DEC VT220 - 8.2.4 Vertical Deflection Circuit

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The

anode

output

is

rectified

internal

to

Tl,

and

filtered

by

graphite

coatings

at

the

CRT

end

of

the

anode

wire,

which

can

connect

or

disconnect

only

at

the

CRT

end.

The

operational

voltage

(+60

V)

used

brightness

control

circuits

is

output

regulated

and

filtered

by

0100

and

L204.

by

the

video

amp

and

from

pin

2

of

Tl

and

The

cutoff

grid

potential

is

taken

from

pin

4

of

Tl

and

regulated

by

L203

and

D202.

The

regulated

potential

(+750

V)

is

routed

to

the

CRT

cutoff

grid

via

a

voltage

divider

network

(R2l9

and

R224,

filtered

by

C219

and

C220),

and

to

R220

in

the

dynamic

focus

circuit.

The

focus

grid

potential

is

picked

off

of

the

focus

potentiometer

(R22l)

with

a

voltage

range

at

R221

from

+500

V

to

-170

V.

The

positive

range

is

developed

from

the

same

Tl

output

used

for

the

cutoff

grid

potential,

while

the

negative

range

is

developed

from

input

from

the

charge

pump

circuit

in

the

horizontal

output

circuit.

The

actual

focus

grid

value

sent

to

the

CRT

is

developed

across

R222

in

conjunction

with

the

value

supplied

by

the

dynamic

focus

circuit

(refer

to

section

8.2.5).

8.2.4

Vertical

Deflection

Circuit

The

vertical

deflection

circuit

develops

the

potentials

supplied

to

the

vertical

windings

of

the

yoke

assembly.

These

potentials

provide

for

deflection

of

the

CRT

electron

beam

during

vertical

trace

and

retrace

times.

vertical

deflection

is

a

linear

current

rise

to

the

vertical

yoke

windings

during

a

complete

scan

of

the

screen

(each

scan

line

of

the

scan

defined

by

horizontal

trace

and

retrace

activity).

A

complete

scan

of

the

screen

occurs

at

a

rate

of

60

vertical

trace

periods

r

second.

Vertical

retrace

time

is

defined

by

a

vertical

sync

signal

input

from

the

video

logic

(VER

IN).

VER

IN

results

in

a

steep

drop

in

the

current

supplied

to

the

vertical

yoke

windings,

returning

the

placement

of

the

CRT

beam

to

the

top

of

the

screen.

The

vertical

deflection

circuit

(Figure

8-14)

consists

of

the

following

circuits.

•

•

8.2.4.1

circuit

periods.

to

move

through

windings

Vertical

processor

Vertical

output

vertical

Processor

Circuit

The

vertical

processor

provides

the

base

control

for

defining

trace

and

retrace

It

develops

a

ramping

output

used

by

the

vertical

output

the

CRT

beam

from

the

top

to

the

bottom

of

the

screen

linear

increase

of

current

flow

through

the

vertical

of

the

yoke

assembly.

8-12

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