Nagra III - The Speed Stabiliser; Speed Stabiliser Circuit Description; Troubleshooting Motor Starting Faults

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8

THE

SPEED

STABILISER

The

Nagra

III

uses

a

new

technique

for

controlling

the

speed

of

the

motor

and

the

following

notes

will

be

helpful

to

those

who

need

to

service

the

machine.

1.

Description

The

block

diagram

herewith

illustrates

the

principle

of

the

servo-

circuit

which

is

used.

The

motor

carries

on

its

spindle

a

phonic

wheel

as

well

as

the

capstan,

which

passes

in

front

of

a

magnetic

head

-

the

tachometer

head.

This

head

is

magnetised

and

the

rotation

of

the

phonic

wheel

induces

an

alternating

current,

the

frequency

of

which

depends

upon

the

speed

of

rotation

of

the

motor.

This

signal

passes

through

the

tachometer

amplifier

where

it

is

shaped

into

a

square

wave

of

constant

amplitude

of

about

6

volts

peak

to

peak.

This

output

is

taken

to

a

frequency

discriminator.

This

operates

the

servo

a

mplifier

which

controls

the

motor

speed.

As

the

voltage

at

the

servo

amplifier

becomes

increasingly

negative,

so

the

motor

current

will

increase

.

The

circuit

described

is

not

self-starting.

Therefore

the

Nagra

includes

a

starter

which

applies

to

the

servo

amplifier

a

large

negative

voltage

when

one

switches

to

"play-back"

from

"stop"

or

from

"test"

to

"record",

and

also

when

the

connection

between

the

socket"

stop"

on

14b

and

earth

is

opened.

This

applied

voltage

causes

the

motor

to

start

rapidly.

The

system

could

function

just

in

this

way,

the

discriminator

being

sufficiently

powerful

to

intercept

and

take

the

motor

under

control

as

the

speed

passes

intothe

range

of

the

discriminator.

However,

the

speed

would

not

be

correct

immediately.

It

would

be

above

normal

for

several

seconds,

that

is,

for

the

time

necessary

for

the

starting

condenser

to

discharge.

To

overcome

this

difficulty

a

special

transistor

discharges

the

condenser

when

the

circuit

of

the

discriminator

begins

to

resonate.

This

gives

excellent

starting.

At

all

times

a

precaution

must

still

be

taken,

the

signal

coming

from

the

tachometer

amplifier

is

a

square

wave,

that

is,

it

contains

about

300/0

of

third

harmonic.

Put

another

way,

during

the

passage

at

1/5

and

above

all

at

1/3

of

the

speed,

the

circuit

of

the

discriminator

could

resonate

and

the

starting

condenser

will

be

discharged.

We

inserted

a

threshold

diode

,

visible

on

the

diagram,

to

avoid

any

discharge

for

a

signal

of

less

than

500/0.

Notice

that

the

accelerator

button

works

directly

on

the

motor

while

the

rapid

rewind

works

through

the

servo

amplifier.

This

can

help

in

localizing

a

fault.

Remember

that

the

rewind

circuit

is

disconnected

when

the

selector

switch

is

on

"record".

II.

Possible

Difficulties:

1)

Faults

in

starting

a)

The

motor

refuses

to

run

even

after

pressing

the

accelerator

button

or

starting

it

by

hand.

Check

the

tension

of

the

brushes

and

examine

the

commutator.

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