A77

Revox A77 Service Manual

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CAPSTAN

MOTOR

Fig.

5.10-49

130

V- (

150

V-)

0V-'

Circuit

1.077.725

consists

of

tachometer

signal

amplifier

( Q

201

to

Q

205 ), the

discriminator,

and

d.c.

amplifier

with

power

stage (

Q

206

to

Q

209 ).

The

tachometer

signal

amplifier

amplifies,

makes

symmetrical

and

limits

the

pick-up

frequency

to

supply

square

wave

pulses

to

the

discriminator.

D.c.

dependent

feedback

stabilizes

the

quies-

cent

points

of

the

amplifier

stages.

The

discriminator

con-

sists

of

an LC-

series-

resonance

circuit

of

utmost

constancy

due

to

type

of

circuit

and

components

used.

Best

drift

characteristics

are

essential

to

guarantee

the

required

maxi-

mum

speed

variation

of .

2 % !

The

discriminator

frequency

can

be

changed

from

1600

Hz

to

800

Hz by

switching

in

capacitor

C

207.

In

addition,

diode

D

201

is

polarized

in

conductive

direction,

thus

requiring

at

the

control

side

but

one

low

current

contact,

it

also

decouples

the

control

line ideally.

V

Operating

Range

of

DC-

Amplifier

500

1000

2000

Fig.

5.10-50

f

The

series

potentiometer

P

201

allows

detuning

the

reso-

nant

frequency

slightly.

This

permits

calibration

of

the

speed

separately.

The

high

speed

is

set

by

adjusting

prima-

rily

the

circuit's

inductivity.

Fig.

5.10 -

50 shows

the

voltage

characteristic

as

a

function

of

frequency

for

the

whole

discriminator.

The

operating

range

of

the

following

d.c.

amplifier

is

given

for

clarification.

The

discriminator

is

temperature

compensated

along

with

the

entire

circuitry.

Fig.

5.10 -

51

shows

the

typical

thermal

characteristic

of

the

whole

system,

measured

inside

the

climatic

chamber

while

motor

was

running.

1600

+0,2,

• 5

'25

••15

•65

C

Fig.

5.10-51

The

signal,

delivered

from

the

discriminator,

is

stripped

of

a.c.

components

from

the

tachometer

frequency

by

in-

tegration (

C

210 &

C

211 ).

The

R

215 -

C

209

member

corrects

phase

error

in

the

control

circuit

and

improves

stability.

D.c.

amplifier

Q

206 -

Q

207,

whose

operating

range

is

shown

in

the

discriminator

characteristic,

has

an

amplification

of

40

dB.

It

brings

the

signal

to

the

value

needed

for

power

amplifier

Q

209.

The

phase

inversion

stage

Q

208

has

little

amplification.

Power

stage

Q

209

consists

of

a

transistor

which

resists

those

peak

voltages

that

can

occur

during

cut-off

and

is

able to

conduct

the

power

loss,

generated

during

operation,

onto

the

heat

sink.

Some

interesting

system

data :

1.

Steepness

of

the

discriminator

and

amplification

factor

of

the

following

amplifier

cause

the

motor

voltage to

traverse

from

0

Vac

to

the

maximum

value

130

Vac

within

3

Hz

frequency

deviation

at

the

1600

Hz

tacho-

meter

signal

frequency.

This

means

that

the

motor

recei-

ves

full

or

no

voltage

for

a

percentage

deviation

of ±.

1%.

2.

Line

voltage

variations

of

20 %

cause

a

deviation

in

speed

of

the

order

of ± .

04 %.

3.

Variations ±

10 %

in

operating

voltage

21 V,

cause

a

deviation

of

speed

by

-±-.

1 %

I

operating

voltage

is

addi-

tionally

regulated

1.

Li

24

1

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Operating Instructions51 pages

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Revox A77 Specifications

General

Brand	Revox
Model	A77
Category	Tape Deck
Language	English

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