Tektronix 2467B

Theory of Operation—2465B/2467B Service
JT
Z-AXIS
OUTPUT
(VZ OUT)
\
+8V TO +73V
+BV
R4413
-AAAr-
13
AC DRIVE
FROM PIN 1
OF T4340
+ 150V
OV
-150V
1
+ 105V
+78V
+8V
W
AA
M
C4343
R
43
4
3
—1( Wv-f-AA/V
R4412
CR4422
R4341
CR4342,
<sn
R4421
■AAAy-
15
PARTIAL U4310
D
A E
]( i D>r-AAA^
JT
/
-1900V
-1920V
-1985V
CONTROL GRID
CATHODE
[•? I DS4410
+42V
CR4331'
•4
^ CR4423
C4422
U4366B
R4364
t—vw-
R4372
R4333
f-AAA(—f-
C4430,
U4332A
+"\ R4332
f-AAAr-
C4332
R4336
♦—vw-
H(-£
rh
Q4331
+42V
FROM ANODE
CURRENT
LIMITER
-15V
6019-06
Figure 3-10. Dc restorer circuit (2467B only).
During the time diode CR4423 is reverse biased (not
clamping the positive peaks), transistor Q4331 is kept
biased in the active region by the charge retained on
C4422 from the previous positive clamping cycle. As the
positive amplitude of the ac waveform at Test Point 72
exceeds the voltage at the collector of Q4331, diode
CR4423 becomes forward biased, and the ac waveform is
clamped at that level. Any current greater than that
required to maintain the clamp voltage will be shunted to
the +42-V supply by transistor Q4331.
Operational amplifier U4332A sinks a time-dependent
variable current away from the base of Q4331 that
modifies the crt grid bias during the first few minutes of
instrument operation. The circuit compensates for the
changing grid drive characteristics of the crt as it warms
up.
At power-up, capacitor C4430 begins charging through
R4333 toward the Positive voltage on pin 7 of U4366B.
The voltage is relative to the setting of grid bias poten-
tiometer R4354. The output of U4332A follows the rising
voltage on pin 3 and after about ten minutes (for all practi-
cal purposes) reaches the voltage on pin 7 of U4366B. As
the output voltage slowly increases, the charging current
through R4332 causes the Grid Bias voltage to gradually
decrease from its power-on level. If instrument power is
momentarily turned off and then back on, the crt cathode
3a-43