Scott 430A,410A Service Manual

Service Manual

Integrated

Stereo

Amplifiers

430A/410A

H

SCOTT

The

Name

to

listen

to.

CONTENTS

Specifications

3

Block Diagram

4

Circuit

Description 5

Adjustment

6

Schematic

Diagram

7

Parts Location

Diagram 9

Exploded

View

10

Replacement

Parts List

12

CAUTION NOTICE

The following safety

precautions must

be

followed

to assure

continued

reliability and

safety

against

fire and shock hazard:

1. Replacement parts used

during servicing of this appliance

must have

identical characteristics as those offered and

recommended

by

H. H.

Scott, Inc.

2. A dielectric test

is to be performed on each appliance

following the

re-assembly and before returning the unit to

the custmer.

3.

The dielectric test

to be performed on H. H. Scott, Inc.

electric components

serviced in the United

States

and

Canada for use in

these countries

shall

consist

of

not

less

than

the following:

*

1)

A

dielectirc tester

designed

to

supply not less than 1

100

volts at

60Hz

and

employing leakage current indicator(s),

is to be

used.

2)

The tester

is to

be

connected per the instructions

enclosed with the

instrument,

or

as follows:

a.

The

tester is

connected to

the power line

receptacle

and the power

switch is

turned on.

b.

Sufficient

time is

allowed for the tester supply

to

stabilize

and

then

the output voltage is adjusted for

1080V.

c. Leads

of the tester,

usually marked

GND and HV,

are connected

between chassis ground and both

blades of the

male plug of

the

power cord.

d. Switch

tester to "test" and observe

leakage

indicator.

Leakage

current must not exceed

0.5mA.

*

Dielectric tests made

by

service personnel in countries other

than

USA

and Canada

must

use test equipment

and procedures

specified

by

the

safety

agency

serving that

country.

SPECIFICATIONS

430A

(410A)

Minimum

Continuous

RMS

Output Power per

channel,

both

channels

driven into 8

Ohms from

20

Hz

-

20

kHz

with

no more than rated THD

45

watts (30

watts)

Total

Harmonic

Distortion

[78

IHF

rated, at 20

Hz

-

20

kHz]

0.08%

(0.1%)

Intermodulation

Distortion

[at rated output,

60:7000Hz; 4:

1]

0.08%

(0.1%)

Frequency

Response

[at 1

watt output, ±ldB]

20

Hz to

20

kHz

Power

Bandwidth

[at

—

3dB]

10

Hz to

30

kHz (15

Hz

to 30

kHz)

Damping

Factor

[at 1

kHz, for

8

Ohm

load]

>40

Input

Sensitivity

[for rated

output]

Phono:

2.5mV

Aux,

Tuner:

150mV

Tape

1

and

2: 150mV

Tape

2

DIN

Input:

150mV

Maximum

Input

Voltage

Phono:

150mV

Aux,

Tuner:

10V

Tape

1

and

2: 10V

Tape

2

DIN

Input: 10V

Signal-to-Noise

Ratio

[shorted

input, IHF

A

network]

Phono,

Ref.

10mV: 85dB

Aux,

Tuner:

90dB

Tape

1

and

2:

90dB

Tape

2

DIN

input: 90dB

Tone

Control

Range

Bass

(100

Hz):

+10dB

Treble (10

kHz):

±10dB

Loudness

Contour

[Volume

Control set to -30dB]

100

Hz:

+7dB

10

kHz:

+3.5dB

Crosstalk

1kHz:

75dB

Channel Balance

[

maximum Volume Control]

0.5dB (0.7dB)

RIAA Tolerance

[78

RIAA rated, 20 Hz to

20 kHz]

+0.7dB

Channel Separation

[78

IHF rated]

Phono

(1

kHz):

60dB (55dB)

Aux, Tuner, Accessory Input, Tape 1 and

2,

Tape

2

DIN Input

(1

kHz):

60dB

Tape

Recording Output Level

[at

rated input

sensitivity

level]

Tape 1 Rec: 150mV

Tape 2

Rec:

150mV

Tape 2 DIN Output: 30mV

AC

Power Requirement*

220V

50Hz

Power

Consumption

370W (250W)

