Elenco Electronics MO-1251 Manual de usuario descargar pdf (Pagina 22)

22

trimmer

capacitor for maximum

signal

strength.

In practice

we have found

that many

sets, particularly

older models and

tube

types,

radiate

so much RF

that, after

the

probe

is tuned,

it can pick

up

enough

signal to work

as

far as two feet

from

the

set.

Alignment

The

decoder was

designed

to be

easy to

align. The values

of

all components

were

selected

so that by

setting each

adjustable

part

to the center

of its range,

it will

be

near its

optimal setting.

Begin

alignment

by

setting all potenti-

ometers

and trimmer

capacitors

to their

center positions.

Supply an input

signal

to

the

decoder

by one of the

circuits

above.

Be sure

that you are

tuned to

a station that

is transmitting

a stereo

signal. Most

TV

stations leave

the pilot

on all

the time and

transmit

a

synthesized

stereo signal

dur-

ing shows

that are not

true stereo.

You'll

need

to monitor

the decoder's

outputs via

headphones

or a

stereo amplifier.

If every-

thing is working,

you

should hear

some

audio from

the decoder,

although

it may

be low in volume

or

highly distorted.

If you're using

the on -board 4.5

-MHz

demos

llator,

you must

adjust

it first. In-

put transformer

T1 is broadly

tuned, so

any

adjustment to it

will have little

effect.

Leave

it centered,

and adjust trimmer

ca-

pacitor

C5 for

maximum audio

output

from

the decoder.

If you're using

the RF probe

for input,

you must

adjust it while

the television

is

operating, so

be extremely

careful not

to

touch anything

inside

the TV set. While

carefully holding

the probe

in a position

where

you can hear

some signal,

adjust

the probe's

trimmer capacitor

for

max-

imum

output. Then

move

the probe

around

to find the point

where

the signal

AUDIO

DEMODULATOR

TO

OUTPUT

STAGE

SJt

STEREO

T' Y MON0

VIDEO

DEMODULATOR

BREAK

HERE

C1

100pF `L

250V

level

is strongest.

Unplug

the TV

set and

attach

the probe

as close as possible

to that

point.

Now

adjust R3 for

maximum

signal.

Then

adjust R19,

the

stereo PLL

adjust-

ment,

rotating it

through its

entire range.

At some

point

the stereo PILOT

LED

should come

on.

Set R19 to

the point

midway

between where

the LED goes

on

and

off. Re- adjust

R3 until

the LED goes

off, then increase

R3

to just

beyond the

point where it

comes

back on.

Set R19

again. You

may need

to increase

the resis-

tance

of potentiometer

R3

a

little

to ensure

reliable PLL

lock up.

Now you

should be hearing

a fairly

good

stereo signal. While listening

close-

ly to the

program material,

adjust

R25 to

where

the sound

becomes

distorted or

noisy. Then

reduce it until

the sound

be-

comes muffled

or dull.

Then set it

midway

between the

extremes.

The matrix

-input

-level controls,

R23

and R49,

affect

overall left/right

separa-

tion. If

everything

is working

normally,

each

control

should be

set to

approx-

imately

the same

position near

the center

of its range.

You may,

however,

wish to

experiment

with their

settings.

While lis-

tening

to stereo

program

material,

alter-

nately

adjust

each to

obtain the greatest

apparent

separation.

Another

method

of adjusting

R23,

R25,

and R49 requires

an

oscilloscope

capable

of X -Y

display.

Connect the

right- channel

output

of the decoder

to the

X input

of the

oscilloscope

and the left

-

channel

output to

the Y input

of the os-

cilloscope.

Depending

on the

signal

you're receiving,

as separation

decreases,

the

display

becomes more

of a straight

line that tilts

one way

or the other.

For

example,

as shown

in Fig. 11 -a,

a

mono

signal will appear

as

a straight line

at a 45-

degree

angle. A good

stereo

signal

R1

1MEG

JI

IF

OUTPUT

DO

NOT

GROUND!

TO

OUTPUT

STAGE

AUDIO

DEMODULATOR

L1

22pH

P1

RF

OUTPUT

C

FIG. 10- INTERFACING

THE DECODER

to a TV or VCR

can be accomplished in

various ways: via the

audio demodulator

(a), at the

output of the

video

demodulator (b),

or indirectly via

an RF probe (c).

FIG.

11-

OSCILLOSCOPE

DISPLAYS

of the

decoder's

left and

right outputs.

Shown in

a

is

a

mono

signal

(L +R);

in

b is a signal with

proper left

/right

separation;

in

c is a

signal

with too much

L -

R.

fills

all four

quadrants

of

the oscilloscope

display

about

equally,

as

shown in

Fig.

11 -b.

A

mostly L -R

signal

appears

as

shown

in Fig.

11

-c.

To adjust

the

decoder

with

a scope,

observe the

pattern and listen

to the sig-

nal. Adjust

R25 to the point

where the

sound is cleanest.

Now alternately

adjust

R23

and R49 for

the most circular

display.

With patience

and experience

with dif-

ferent types

of

program

material, you'll

quickly learn

how the controls

affect the

sound,

and thus find the

best setting for

each.

When you're

satisfied with your

adjust-

ments,

assemble the

decoder and your

TV

set, sit back,

and enjoy the new stereo

-TV

shows.

R -E

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