Elenco Electronics MO-1251 Manual de usuario descargar pdf (Pagina 21)

AF

RF

S1

SOURCE J

J2

MPX

- AUDIO

INPUT

J1

RF

INPUT

J3

LED2

J4

el I

POWER

LEFT

RIGHT

T

CHANNEL

CHANNEL

CC

LINE

LINE

W

(

OUTPUT

OUTPUT

HEADPHONE

VOLUME

T2

i

S2

POWER

FIG. 8 -THREE

JUMPERS AND ALL OFF

-BOARD

components

mount as shown

here.

R19

J2

J1

J4 J3

T1

T2

F1

1/4

250V

117

VAC

o

J5

HEADPHONES

between

the video and

the audio

demod-

ulator

sections;

newer sets may

use a ce-

ramic filter.

In either

case, solder

one lead of

a 100 -

pF capacitor

to the output

side of the

transformer

or

filter. Cut a

length of

shielded

cable that

is long enough

to reach

from the

capacitor to the

rear of the

set.

Prepare

one end by

completely removing

about one inch of

the braid. Cover

the part

of

the cable

where the shield ends

with

tape or

heat -shrink tubing.

There must

be

no possibility

of the shield

wire's

touching

any part in

the TV.

Now solder

the center

lead to the free

end of

the 100 pf capacitor.

Dress the

capacitor and the

cable so that they

don't

touch any other

parts. Locate

a conve-

nient,

non -conductive

place on the

rear

cover of the set

and mount the

RCA jack.

Do not

mount the jack on any

metal

part of the

set. Finish the

installation by

soldering the I-

megohm resistor

and the

shielded cable to

the jack.

RF probe

The RF probe

is

probably

the best

inter-

face to use if you're not

familiar

with

the

inner

workings of TV's and

VCR's. Your

set needn't be modified

in any way, and

you

don't have to deal

with high

voltages.

However,

you'll almost certainly

have to

remove the

cabinet in order to pick

up the

RF signal.

In addition, you'll

have to

F1

build

the 4.5 -MHz

demodulator section

of the decoder,

but in that case,

replace

0.01 -11F input capacitor

Cl on the decoder

board

with

a

wire

jumper.

The basic idea is to build a small

anten-

na that is tuned

to 4.5 MHz and is placed

as close as possible

to the TV's

audio

demodulator.

The antenna will pick

up

the

RF

signals

that are naturally

radiated in

the set.

The circuit

is very simple, as shown

in

Fig. 10 -c. Use several

drops of quick -set

glue to hold the

coil to a stick. Then solder

the capacitor

close to the body of

the

coil.

Cover the assembly

with

heat

-shrink tub-

ing to help hold it together

and to provide

insulation. Cut a small

hole in the tubing

so you can adjust

the trimmer capacitor.

Then attach a

length of shielded

cable

about six feet long, and

terminate it

with

an RCA plug.

Finding the optimal

location for the

probe requires that the

decoder be opera-

tional. On the other

hand,

you

can't make

the decoder operational

without an

input

signal from the probe.

That leaves you

in

a

bit of a dilemma.

The

best solution

is to

locate the audio

demodulator

in the television.

Then use

a

rubber band or a piece of

tape to secure

the probe close to

that portion of

the cir-

cuit. Temporarily

remove any shielding,

if necessary. Now you should

be able to

get enough signal to

align the decoder,

after which you can

go back and reposi-

tion the probe and

adjust the setting of the

R25

LEDI

FIG.

9 -THE COMPLETED

DECODER appears

as shown

here. The board

shown

was an early pro-

totype,

so

it

doesn't

match the parts -placement

diagrams (shown

in Fig. 7 and

Fig.

8) exactly.

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