Elenco Electronics MO-1251 Manual do Utilizador descarregar pdf (Página 18)

TOD. T. TEMPLIN

STEREO

DECODER

Now that we know

the theory

behind MTS transmission

and decoding,

let's

build a decoder!

Part

IN THE

FIRST PART

this

article we

showed

the complete

set

of schematic

diagrams

(in Fig.

3 -Fig. 6) while

discussed

the

decoder's

theory

operation.

However,

due

to a printing

error,

a line

connecting

R13,

R14,

and pin

7 of IC4 was

deleted

from

Fig. 3,

the decoder

stage

schematic.

After you

back to

your January

issue

and draw

the

line in, you'll

be ready

start building.

But

before purchasing

any

parts,

read

the section

on interfacing

be-

low; you

may not

need the

board -mounted

demodulator

and its

associated

compo-

nents,

depending

on how you

interface

the

decoder

with

your

TV or VCR.

To build

the decoder,

it's best

to use a

PC board.

If you wish

to etch your

own,

the foil

pattern

is shown in

Service.

Otherwise you

can buy

a board from

the

source mentioned

in the

Parts List.

However you

obtain

a board,

before

beginning construction,

inspect

it care-

fully

for shorted

and

open traces,

and

make

sure that

the copper

is clean.

necessary,

rub

with

steel wool

and then

clean it with

soap and water.

When

the board

is in good

shape,

start

stuffing

it, as

shown in

Fig. 7 (which

shows

all

board -mounted

components)

and Fig.

8 (which

shows all

off -board

components

and

the three jumpers).

First

insert

the low -profile

components,

and

then

work

to the larger

components.

sure to

observe the

polarity

of all semi-

conductors

and electrolytic

capacitors

one

mistake could

be deadly!

When

the board is

stuffed,

clean flux

from the foil

side and

check your work

once

more. Then

mount the

board in

case, as

shown in Fig.

Interfacing

Before

building

the decoder, you

should

determine

how you'll interface

with

your TV

VCR.

If your TV

or VCR

has

a MPX audio

-output

jack,

then you

can

simply

connect

the decoder's

MPX

input

to that jack.

In that

case, you won't

need to

buy parts for,

or build the 4.5-

MHz demodulator.

However,

few late

model sets include

such a jack,

so you'll

probably

have

to build and connect

a spe-

cial interface

circuit.

Doing

so may void

any warranty

that is

in effect,

so don't

undertake

any

modifications

to your

set

unless you're

quite

sure you know what

you're

doing -or

are willing

to accept the

consequences.

We'll

present

several ideas

for interfac-

ing the

demodulator;

whichever

you

chose,

be sure you never

work

on any

device

while it is plugged

into

a 117 -volt

power

outlet.

Many TV chassis

are

extremely

dangerous

because

they do not

have power

transformers

to isolate

them

from the

AC power line.

Sets that lack

such

a transformer

are said

to be hot

chassis

types, because

there may

be a 117

volts

between the chassis

and ground.

Converted

VCR

output

This

is probably

the most

difficult

op-

tion physically,

because you must remove

the case

of your VCR

and drill

a hole in

the

rear panel

to mount

a small

SPST

switch. You must

also locate

the 75 -Rs

audio de-

emphasis capacitor

in the tuner

section,

and lift the leg

that goes

ground.

To find that

capacitor, you'll

probably

need a copy

of the schematic

diagram for the

tuner section

your

VCR.

Your dealer's service

department

may have

that information,

and you may

be able to ask a technician

there for help

locating

the capacitor.

The de- emphasis

capacitor is always

located close

to the audio -demodulator

IC. The

capacitor forms part of a

series

RC network;

one leg goes

to ground, and

the other

is connected

to a resistor

that's in

series with

the

audio path

through

the

circuit.

In some

sets

one IC may perform

both

audio

and video

demodulation.

After

locating

the proper

capacitor,

re-

move

the

grounded

leg.

Then prepare

piece

of shielded

cable that

is long

enough

reach

from the

capacitor

to the

rear -

panel switch.

As shown

in Fig.

10 -a,

sol-

der

the shield

to the

hole from

which

the

capacitor's

leg was

removed,

and the

cen-

ter conductor

the free leg.

Connect

the

other

end

to the

switch.

Now,

when

the switch

is in

the STEREO

position,

the

capacitor

is disconnected

from

the

circuit.

That

allows the

high -

frequency

portion

of the

audio signal

that

contains

the pilot

and the L -R

signals

pass through

the

remainder

the circuit-

ry and

appear

at the VCR's

regular

audio

output

jack.

Closing

the

switch returns

the recorder

to normal

MONO

operation.

Because we

tapped

the

demodulated

audio

directly,

ICI

and associated

compo-

nents

can

be eliminated

from

the

de-

coder's

PC board.

addition,

you

can

use

that

technique

with

a TV

or a

monitor,

but

only

it is not

a hot

-chas-

sis type.

output jack

Conversely,

the following

technique

may be

used on

a TV with

a hot chassis.

You'll

have

build the 4.5

-MHz

section

the decoder.

Before

beginning

con-

version,

obtain

a copy of

the schematic

diagram

your

set.

What

you're

looking

for is

a place

to pick

up the 4.5

-MHz

audio

IF signal

before

it is demodulated.

Locate

the audio

-demodulator

section

of the TV

set; it

should look

something

like Fig.

-b. In many

cases,

the circuit

will

look

similar

to the

demodulator

cir-

cuit

in the

decoder.

Older sets will proba-

bly

use a 4.5

-MHz

transformer

1 2 ... 13 14 15 16 17 18 19 20 21 22 23 ... 85 86

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