DEC VT220 Technical Manual page 181

Output
voltage
current
vertical
from the vertical processor circuit is applied to the
divider network. The resulting voltage drop caused by
flow through R3l4 and R3l5 is reflected back to the
processor circuit at R3l3.
During vertical trace, the output capacitors are incrementally
charged, with a resulting incremental flow of current through the
vertical windings of the yoke assembly. This moves the CRT beam
from top to bottom of the screen during the trace.
At the start of retrace, the input from the vertical processor
returns to zero. This results in C3ll and C312 discharging through
the vertical windings of the yoke assembly. This reverse in
current flow quickly moves the CRT beam back to the top of the
screen.
8.2.5 Dynamic Focus
The CRT beam travels further when deflected to a corner of the
screen than when scanning near the center. The dynamic focus
circuit optimizes focus in the corners.
The dynamic focus circuit (Figure 8-17) consists of the following
components/circuits.
• Operating voltage network (0203 and C22l)
• Horizontal component coupling (C223 and R230)
• vertical component coupling (C3l3 and R316)
• Focus transistor (Q203)
• Bias resistors (R225-R229)
• Output coupler (C222)
Li i '
600V FROM TO
HORiZ
---r::
YOKE
OUTPUT ASSY
IC2161 C223 (Jl, PIN 21
.047
R230
lK
TO
CRT
FOCUS
GRiD\
R227
75K
R225
270K
R316
470
D203
RG!j
- - - - , - - - - .. TO
FLYBACK
TRANSFORMER
iTl, PiN 6:
C313
47
10V
---l1---1-"'-"-;
'Q202i
FROtio~
HOP
vERTiCAl-
OUTPUT
(C3L?
FROM
Figure 8-17 Dynamic Focus Circuit Diagram
8-15