Zenith D952X - Page 20

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Before

aligning

or

servicing

Multiplex

Circuits

be

certain

that

RF,

IF,

and

Ratio

Detector

are

correctly

aligned

and

that

operation

is

normal

on

monaural

FM

signals.

^

1IA

_

„

zx

FM-MULTIPLEX

ALIGNMENT

PROCEDURE

CHASSIS

-

10ZT30,

11AT30,

15WDR50Z1,

20AT21,

20AT21Z,

20AT30,

20AT30Z,

20AT31Z,

21BT30,

21BT34,

(Z1

),

25AT20

25BT22

25WDR50,

(Z

1),

27BT30,

29CT20,

29CT21

,

(Z

1),

(Z2),

29AT24,

(Z1

),

(Z2),

29CT30,

35WDR50.

STEP

CONNECT

SCOPE

AND/

OR

VTVM

CONNECT

GENERATOR

DUMMY

ANTENNA

INPUT

SIGNAL

FREQUEN

CY

BAND

SET

D

^

L

ADJUST

PURPOSE

NOTE:

PL

ACE

BANDSWI

TCH

IN

FM

POSITION

AND

STEREO/MONO

SWITCH

IN

STEF

EO.

USE

RF

FREQUENCY

NEAR

CENTER

SCALE

1

**

Test

Point

"M"

Composite

Amp

VTVM(AC)

Test

Point

"A"

FM

ANT.

300

Ohm

98MHz,

67kHz$CA

FM

98MHz

(T301**)

Adjust

67kHz

Trap

for

minimum.

2

Test

Point

"N"

Doubler

VTVM(DC)

98MHz,

10%

Pilot

T301

,

T302,

R308

(T302,

T303,

R309**)

Adjust

19kHz

Amp

and

Doubler

for

maximum.

NOTE

—

Adjust

this

step

with

Test

Point

"T"

Shorted

Adjust

mute

control

at

same

time

so

voltage

doesnot

exceed

—

.2

volt.

Voltage

must

be

kept

at

minirtidrn

for

proper

alignment.

If

stereo

indicator

lamp

lights

up,

readjust

mute

control

to

extinguish

lamp

and

con



tinue

adjustment

of

transf.ormers

for

maximum.

3

98MHz,

5%

Pilot

R308(R309**)

Adjust

mute

control

to

pointwhere

stereo

lamp

lights

up.

4

"L"

Audio

Output

(Aft^r

38kHz

filter)

VTVM

(AC)

98MHz,

10%

Pilot

L+R,

L-R,

(Mod.

L

Only)

T303(T304**)

Adjust

38kHz

Detector

for

maxi



mum

at

"L"

Output.

NOTE

—

"L"

Output

should

be

approximately

10

(or

greater)

times

"R"

Output.

5

"R"

Audio

Output

(After

38kHz

filter)

VTVM(AC)

Check

"R"

channel

output

**FOR

CHASSIS

29AT24,

(Z1

),

(Z2)

ONLY.

3.

Sequentially

connect

your

scope

to

the

base

of

composite

amplifier,

base

of

19kHz

amplifier

and

collector

of

19kHz

amplifier.

The

amplitude

of

the

19kHz

signal

should

greatly

increase

as

you

proceed

along

the

19kHz

chain.

Doubler

and

Subcarrier

Signal

Tracing

To

determine

if

the

doubler

is

functioning,

place

your

scope

at

the

junction

of

the

two

diodes

and

you

will

see

38kHz

DC

pulses.

Placing

the

scope

at

the

collector

of

the

subcarrier

amplifier,

you

should

see

a

38kHz

sine

wave

which

will

indicate

that

the

subcarrier

amplifier

and

associated

ringing

circuitry

is

functioning

properly.

Multiplex

Detector

Signal

Tracing

1.

Move

the

19kHz

pilot

amplitude

control

to

10%.

*

2.

Move

the

L

-

R

generator

switch

to

L

-

R.

You

should

see'

equal

amplitude

1000

hertz

sine

waves

at

both

L

and

R

outputs.

