DEC VT220 - 8.2.7 CRT Device; 8.4 Schematic Reference Information

The voltage divider network, which includes the brightness
thumbwheel (Rlll), provides for a variable bias (from +9.5 V to
-80 V) to the intensity grid of the CRT. The bias is developed
from -170 V input from the charge pump in the horizontal output
circuit (refer to section 8.2.2.3) and +60 V input to the
reference voltage network from the video amp operational voltage
developed by the flyback transformer.
C103 provides for bypassing transients that occur as a result of
internal CRT arcing, as well as for bypassing video signals from
the intensity grid that would interfere with the frequency
response of the CRT cathode.
8.2.7 CRT Device
The CRT device (Figure 8-19) provides for actual output to the
operator. Video input to the monitor circuits is converted to
emitter potential to the CRT emitter. FOcus, intensity, and cutoff
potentials applied to the three grids of the CRT inhibit (cut off
input from flyback transformer circuit), refine (focus i t from
the focus circuit of the flyback transformer coupled to the value
developed by the dynamic focus circuit), and contr 1 the
brightness (intensity input from brightness control) of the
display generated by the CRT beam from the emitter potential
provided by the video amp.
I
I
I
I
I
I
I
I
_ --J
r-----,
CRT
FROM
VIDEO EMiTTER
AMP
R108
FROM
FLYBACK ANODE
TRANSFORMER ---------+--+++---++
FOCUS GR '0
7
I ALL
II,
cI
I! :=
i
I RED
,
CUTOFF GRiD
I
INTi;NSITY GRiD I
I
......
GRN
.. ~
I
I
3 I
BRN
I
'17V~
I
BLK
r~ II
I
I
I
WHT
i
~
I
I
I
I
L
I
---
FROM
DYNAMIC
FOCUS
IA227'
F ROfl~~
FLYBACK
TRANSFORMER
(R224)
FROM GND
BRIGHTNESS f7'\b
CONTROL V
IR111i
Figure 8-19 CRT Device
8-17