorer charge C855 to
a
voltage equal to the initial
level plus the amplitude of the clamped square-
wave input.
The added charge held by C855 sets the control-
grid bias voltage. If more charge is added to that
already present on
C855, the control grid becomes
more negative, and less
crt writing-beam current
flows. Conversely, if less charge is added, the
control-grid voltage level becomes closer to the
cathode-voltage level, and more crt writing-beam
current flows.
During periods that C854 is charging, the crt
control-grid voltage is held constant by the long
time-constant discharge path of C855 through
R860.
Fast-rise and fast-fall transitions of the Z-Axis out-
put signal are coupled to the crt control grid through
C855 to start the crt writing-beam current toward
the new intensity level. The DC Restorer output level
then follows the Z-Axis output-voltage level to set
the new bias voltage for the crt control grid.
Neon lamps
DS858 and DS856 protect the crt from
excessive grid-to-cathode voltage if the potential on
either the control grid or the cathode is lost for any
reason.
High-voltage multiplier U975 uses the
2-kV winding
of T902 to generate 12
kV to drive the crt anode. An
internal half-wave rectifier diode in the multiplier
produces -2
kV for the crt cathode. The -2-kV
supply is filtered by a low-pass filter formed by
R975, C975, C976, R976, R978, and C979. Neon
lamp
DS870 protects against excessive voltage
between the crt heater and crt cathode by
conducting if the voltage difference exceeds
approximately 75 V.
Focus voltage is also developed from the
-2-kV
supply by a voltage divider formed by R894, R892,
FOCUS potentiometer R893, R891, R890, R889,
R888, R886, and Q885. The focus voltage tracks the
intensity level through the action of
Q885. The
emitter voltage of
Q804, set by the INTENSITY
control, is applied to the emitter of Q885 through
R885. When the emitter voltage of Q804 changes,
the current through Q885 changes proportionally and
alters the voltage at one end of the FOCUS control.
POWER
SUPPLY
The Power Supply circuitry (diagram 7) converts the
ac-power-line voltage into all the voltages required
by the instrument. It comprises the Mains lnput
Board, Transformer, Preregulator, Series Pass, and
Inverter circuits.
Mains lnput Board
The power switch (S901) connects the ac-power
line
to
the primary winding of the toroidal wound
input transformer,
T901, via fuse F901, filter
components
L901, L902, C903, C904, C905, and
VOLTS SELECTOR switch
S902. The secondary out-
put is rectified and smoothed by
CR901, CR902,
CR903, CR904 and C900. With an ac-input voltage
of 240 V, there is approximately 60 V between
W903-pin
1
and W903-pin 2 at full load.
LINE SYNC. The additional components on the
Mains lnput Board produce a Line Sync signal for the
Trigger circuit. Transistor
Q900 is a floating
differential amplifier with a dc bias network
comprising
R905, R904, and R902. Resistors R906
and R903 apply
a
small line-frequency signal from
the secondary of
T901 to the base-emitter junction
of
Q900. The resultant collector current of Q900 is a
line-frequency, sine-wave signal that is fed via
W903-3 to the Main board.
Preregulator
The 60-V power supply from the Mains lnput board,
is applied to the Preregulator circuit formed by
U910, Q913, and associated components. Zener
diode
VR910 and R910 reduce the incoming supply
Scans
by
ARTEK
MEDIA
==-
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