DEC VT220 - 8.2.5 Dynamic Focus

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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)

Lii'

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

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