Dimensions

17-3/4"W,

5-1/4-H, ll-3/4"D

(17-3/4",

5-1/4",

8-1/2")

430W,

132H, 300D

(430,

132,

217)

mm

Net

Weight

18.9

lbs

(15.5

lbs)

8.5 kg

(7.0

kgs)

*

AC

Power:

Units for Great Britain:

240V,

50Hz

Units for

USA

and

Canada:

120V,

60Hz

Class

2,

double isolation

system employed

3

CIRCUIT

DESCRIPTION

Design

Philosophy

on

430A and 410A

When using

a

high

gain

wide band

open loop

operational

amplifiers, it

is

possible

to

design

a

passive

network

providing a

negative

feedback to

control the

amplifier gain

and

frequency

response.

In

fact it

makes

easy

the

reproduction

and

repititivity

of the

wanted

results.

We

can

define

the gain

of

such an

amplifier as

follows.

Zm

o 'VW-

Vln

O

r

OP

AMP

3

PHONO IN R CH

C4

[PHONO AMPL tFIER USING OP AMP]

1)

Vo

=

-Vin-

2)

Rin

=

Zf

Zin

(Inverting Amp.)

OOUT

3)

Vo

=

Vin

•

1

+

Zin

c

—

R

in

^

Zf

Zl

Zino

•

Gopenloop

G

closedloop

(Non

Inverting)

Where

Ri

n

is

the

input

load

resistor. Z

m

is

the open loop

input

impedance

multiplied

by

open

loop

gain divided

by

closed

loop

gain.

Example:

If

open

loop gain

is

10,000

(i.e) 80dB)

and the

closed

loop gain is

100

(i.e

40dB).

For

an

amplifier

having

Zin

=

10K ohm,

the

equivalent

Zin

c

=

Rin

//

1000K

ohm.

Phono

Equalizer:

The

phono non

inverting

amplifier

equalizer

consists

of an

operational

amplifier

with a

feedback

network

to

fullfil

the RIAA

equalizing

requirements.

Zf

=

R8

//

X

C

6

+

RlO

»

*c8

Zl

=Ri2

+

Xci2

1

1

where

X

c

=

2TTf-C

6.28 f-c

f

=

frequency

c

=

capacitance

in Farads

R

=

Resistance

in

Ohms

The

above

network

provides a

gain

of 60

at

1kHz

and

a gain

variation

as

function

to

the value

of

Zf

and

Z\ at

any

given

frequency.

For

example at

100Hz the

gain is 265

or

+12.9dB

referred

to

1kHz gain, at

10kHz

gain is

about

13

or

-l3.7dB

referred

to

1kHz

gain.

Premain

Amplifier

The

premain

amplifier is an

operational

amplifier built

by

discrete

components. It is

directly coupled to

the load (i.e

speakers).

The high gain open

loopisprovided

by

the dual

differential

amplifiers and the boostrap

capacitor.

The

complementary

output

drivers/buffers provide a

symmetrical

output drive.

The amplifier is

controlled

by

multiple

feedback

networks:

DC

feedback (R93

+

R95) on

one channel (R94

+

R96)

on

the other are directly

coupled

between

output and

negative

input. The

effect of the DC

feedback

is

for longterm

stability and

unity DC gain.

The effect of

this

feedback

is

minor at

frequencies over

5Hz.

The

multipole

AC

feedback

network (with the bass and

treble

potentiometers

incorporated)

provides

a

constant gain

with no

effective

gain variations

of tone potentiometers. At

100Hz

the bass

potentiometer allows

gain control of ±10dB. At

10,000Hz

the

treble

potentiometer

allows

gain

control

of

±!OdB.

The amplifier has a

current limit network

that

limits

excessive current

loading.

A

fuse is provided

to

prevent

damage to

speakers if

the amplifier fails.

Tone Control

The tone control is a

negative feedback type whicli

uses the

power amplifier

stage

as

the active element. That

is,

the

gain of the power

amplifier stage

is

controlled

by

the

tone

controls circuitry. At

1kHz,

the position of

the tone

controls

has

little

effect

on

the

gain, as C5

3

impedance

is

high,

removing VR2

from the circuit, and CI,

C3

impedance

is low,

effectively short

circuiting VR1.

