Mitsubishi Electric MUX-25TV - E1 User Manual download pdf (Page 56)

Why No

Converter

Project

Some

readers

may be wondering

why

are

not providing construction

de-

tails

for

complete

stereo TV converter.

There are

good reasons

for not doing

so.

The primary

reason

is that the com-

ponents

needed to make

such

con-

verter

are

very

difficult to

obtain, be-

cause stereo

uses

an integral

noise -

reduction system.

This effectively

pre-

vents

utilization

of low

-cost FM stereo

decoder

chips.

A special stereo

TV de-

coder chip,

like National Semicon-

ductor's

LM -1884

or Panasonic's

-6291

noise -reduction

chip

must be

used.

Unfortunately,

both are

new

that they

are

not available

through

regu-

lar electronics

parts

distributors.

A second

reason

for not including

plans to build your

own stereo

converter

stems

from

the fact

that

stereo

TV con-

verters

are

complex

and

require many

adjustments

to be

made to get

them

working properly.

A special

stereo TV

alignment

generator

required for

alignment,

without

which you

would

not be

able

to align

"by ear" as you

can

with an FM

radio

stereo decoder.

Mak-

ing matters

worse is

the fact that

this in-

strument

not yet

on the general

market -even

you

did

not mind pay-

ing the price

for it just

to build your

own

stereo

TV converter.

volts. Plug

TI into

outlet

and

touch the

meter's positive

probe to

11 and

negative probe

project ground.

If you don't

obtain a

reading between

11.5 and

volts,

once again

check

for

wiring

errors

and,

if you don't

find

any,

one or

more bad

components.

Making the Connection

You're

now

ready to connect

the

project to

your

TV receiver.

This

en-

tails connecting

the INPUT cable

the

sound

i -f

section

inside

your

set,

after

which you

mount the project

the

set's

rear panel

with

6 -32

x % "

machine hardware.

Keep firmly

in mind that there are

dangerous

high

voltages

present

in-

side

your

TV set, even

with the power

Sound

takeoff

transformer

wi,

project

RG -174

coax

cable

Sound

irr

(A) TRANSFORMER

COUPLED

IF HOOKUP

4.5mHz

amp

RG 174

coax cable

I,rn¡ecl

Sound

(B)

FILTER COUPLED

IF HOOKUP

Fig. 5. Shown

here

is whereto

tap into the

two circuits typically

used in TV receiv-

ers.

Check your

set

determine

which

circuit it has, and

then use the appropriate

schematic

to make

the connections

to the project.

turned

off. In and

around some

the

circuits,

some of these

voltages

are potentially

lethal. So before

do-

ing

anything

else,

disconnect

the

set's

power

cord

from the ac

line. If at all

possible,

wait several

hours before

removing

the

back

panel

from the

set.

When

you do remove

the back

panel,

keep as far away

as possible

from the picture

tube area

and

the

high

-voltage power

supply. Even

after

hours without

power, these

areas

may still

have enough

voltage in

them

to give you a

nasty jolt.

Figure

5 shows

how the connec-

tions are actually

made to your

set's sound

detector circuit.

Note

that some sets use

a transformer

(cir-

cuit

A), while others

use a

4.5

-MHz

ceramic

filter (circuit

B). If you can

do so, obtain

the schematic

diagram

for your

TV receiver. If

you

can't,

you may

have to seek out the

help of a

professional TV technician.

It's

to you to determine exactly

where in your

particular set to

make

the INPUT cable connections.

Regret-

fully, there are

many different

models of

TV receivers on

the

market

that

can't

give you

more than

very

minimal assistance

in this article.

You might check

the

inside of your

set's back

panel

for a

module or tran-

sistor/IC

placement

label. This

will

often

get you

to the

right place

fast.

You

can

then

remove

the module

access

the

foil side

of the board.

Your

first step

is to

locate the

transformer

filter. Then

pass

the

free end of the cable

through

hole in

the back

panel (you might have to

drill

it) and connect the shield to the

coil's shield

a transformer

set, or

to the center terminal

filter set.

The cabel's "hot"

lead then goes to

the IC's input point.

Before reassem-

bling

your TV

set,

check for shorts.

Once the connections

have been

made,

mount

the project

top on the

back panel

of the set and

remount the

back

panel on the

TV set. Choose a

location as

far

as possible

from the

set's

horizontal output section

and

isolated from the

metal chassis.

Now take a

few moments to

make

a final safety check.

Set your

multi -

meter to

its highest resistance

range

and touch one probe

to the shell of

the phono

plug and the other

probe

to first one and

then the other prong

the TV set's line cord. If

you

ob-

tain anything

but

infinity

reading

either case,

find the cause and cor-

rect it immediately.

Alignment

Setting

up the project

for proper

op-

eration

is a quick

and easy procedure

to perform.

You

need

audio

am-

plifier (with speaker)

and an

insulat-

ed alignment

tool.

not

use

a jewel-

er's screwdriver

or any other

screw-

(Continued

page 96)

/ MODERN ELECTRONICS

/ April

1986

Say You Saw It

In Modern Electronics

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