DEC VT220 - 8.2.6 Brightness Control

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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

47K"

47

!

'

100V

ro I

F~

VIDEO FLYBACK

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

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