Bass Control: As

the frequency

decreases below

1

kHz,

the impedance

of CI and

C3

increases

proportionately.

Thus

at

very low

frequencies, the gain is

mainly

determined

by

the

position

of the bass

control

VR1.

Rotating

VR1

toward R3 will

boost the low

frequencies, while

turning

it

towards

C4

will

cut the bass.

Treble Control: At

high frequencies,

as at

1kHz,

\/Rl is

effectively short

circuited.

At

these

frequencies,

\

owever

C53 and C55

impdeance decreases, so that VR2

Is

comes the

the main

control of

the amplifier

gain. Rotating VR2

towards

C53

cuts

the treble response.

5

Power

Amplifier

This

circuit

is

an

OCL,

pure

complementary

amplifier.

The

input stage

consists

of

two

differential

amplifiers

(Ql and

Q.3/Q.5).

The

first

differential

amplifier

(Ql

)

is a

matched

transistor

pair

in

one

package

providing

excellent

common

mode

rejection

and

low DC

offset.

Q5

acts as

the

voltage

amplifier

providing

voltage

swing to

nearly full

plus and

minus

supply.

Current

gain is

then

provided

by

the

fully

complementary

Darlington

pairs

of

Q7

and Ql

(Power

transistor)

for

the

positive

swing,

Q9

and

Q3

(Power

transistor)

for

the

negative

swing.

The output

stage

bias

is

set

by

the

double

diode

Dl and

RV1. As

previously

described,

amplifier

gain is

set

by

tone

control

circuitry.

The driver

and

output

stage is

protected

from

short

circuit

and

overload

by

transistors

Qll and

Q13,

which

short out

the driving

signal

when

current

through the

output

transistor

reaches

an

excessive

level.

Power

Supply

The main

power

supply consists

of

a

full wave bridge

rectifier and

two

6800uF

capacitors. The

B+

and

B-

regulators

(zeners, D31 & D32)

supply

stabilized voltage

for the low

level

circuitry.

Unwanted

transients are

eliminated

by

circuitry

consisting

of

Q16

which performs a

muting

function

when the

unit

is

switched ON

or OFF.

The base bias

of

Q16

is

given

by

two

different

circuits; one

normal

positive

line voltage circuit

having

a

large

time

constant, and

another

negative supply

voltage circuit

having

a

small time

constant

(C63/R84). At

turn on,

the negative

voltage is

immediately

supplied

to

the base

of

Q16

because

of its smaller time

constant, and

this makes

Q16

cut off.

Then

C61

is

gradually charged up by

the

normal

positive

power

line voltage and

when the charged

level

is

reached to

a

proper level,

the power

line

switching

transistor Ql 5

is

turned on

and

supplies the power to the

differential

amplifiers. At

turn off, the

negative

base

supply

voltage

to

the

Q16

is

immediately decreased to

zero because

of

its

small time

constant,

then

the

base bias

is

supplied

from the

positive power

line voltage only, and

Q16

is

turned on

immediately, resulting

in shorting the

Ql

5

base to

the

ground and eliminating the supply for the

differential

amplifiers,

stoping

amplifier

operation

immediately.

ADJUSTMENT

Equipment

Required

Audio

signal

generator

DC voltmeter

Speaker

load

resistors, 8

Ohms, 100W

Digital

voltmeter

The

following

adjustments

are the same

for

both

left and

right channels.

Bias

Adjustment

1)

Connect

8

Ohm

resistors to

the

speaker

"A"

terminals,

and

set

the

Speaker

Mode

switch to

"A"

position.

2)

Turn

the

Volume

control fully

counter-clockwise.

3)

Turn

RV1 fully counter-clockwise.

4)

Set digital voltmeter

to most sensitive voltage

range.

Connect probes across

TP1 and TP2

(Voltmeter bias test

point, L

channel).

Turn unit

on. Let it idle for 10 seconds.

Adjust RV1 for

40mV across the resistors.

5)

Perform the same procedure

for the right

channel,

except

measure

voltage across

TP 3 and TP4 (Voltmeter

bias

test point,

R

channel).

Adjustment

is made with RV2.

6)

Leave the amplifier

on for

about 30

minutes, then.

recheck

measurement.

A tolerance of +25% is

acceptable.

Readjust if necessary.

6

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