DEC VT220 - Page 174

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Q202

is

turned

on

and

off

by

current

flow

through

the

secondary

of

T201

in

the

horizontal

driver

circuit.

During

the

second

half

of

retrace,

Q202

is

turned

on

to

saturation.

At

other

times,

Q202

is

backbiased

by

T20l,

quickly

turning

Q202

off

to

minimize

dissipation.

The Q202

collector

voltage

is

sourced

at

+12 V

and

boosted

through

the

collector

voltage

network

to

+15.5

V.

The

charge

pump

circuit

inverts

the

large

positive

voltage

swings

at

the

collector

of

Q202

into

-170

V

bias

used

by

brightness

control

and

dynamic

focus

circuits.

The

horizontal

linearity

circuit

provides

adjustable

(L20l)

control

of

current

flow

through

the

horizontal

windings

of

the

yoke

assembly

during

trace

and

retrace

scanning.

L20l

provides

variable

inductive

reaction

for

adjusting

current

flow

and

varying

the

trace

width.

L202

provides

nonlinear

reactance

that

varies

with

es

in

yoke

current.

R217

and

C212

provide

correction

for

nonlinearity

caused

by

the

decaying

exponential

rate

of

current

increase

in

the

yoke.

The

output

components

switch

yoke

current

back

and

forth

between

C213

and

C216.

Figure

8-8

provides

an

illustration

of

the

waveform

developed

at

the

horizontal

wi

ings

of

the

yoke

assembly

during

trace

and

retrace

riods.

Points

T0

through

T3

are

identified

in

this

figure

as

references

within

the

waveform.

Figures

8-9

through

8-12

provide

illustrations

and

descriptions

of

the

current

flow

conditions

that

result

in

the

waveform

shown

in

Figure

8-8.

N-~"'"

I

THROUGH

YOKE

-0

AMPS

t

~m===~~===-1r!=:l_OIlOLTS

r

l.

L

IIOLTAGE

i I

II

ACROSS

I I

YOKE

T,j;3

TO

T2

..

..-..

Figure

8-8

Horizontal

Deflection

Waveform

8-8

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