Dual KA 12 L handleiding

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Functions

AFC

L._

L

FM

section

The

FM

circuits

are

equipped

with

two

FETs

(HF

input

stage

T

101,

Mixer

T

102).

T

103

produces

the

oscillator

voltage.

The

use

of

the

two

FETs,

a

separate

oscil—

lator

stage,

and

tuned

gate

and

drain

cir—

cuits

gives

the

best

results

with

respect

to

sensitivity,

noise,

and

large

signal

handling.

FM

-IF

It

was

decided

to

use

two

ICs

(I

301,

I

302),

a

ceramic

filter,

and

an

LC

bandfilter,

rather

than

discrete

componente.

The

IF

signal

is

extracted

at

the

drain

of

the

mixer

by

an

LC

—matched

ceramic

filter

and

passed

to

the

input

of

IC

301

(TA

991,

pins

2

and

4).

The

amplification

is

approxi—

mately

70

dB).

The

IF

signal

is

split

at

pin

B.

One

part

is

feed

to

the

second

IC

(I

302)

for

furt—

her

amplification,

and

the

other

part

is

fed

to

the

AGO

circuit.

I

302

(TBA

120),

as

well

as

amplifying

the

signal

a

further

60

dB,

also

limits

the

signal

correctly.

The

coincidence

demodulator,

which

is

integra—

ted

in

the

TBA

120,

produces

the

AF

signal.

Control

The

FM

IF

voltage

at

the

output

of

I

301

(TA

991)

drives

the

transistor

T

301

(BF

254),

whose

collector

circuit

is

tuned

to

the

frequency

of

the

ceramic

filter.

,

The

diode

D

301,

on

the

secondary

side

of

this

filter,

rectifies

HF,

and

produces

the

AGO

voltage.

The

following

details

are

important

for

un—

derstanding

this

operation:

The

negative

AGO

voltage,

produced

by

diode

D

301,

is

fed

to

the

transistor

T

302

(BC

208

A).

Which

is

biassed

on

when

no

aerial

input

is

present.

When

a

signal

of

more

than

1.5

µV

is

received,

a

negative

reference

voltage

is

produced

at

the

base

of

T

302,

which

is

proportional

to

the

field

strength

of

the

input

signal.

T

302

is

an

NPN

transistor,

and

is

biased

off

proportionally.

A

corres—

ponding

positive

voltage

appears

at

the

col—

lector,

and

is

used

to

set

up

the

stereo

threshold

and

to

prevent

the

stereo

indica—

tor

lamp

from

lighting

when

the

signal

is

noisy

or

too

weak.

The

returning

emitter

current

operates

the

field

strength

meter.

D

301

also

supplied

the

AGO

voltage

for

the

integrated

circuit

I

301.

This

voltage

is

fed

to

pin

5

of

the

IC

via

filter

circuits.

Stereo

Decoder

The

integrated

circuit

I

401

(TBA

450)

used

in

the

stereo

decoder

operates

on

the

ma—

trix

principle.

The

38

kHz

switching

fre—

quency

is

obtained

by

doubling

the

pilot

frequency.

I

401

contains

the

lamp

driver

and

the

two

triggers

for

mono/stereo

chan—

geover.

One

trigger

operates

at

0.9

V,

whi—

le

the

second

is

actuated

by

the

19

kHz

pilot

frequency.

At

pin

7

(left

channel)

and

pin

8

(right

channel),

the

stereo

sig—

nal

is

available,

which

is

then

amplified

to

about

800

mV

by

the

following

transistors

T

401

and

T

402.

The

AF

output

voltage,

from

which

the

AFC

voltage

is

obtained,

contains

a

DC

compo—

nent,

which

is

also

true

for

the

AFC

refer—

ence

voltage.

This

is

equalised

on

the

one

side

by

the

voltage

divider

R

321,

R

341,

R

323

and

on

the

other

side

by

the

stabili—

sed

voltage

available

at

MP

9.

The

AFC

vol—

tage,

which

controls

the

variablecapacity

diode

D

101,

is

adjusted

by

means

of

R

341

(variable).

AM

section

The

AM

HF

circuits

are

built

in

a

conventio—

nal

manner,

with

a

separate

oscillator

and

a

gain

—controlled

HF

transistor.

AM

-IF

The

coupling

of

the

HF

circuits

to

the

IF

amplifier

is

carried

out

by

means

of

an

LC

—

matched

ceramic

filter.

The

IF

amplification

is

carried

out

by

I

301

(TAA

991),

whose

in—

put

is

switched

across.

At

the

output

there

are

two

LC

filters

in

series.

The

signal

is

extracted

selectively.

The

reference

volta—

ge

is

available

at

the

anode

of

the

diode

D

302,

while

the

AF

voltage

passes

through

the

decoder

and

is

amplified

in

the

follo—

wing

AF

amplifier

stages

T

401

and

T

402

to

about

800

mV.

Indicating

meter

The

two

rectifiers

(455

kHz

and

10.7

MHz)

are

connected

in

series.

Current

is

supp—

lied

via

R

315

from

a

negative

potenial.

Thus

a

voltage

is

developed

across

R

315,

which

is

connected

in

a

bridge

circuit

with

R

316,

the

meter,

R

314,

and

T

302

(BC

238

A).

With

no

input

signal,

the

meter

is

not

de—

flected.

When

a

signal

is

received,

the

bridge

becomes

unbalanced,

and

the

meter

shows

an

indication

proportional

to

the

field

strength

of

the

signal.

Power

supply

The

HF

circuits

have

a

separate

power

supp—

ly.

The

AC

voltage

from

the

mains

transfor—

mer

is

rectified

by

D

501

and

D

502

(full

—

wave

rectification)

and

stabilised

by

T

501

and

the

Zener

diode

D

503.

LFsection

AGC

amplifier

The

volume

can

be

adjusted

by

a

tandem

po—

tentiometer

(logarithmic).

This

potentio—

meter

also

has

connections

for

a

physiolo—

gical

control

contour.

T

10

is

used

to

match

the

signal

to

the

high

impedance

volume

con—

trol.

The

signal

is

extracted

via

C

13

and

is

passed

to

the

following

bass

control

P

11,

treble

control

P

12,

and

the

balance

control

P

13.

Power

amplifier

The

signal

is

passed

to

the

base

of

T

30

via

C

30.

T

31

amplifies

large

signals.

The

negative

feedback

via

R

35,

R

34,

R

33,

and

R

43

determines

the

gain

of

the

amplifier.

R

33

is

variable

and

is

used

to

symmetrize

the

two

channels.

The

idle

current

of

the

complementary

final

stage

T

32,

T

33

is

sta—

bilised

by

R

38,

R

41,

and

D

30.

The

AF

sig—

nal

is

fed

via

C

35

to

the

headphone

socket

and

the

loudspeaker

outputs.

Power

supply

The

four

diodes

D

31

—

D

35,

which

are

con—

nected

as

a

bridge

rectifier,

and

the

smoo—

thing

capacitor

C

36

supply

the

amplifier

with

voltage.

3

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