DEC VT220 Technical Manual page 182

D203 and C22l rectify and filter an operational voltage input of
+160 V supplied to the base and collector of Q203 through the bias
resistors.
The horizontal component of the focus value is coupled to the base
of Q203 by C223 and R230, and developed from the charge condition
of C216 (refer to section 8.2.2.3, and Figures 8-9 through 8-12)
during scanning. The vertical component of the focus value is
coupled to the base of Q203 by C313, and developed by the charge
condition of C31l and C3l2 during vertical deflection (refer to
section 8.2.4). The output resulting from-the biasing of Q203 is
coupled to the focus grid potential developed by the flyback
transformer circuit through C222.
8.2.6
Brightness Control
The brightness control circuit allows the brightness of the
display at the CRT to be adjusted by the operator.
The brightness control circuit (Figure 8-18) consists of the
following components/circuits.
• Reference voltage network (D103, Rl13, C104, and C107)
• Voltage divider (Rl10-Rl12)
• Bypass capacitor (C103)
....
Ri12
47K
R i!1
lOOK
",
... 9
TO
I
r-
I
- - - - - t - - - - - - ..
CRT
I
i
r
(INTENSITY
GRIDI
I
i
'
0103
1
Cl04
i
I
IN4004 "'" 22
I
1.
Cl03 l00V
1"
01 ...--
I
l00V
I
~
RllO R1l3 Cl07
lOOK 4 7 K " 47
!
'
100V
ro
I
F~
VIDEO
FL
YBACK
AMP
_ - - t - - - I - - - . . . . l - - - I - - - - ' - - - - -
TRANSFORMER
IR 109! 101001
GND b
l - - - t - - - l . . - - - - - l . - - - - - _
GND
0
170V
FROM
HORIZONTAL
OUTPUT
ICHARGE PUMPI
Figure 8-18 Brightness Control Circuit Diagram
8-16