Tektronix 2467B - Page 134

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
+ 150V
OV
-150 V
+15V
R1991<
C1990.
m
n
\
+8V TO +73V
+BV
R1913 R1922
■AAArAA/V-
13
+105V
+78V
+8V
wnwi
AC DRIVE
C1
,
9
P° B1950
FROM PIN 7 1 f V\A/ f VW
R1920
CR1930
R1953
R1941
t—wv
CR1953
15
PARTIAL U1830
D
-le
c
-le
B
A E
JT
A
-1900V
-1920V
■1985V
CONTROL GRID
DS90
DS91
CATH0OE
+42V
CR1990
i
^ CR1950
C1971
U1B90A
+\. R1992
C1991 f-
R1994
"^
Q1980
+ 42V
+ 42V
GRID BIAS I
Rle81
R187
8
|*^VV-
J
6019-11
Figure 3-9. Dc restorer circuit (2465B only).
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