Marantz 105B User Manual download pdf (Page 4)

---

.........:.:.

The 10.7

MHz

front

end

output

is

led

to

the

next

IF section .

The

IF section consists

of

five

stage

of

I F

amplifier

and one stage

of

sub I F

amplifier.

Two

pieces

of

ceramic

filters

are also used

to

obtain

high

selectivity,

a

pair

of

symmetrical

diode

limiters

are also

employed

for

the

best

limiting

characteristics,

improved

capture

ratio

and good

AM

suppression. A

part

of

I F

amplifier

H 1

05

output

is

rectified

by the

diode

H 115, H

116

and

its

DC

output

is

fed back

to

the gate

of

FET

RF

amplifier

to

decrease the gain

with

increased signal strength.

2.1

Muting

and

Auto-Stereo

Switching

Circuits

The

muting

circuit

consisting

of

all solid-state electrical

switchin

g

has

been

incorporated

in the

Mode1105B.

The DC vo Itage

obta

i ned

by

rectifi

ng

the

su

b I F

output

signal

from

the

H

109

is

app I ied

to

the

base

of

H

11

0 and

turns

on

it,

if

th

e sub I F

output

is

greater than

predetermined

level

(muting

threshold

level).

When

H110

turns

on,

the

muting

switch

transistor

H111

is

turned

on,

thus

decreasing

the

emitter

collector

resistance

to

near zero

ohm

and

allowing

emitter

current

path

to

the

Final

IF

ampl

ifier

H1 08.

When

the

input

signal

is

lower

than

the

predetermined

level, the

DC

output

obtained

is

small

and can

not

turn

on the

H110,

thus

the

H110

keeps its

turn

off

state and

this

makes the switch

transistor

keep H111

turn

off,

then

no

emitter

current

is

supplied

to

the

H108

and signals

below

the

threshold

level are

muted

out.

The

muting

threshold

level can be varied

by

adjusting

the

trimming

resistor R 153.

The

DC voltage

obtained

is

also used

to

make

the

Auto-Stereo

switching

transistor

H

112

turn

on

and

off.

2.2

MPX

Stereo

Deco

di

ng

Circuit

A Non-equalized

audio

signal

from

the

FM

detect

or

is

applied

through

the

phase

adjuster

network

of

C148

and R161

to

input

termin

al

pin

CD

on

the

MPX decoder IC

Hl14.

The

MPX

decoder

Ie

consists

of

a stereo decoder and postampl

ifier

for

the

ou

tput.

The

right

and

left

channel signals decoded

by

the

stereo decoder H

11

4,

appear

at

pin

@

(right

channel) and

pin

QJ)

(left

channel), respectively. These signals are

passed

through

the

low-pass

filters

and

de-emphasis

networks

to

eliminate

undesirable residual

switching

signals and are

then

delivered

to

postamplifier

input

pin

®

(right

channel) and

pin

(j)

(left

channel), respectively.

The

signals

ampl

ified

in

the

postamplifiers

to

the

required

level·

(approximately

10

dB)

are delivered

to

pin

®

(right

channel) and

pin

®

(left

channel), and are

then

passed

through

C169 and

C168

to

pin

terminals

Jl14

and

Jl15,

hence,

through

fu

nc

tion

switches

to

the

"OUTPUT"

terminal.

Pin

(j])

on

the

MPX

IC

Hl14

is

connected

through

R1

63

to

the

collector

of

the

autostereo

switch

transistor

Hl12,

which

turns

on

or

off

acc

ording

to

the

incoming

FM

signal

strength,

thereby

automatically

switching

between

the

stereophonic

and

monaural

operations

.

The

H

112

turns

on

or

off

in accordance

with

whether

the

F M

si

gnal strength

is

more

or

less

than

approximately

25~V.

2.3 Suggestion

for

Trouble

Shoo

ting

of

FM

T

uner

2.3.1

Symptom:

No

FM Recept

io

n

First

turn

on

the

power

switch

and

try

to

t une FM

stations

. R

otate

the

fly-wheel

tuning

knob

slowly

and observe

the

FM

TU

N

IN

G meter.

If

the

TUNING

meter

deflect

at several

frequencies received, the

tuner

circuits

preceding t he

discriminator

cir

cuit

may

have no fai lure.

When

no

reading

is

obtained

in

the

meter, check FM local

oscillator

circuit,

usi

ng

a RF

VTVM.

The

normal

local

oscillator

voltage

is

one

or

two

volts

(rms)

at

the

tuning

capacit

or

, depending on

the

tuning

capaCitor

position.

If

the

local

oscillator

voltage

is

normal

,

next

check all voltage

distribution

in

the

FM

Front

End and I F

amplifier

unit

and

compare

them

wit

h those shown in

the

circuit

diagram. When

TUN

I NG

meter

deflects

but

no

so

un

d

is

obtained,

check

audio

circuits,

using high sensitive oscilloscope.

2

1 2 3 4 5 6 7 8 9 ... 18 19

Comments to this Manuals

No comments