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4-2

VIDEO

OUTPUT

The

video

DC-coupled from video driver

QB05

and

Contrast control

RB15

to the base of video out-

put

transistor Q201.

The

beam limiter current is used to control the

amount

coupling. Beam current limiter tran-

sistor

QB07

and

its associated components sense

the

anode

current

the tertiary of

the

horizontal

output

transformer. As

the

beam

current increases, so does

the

collector

current

QB07. This collector current

is fed to

the

emitter of Q201,

which

increases

the

collector voltage of

the

video

output

transistor. In-

creasing

the

collector voltage decreases

the

beam

current.

This circuit allows for a

maximum

of 90%

cou-

pling. Because this circuit will

not

limit the

maximum

beam

current at a sufficiently

low

value,

diode

CRB02

is required. This diode is biased off

until

the

beam current reaches 210 microamps. At

this point, the diode is allowed to conduct. The

emitter impedance is lowered considerably,

which

increases the collector current. This limits the

maximum

beam current.

SPOT BURN

PROTECTION

When

the

Monitor is

turned

off,

CRB03

and

CB09

keep the collector voltage high. This keeps

the

CRT

biased off, preventing

spot

bum.

VERTICAL

SWEEP

CIRCUIT

The vertical sweep circuit is a self-oscillating

DC-

coupled

ramp-generating circuit

that

uses com-

plimentary

push-pull

class B

output

transistors, a

driver transistor, a differential amplifier transistor,

and

oscillator transistor.

The

emitters of the

output

stage are fed back to the

input

through C613,

R61B,

R617,

and

Q601. The dif-

ferentiated, positive-going fly back

pulse

from

the

emitter charges C606

and

C60B

through Q601. The

capacitors discharge through

their

respective resis-

tor networks. Capacitor C606 discharges during the

trace interval to 0.6 volts below

the

emitter voltage

of Q601. At this point, Q601 conducts

and

turns

off

the

amplifier stages. This causes

the

yoke voltage

to fly

and

repeat the cycle.

The

presence of a sync signal causes Q601 to con-

duct

slightly before

the

voltage

C606 decreases

to 0.6 volts below the Q601 emitter voltage, bringing

the

circuit timing into sync

with

the

sync signal.

Capacitor

C60B

discharges linearly through its resis-

tor network because this network is

returned

to yoke

current sensing resistor R624, where a

ramp

voltage

appears of the same

amplitude

as the ramp voltage

across

C60B.

A constant voltage appears across the

discharge resistor

and

maintains the constant dis-

charge current from C608. Capacitor C608 provides

a linear, negative-going ramp voltage of average

value, established

R60B

and

R609, to the base

of Q602.

The

signal to

the

emitter of

the

differential

amplifier comes from the yoke

return

circuit.

The

ramp

voltage across R624 has S-correction

its waveform as required

the

yoke

current

to pro-

duce

linear pictures

the CRT. Across C609 is

inverse S-correction signal,

which

is derived

through

the

shaping network of R620, R621,

and

C615. This adds to the ramp-plus-S-correction signal

appearing across R624 to

produce

a linear ramp at

the

emitter of Q602. This linear

ramp

is compared

Q602

with

the linear

ramp

across

C60B.

The

dif-

ference

between

the two is

coupled

to the succeed-

ing amplifiers stages. This returns

the

yoke current

the

desired current for producing a linear picture.

Brand	Zenith
Model	SM-ZVM-122
Category	Monitor
Language	English

Zenith SM-ZVM-122 User Manual

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