Circuit Description—434
signals present at the amplifier inputs into current varia
tions to drive the output portion of the Unblanking
Am plifier. CRT intensity variations are accomplished in the
following manner.
When a sweep starts, the DC level present at pin 2 of
U940A steps negative, resulting in a reduced current flow
from pin 9 of U940A. This reduction in current increases
the forward bias on Q954 and reduces the forward bias on
Q955. Q954 conducts harder, resulting in a positive move
ment at the emitter of Q954. This positive movement is
coupled through C942 to the emitter of Q940 and starts a
positive movement at the collector of Q940. A t the same
time that this is occurring, Q955 is decreasing its conduc
tion through R955 and Q950, which causes a positive
movement to occur at the collector of Q950, which
reinforces and sustains the positive movement started by
Q940. A t the end of the sweep, conditions are reversed.
The DC level at pin 2 of U940A steps positive, causing
increased current flo w from pin 9 of U940A and the collec
tors of Q940 and Q950 move negative. The variations in DC
level at the collectors of Q940 and Q950 are connected to
the DC Restorer circuit.
To reduce the possibility of burning the CRT phosphor,
the em itter voltage of Q940 is changed by the TIM E /D IV
switch, S700A. In the 50 ms/DIV position and up, the
TIM E /D IV switch applies +115 volts to the junction of
R943 and VR944. In all switch positions below 50 m s/DIV,
+ 115 volts is no longer connected to the junction of R943
and VR944. Instead, the em itter o f Q940 is returned to
+ 115 volts through R942, R943, and VR944. This
produces a less positive level at the em itter of Q940 to lim it
the collector level of Q940 and Q950 which protects the
CRT phosphor in these switch positions.
DC Restorer Circuit
Q924 and Q925 form a complementary amplifier, each
side of which conducts on a half cycle of the waveform
coupled through T920 from the secondary of the power
supply transformer. The voltage on the collector of Q924 is
determined by the output of the Unblanking A m plifier. On
the positive half cycle, Q924 saturates and the common
emitters seek the Q924 collector voltage. On the negative
half cycle, Q925 saturates and the common emitters go to
ground potential.
C924, C925, CR926, CR928, and R928 form a DC
restorer network. A ll DC voltage levels in this circuit are
referenced to the negative high-voltage potential. The
voltage difference across R928 approximately equals the
voltage swing present on the common emitters of Q924 and
Q925. The upper end of R928 is more positive than the
lower end, but negative w ith respect to the CRT cathode.
Varying the drive signal to the Unblanking A m plifier
changes the voltage level at the am plifier output, which in
turn varies the amplitude of the voltage swing on the
common emitters of Q924 and 0925, thereby resulting in
changes to the CRT bias. CR924 and CR925 lim it the
positive excursion that can occur at the collector of Q924
and the common emitters of Q924 and Q925 respectively.
CRT Grid Bias adjustment R980 determines the level of the
range of adjustment of the INTENSITY control. CR935
provides protection to the CRT if the grid tries to rise more
positive than the cathode.
High Voltage Regulator
Feedback from the high-voltage circuit is connected to
pin 20 of integrated circuit U940B. Any change in this
sample of the high-voltage level produces an in-phase error
signal at pin 17 of U940B which is amplified and inverted
through Q989, Q990, and G995. Regulation occurs as
follows:
If the negative high-voltage level attempts to go positive
(less negative), a sample of this positive-going voltage is
applied to pin 20 of U940B. This results in a positive-going
error signal at pin 17 of U940B which is applied to the base
of Q989 through CR982. Q989 amplifies and inverts this
error signal, which receives further amplification through
Q990 and Q995. The resulting negative movement at the
collector of Q995 is applied to the positive pole of the
minus high-voltage supply, and is of sufficient amplitude to
maintain the output level of the supply.
VR985 through VR990 prevent more than 900 volts
from appearing across Q989, Q990, and Q995 to prevent
exceeding the maximum voltage rating of these devices in
the event of circuit failure. CR983 prevents the base of
Q989 from going more negative than about —0.5 volt.
R963 adjust the negative high-voltage power supply for the
proper output voltage.
CRT Control Circuits
Focus of the CRT display is determined by the Focus
control R82B. This control is part of voltage divider
R962C, R968, R82B, R83, R962D, R962E between the
negative high-voltage and ground. Therefore, the voltage
applied to the focus grid is more positive (less negative)
than the voltage on either the control grid or cathode.
R968 sets the range of adjustment of the Focus control.
The Focus control is ganged with the front-panel INTEN
SITY control, and is autom atically adjusted when changes
are made in the INTENSITY setting. The ASTIG adjust
ment R85, which is also adjusted to obtain a well-defined
display, varies the positive level on the astigmatism grid.
Geometry adjustment R967 varies the positive level on the
horizontal deflection plate shield to control the overall
geometry of the display. The TRACE ROTATION adjust
ment R80 controls the current through L100, which varies
the magnetic field around the CRT to horizontally align the
CRT trace.
3-23
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