Theory of Operation-2225 Service
the 2-kV winding is referenced to the emitter of
(2804, so that the output of the multiplier (12 kV) is
reduced slightly at high intensity levels.
Z-Axis
Beam Find
When the BEAM FlND button is pressed, the BEAM
FlND line goes to about -8
V.
This voltage level will
shunt about 1
mA from the Z-Axis Amplifier, over-
riding any other current combinations to unblank the
trace.
DC
Restorer and Multiplier
The DC Restorer circuit sets the crt control-grid bias
and couples the ac and dc components of the
Z-Axis Amplifier output to the crt control grid. Direct
coupling of the Z-Axis Amplifier output to the crt
control grid is not employed due to the high potential
differences involved. Refer to Figure 3-5 during the
following discussion.
Ac drive to the DC Restorer circuit is obtained from
pin 4 of
T902. The drive voltage has an ac peak
amplitude of about 1
OOV, at a frequency of about
20
kHz and is coupled into the DC Restorer circuit
through C853 and
R853. The cathode of CR851 is
biased by the wiper voltage of Grid Bias
potentiometer
R851, and the ac-drive voltage is
clamped whenever the positive peaks reach a level
that forward biases
CR851.
The Z-Axis Amplifier output voltage, varying with
display intensity between +10 V and +75 V, is
applied to the DC Restorer at the anode of
CR853.
The ac-drive voltage holds CR853 reverse biased
until the voltage falls below the Z-Axis Amplifier
output voltage level. At that point,
CR853 becomes
forward biased and clamps the junction of
CR851,
CR853, and R854 to the Z-Axis output level. Thus,
the ac-drive voltage is clamped at two levels to
produce a square-wave signal with a positive
dc-
offset level.
Figure
3-5.
Simplified diagram of the
DC
Restorer circuitry.
3-1
5
I
t75
v
C855
Scans
by
ARTEK
MEDIA
r>
CONTROL GRID
O
--
-
-
ZZ
f1
ZZ
I
f
Z-AXIS
OUTPUT
-
CR853
R858
CATHODE
R860
ZZ
CR855
CATHODE
t85
V-
'
ti0
v-
ti00
v,
CR851 R850
1
R854 C854
4)
I
/
CR854
6.2
M
VOLTAGE
,
,
SUPPLY
-
-
FROM T902 (r-2
kV)
6299-30
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