Zenith J584W - Page 25

80 pages

Save Page as PDF

To Next Page

To Next Page

To Previous Page

To Previous Page

Loading...

FIGURE

9

-

CHASSIS

3WJR52

FM

IF

IC

BLOCK

WITH

EXTERNAL

COMPONENTS

CERAMIC

FILTERS

Referring

to

Figure

8

there

are

only

two

tuneable

LC

devices

in

the

FM-IF.

One

is

the

1st

IF

Transformer

(T201)

and

the

other

is

the

Quadrature

Coil

(L203).

Y201

is

the

ceramic

filter,

used

in

the

FM-IF.

This

reduction

of

tuneable

circuits

should

simplify

alignment

in

the

field,

if

alignment

becomes

necessary.

Being

highly

selective,

ceramic

filters

provide

ap



proximately

90%

of

the

IF

selectivity.

Ceramic

filters,

as

manufactured,

are

fixed

frequency

devices

and

fall

into

one

of

five

overlaping

groups,

based

on

the

center

frequency

of

each

filter.

Table

B

shows

the

nominal

center

frequency

of

each

filter

and

its

frequency

range,

etc.

When

you

align

this

type

of

IF,

follow

the

instructions

in

the

alignment

procedure.

Remember,

you

must

set

the

signal

generator

to

the

frequency

of

the

ceramic

filters,

then

align

the

chassis

(T201,

L203,

etc.).

Do

not

assume

that

your

generator

is

at

the

correct

center

frequency

until

you

have

checked.

FM-IF

INTEGRATED

CIRCUIT

IC201,

the

FM-IF

Integrated

Circuit,

consists

of

three

IF

Amplifier

stages

with

Level

Detectors

for

each

of

the

stages,

a

Doubly-Balanced

Quadrature

Detector,

AFC

Amplifier,

Audio

Amplifier,

and

Internal

Voltage

Regulators.

Outputs

of

IC201

are

(See

Figure

9):

1.

Delayed

AGC

(pin

15).

2.

AFC

and

FM

Meter

(pin

7).

3.

Composite

Audio

(pin

6).

FM

-

AGC

Delayed

FM

AGC

Voltage

is

derived

from

circuitry

located

within

FM-IF

IC201.

Output

of

the

1st

IF

Amplifier

in

IC201

is

amplified

by

its

Level

Detector.

Since

the

1st

IF

Amplifier

is

the

last

IF

Amplifier

to

go

into

limiting,

the

Level

Detector

will

develop

a

current

which

will

appear

as

a

voltage

across

R203

at

pin

15

of

the

IC.

This

is

the

Delayed

AGC

Voltage

which

is

applied,

via

R2

and

R202,

to

Gate

2

(G2)

of

the

FM

23

TABLE

B

-

CERAMIC

FILTERS

-

CHASSIS

3WJR52

PART

NO.

COLOR

CODE

NOMINAL

CENTER

FREQUENCY

FREQUENCY

RANGE

224-2

Black

10.64

MHz

10.61

to

10.67

MHz

224-2-01

Blue

10.67

MHz

10.64

to

10.70

MHz

224-2-02

Red

10.70

MHz

10.67

to

10.73

MHz

224-2-03

Orange

10.73

MHz

1Q.79tBltt.76

MHz

224-2-04

White

10.76

MHz

10.73

to

10.79

MHz