Mitsubishi Electric MUX-25TV - E1 User Manual download pdf (Page 52)

Project

Add

Stereo

Sound

To Your

TV Receiver

This

device

readies

any

TV

receiver to

use

a

commercial stereo

TV converter

for

full- spectrum

stereo

sound

reproduction

By Gary

McClellan

Stereo TV sound

is here! Once

you

hear it,

you'll

agree

that

stereo

sound

does for the

1980s

what

color

pictures did

for the 1960s.

The

bad

news,

however, is that or-

dinary

TV receivers can't

decode the

stereo

signal, and

those that can are

currently

quite expensive.

The good

news' is that

there are already

on the

market

a

number of converters

that

can

provide

stereo

sound -but

only

if

your set

is

"stereo-

ready."

However,

you can

ready any

TV

set to decode

stereo through a

suit-

able converter

using

the simple cir-

cuit described

here.

A stereo -ready

capability

obviates

the need to buy a

new set to take advantage

of stereo

sound.

By

itself, this project

won't

give you

stereo sound

directly

(you

need a stereo

TV converter

for that).

But

it does provide

high

-quality

mono sound directly

and prepares

your

set to use a stereo

converter

that

will

let

you

enjoy

full- spectrum

stereo sound.

Installation doesn't

affect

normal

operation

-it

enhances

it. A built -in

squelch circuit

mutes annoying

in-

terstation

noise, and a

level control

matches

the project

to the

various

stereo

converters

now available

to

obtain

optimum performance.

Installation

entails

only two solder

connections

to your

TV set.

A few

adjustments

you

do by ear

then get

the system

in proper operating

order.

Project cost

is just $25,

which gets

you both superb

mono sound and

stereo -ready

capability.

Add another

$100 to $200

for

a

stereo

TV

convert-

er, and you

get

full-

spectrum

stereo

sound as

well.

About the Circuit

This project performs

several dif-

ferent tasks

in

order

to make your

TV

receiver

"stereo ready."

Its first task

is to electrically

isolate the TV chassis

from

your sound system.

This

is

mandatory

for

both

safety and

tech-

nical

reasons. To keep costs

down,

modern

TV receivers no

longer have

power

transformers and,

thus,

have

their chassis

"grounded"

to one side

of the ac

line. This being

the case, up

to

117

volts

ac

can appear

on the

chassis,

making it a potentially

lethal

shock

hazard if you

touch any ex-

posed

metal parts that are electrically

connected

to the "hot"

chassis.

Therefore, this project

ensures

full

electrical

isolation

from the TV

re-

ceiver.

It

also

blocks the

TV receiv-

er's 15,734

-Hz horizontal sweep

sig-

nal from entering the stereo

TV con-

verter to prevent

false triggering

that

would

spoil

both

mono and stereo

sound

reception.

The

project

amplifies and

detects

the 4.5 -MHz sound

i -f signal as

it

comes

from the

TV

set's

i -f strip.

This amplification

is

essential

be-

cause

the existing

sound detector cir-

cuit,

lacking the

wide bandwidth of

the project,

would prevent stereo

re-

ception.

The project also

outper-

forms the sound detector

in your

TV

set by providing

more gain at

4.5

MHz and

less

distortion

and provides

a squelch

circuit to boot.

As

shown

in

Fig.

1, the project

taps

into the

TV set's

i -f

amplifier

section

just

before

the

regular

sound-

detec-

tor circuitry.

This

has little or

no

ef-

fect on the existing

circuitry or

the

set's

normal sound.

At this point, the

60

/ MODERN

ELECTRONICS

/ Apri( 1986

Say

You

Saw

It In Modern

Electronics

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