Tektronix 2235A Instruction Manual page 66

Theory of Operation
-
2235A Instruction
Figure 3-5. Simplified diagram of the Dc Restorer circuitry.
+ 1 0 V T O + 7 5 V 3
-)
I
+ l O V
C855
The Dc Restorer is referenced to the -2 kV crt cathode
voltage through R858 and CR854. Initially, both C855
and C854 will charge up to a level determined by the dif-
ference 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.
Z-AXIS OUTPUT
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 aeross C854 is equal to the ampli-
tude of the positive transition. The negative transition is
coupled through C854 to reverse bias CR854 and to for-
ward bias CR855. The increased charge of C854 is then
transferred to C855 as C854 discharges toward the
4)
CONTROL GRID
Z-Axis output level. Successive cycles of the ac input to
the Dc Restorer will charge C855 to a voltage equal to
the initial level plus the amplitude of the clamped
square-wave input.
I
DS856
R860
CATHODE
6.2 M
CATHODE
DS858
VOLTAGE
+
=
-
FROM T948, SUPPLY
PIN 16 (fir-2 kV)
i
7683-27
The added charge on C855 determines 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 will flow.
Conversely, if less charge is added, the control-grid
voltage level will be 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.
Scans
by
ARTEK M . =>