3.

Move

the

L+R

switch

to

L+R

and

look

at

the

L

audio

output,

and

measure

the

magnitude

of

the

1000

Hertz

sine

wave.

If

the

multiplex

detector

and

preceeding

circuitry

are

aligned

properly,

the

magnitude

of

the

wave

form

at

L

should

be

greater

than

at

R.

If

all

the

waves

are

similar

in

form

and

magnitude

to

those

indicated,

then

it

can

be

assumed

that

the

multiplex

portion

of

the

receiver

is

functioning

properly

and

the

problem

lies

ahead

of

this

in

the

FM

receiver.

If

any

of

the

wave

forms

are

missing

at

a

latter

point

but

are

apparent

at

a

previous

point,

then

something

is

amiss

in

the

circuitry

between

the

two

test

points.

GENERAL

TROUBLE-SHOOTING

PROCEDURE

Should

a

problem

arise

in

aligning

the

FM

multiplex

portion

of

the

receiver,

the

technician

must

determine

whether

the

difficulty

lies

in

the

RF,

IF,

limiter

and

ratio

detector

portions

of

the

receiver,

or

whether

the

difficulty

lies

in

the

multiplex

portion.

A

multiplex

generator

is

an

excellent

signal

tracing

device

to

determine

if

the

multiplex

section

of

the

receiver

is

functioning

properly.

The

composite

output

of

the

multiplex

generator

can

be

injected

at

the

output

of

the

ratio

detector.

To

reduce

possible

extraneous

signals

coming

through

the

ratio

detector,

short

the

ratio

detector

primary

with

a

jumper

lead.

The

wave

forms

and

their

magnitude

may

vary

slightly

from

chassis

to

chassis,

however,

they

are

quite

indicative

of

what

will

be

seen

when

signal

tracing

the

multiplex

circuitry.

67kHz

Signal

Tracing

1.

Turn

generator

pilot

carrier

amplitude

control

to

zero.

2.

Move

L+R

and

L-R

switches

to

OFF

position.

3.

Move

67kHz

generator

ON.

Sequentially

connect

an

oscilloscope

to

the

input

and

output

of

the

67kHz

trap.

The

67kHz

signal

at

the

output

of

the

trap

if

it

is

properly

nulled,

will

be

much

smaller

than

at

the

input.

The

voltage

ratio

should

be

approximately

20

to

1,

input

to

output.

•

19kHz

Signal

Tracing

1.

Move

the

67kHz

generator

OFF.

2.

Move

the

generator

19kHz

pilot

carrier

amplitude

control

to

10%

position.

FM

-

MULTIPLE

ALIGNMENT

PROCEDURE

-

CHASSIS

4WDR50X,

(XI),

MODEL

D538

Before

aligning

or

servicing

Multiplex

Circuits

be

certain

that

RF,

IF

and

Ratio

Detector

are

correctly

aligned

and

that

operation

is

normal

on

monaural

FM

signals.

18

STEP

CONNECT

SCOPE

AND/

OR

VTVM

CONNECT

GENERATOR

DUMMY

ANTENNA

INPUT

SIGNAL

FREQUENCY

BAND

SET

DIAL

TO

ADJUST

PURPOSE

NOTE-PL

ACE

BANDSWITCH

IN

FM

STEREO

POSITION.

USES

RF

FREQUENCY

NEAR

CENTER

SCALE.

1

2

Test

Point

"M"

Test

Point

"A"

FM

ANT.

300

Ohm

98MHz,

10%

Pilot

FM

98MHz

L302

L303

Adjust

19kHz

Amp

for

maximum.

3

"L"

Tape

Output

98MHz,

10%

Pilot,

L+R,

L-R,

(Mod.

L

Only)

L301,

L302,

L303

Adjust

for

maximum

L

Channel

reading.

4

"R

”

Tape

Output

L303

Adjust

for

minimum

R

Channel

reading.

5

Repeat

Steps

3

and

4

for

minimum

change.

To

provide

maximum

separation.

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