DEC VT220 - 9.2.4 DC Input Components

+5 V, +12 V, and -12 V potentials are applied as inputs to the dc
ok circuit. When all three inputs are present to turn on 0499,
0400, which is mounted on the front panel of the VT220, and
connected to 0400's collector via J7, it will light.
9.2.4 DC Input Components
The dc input components located on the logic board provide for
additional filtering of dc potentials to be used by the monitor
module logic board components, and for the +12 V potential to be
output to the LK201 keyboard module. They also develop a fourth dc
potential, -6 V, from the -12 V input.
The dc input components (Figure 9-11) consist of the following
components.
• +5 V filter capacitors (C4l and C47)
• +12 V filter capacitor (C42)
• -12 V components (OS, C43, and C45)
• -6 V regulation components (R42 and 04)
• Individual IC +5 V decoupling capacitors (Cl, C3, C4,
Cll, C14-C39, and C48-C54)
-12 VA is an unfiltered version of the -12 V input to the logic
board.
PART OF
/" J1
C45
10/LF
D4
1N753A
I 1 .+5V
..L
'"'41 C1,C3C4,C11
~ Cl4-C39,C48-C54
,22}J.F ,01,uF
C471
2200}J.F "T'
t---,..--f--f---+----- +12V
+5V
+5V
+5V
+12V
+5V
GND
GND
GND
GND
-12V 21 ...... _-.. ...... -+--...I---..L---_ -12V
FROM
PS AND
MONITOR
BOARD
t------'--------_-6V
....... -----------_-12VA
Figure 9-11 DC Input Circuits Diagram
9-14