Theory of Operation—2465B/2467B Service
Operational amplifier U1890A sinks a time-dependent
variable current away from the base node of Q1980 that
modifies the crt control-grid bias during the first few
minutes of instrument operation. The circuit compensates
for the changing drive characteristics of the crt as it
warms up.
At power-up, capacitor C1990 begins charging through
R1991 toward the +15 V supply. The output of U1890A
follows the rising voltage on pin 3; and after about ten
minutes (for all practical purposes), it reaches +15 V. As
the output voltage slowly increases, the charging current
through R1992 causes the Grid Bias voltage to gradually
lower about ten volts from its power-on level. The charge
on C1990 dissipates slowly; therefore, if instrument power
is turned off and then immediately back on again, the out-
put of U1890A will still be near the +15 V limit rather than
starting at zero volts as when the crt was
cold.
Z-AXIS DRIVE LEVEL. The variable-level Z-Axis signal
(VZ OUT) establishes the lower clamping level of the ac
waveform applied to the High Voltage Module. When the
amplitude of the waveform drops below the Z-Axis signal,
CR1930 becomes forward biased, and the ac waveform is
clamped to the Z-Axis signal level. The VZ OUT level may
vary between +8 V and +75 V, depending on the setting
of the front-panel INTENSITY and READOUT INTENSITY
controls.
Z-AXIS
OUTPUT
(VZ OUT)
OF T1970
+ 15V
R1991
JT
\
+8V TO +73V
+8V
R1913 R1922
■AAAHWV
13
R1920
+ 105V
+78V
+8V
ii\rm
CR1930
AC DRIVE
C
)
a
?° 01950
FROM PIN 7
j-—
£ WV f WV
R1953
CR1953
R1941
■AAAr
15
PARTIAL U1830
0
"if
c
B
A E
-Or-VW-*
1900V
-1920V
n:
1985V
CONTROL SRID
DS90
DS91
CATHODE
+ 42V
CR1990
i
01990^1^
U1B90A
+\ R1992
f-A/VV
C1991
R1994
♦ WH
+ 42V
GRID BIAS I
B18B1
R1878<*
J
VVV
J
\7 CR1950
1971 C1971
Q1980
+42V
6019-11
Figure 3-9. Oc restorer circuit (2465B only).
3a-38
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