Nagra III - Wow and Flutter Issues

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10

Cause:

The

servo-

amplifier

passes

current

to

the

motor

even

when

the

discriminator

does

not

call

for

it.

This

is

generally

caused

by

thermal

current

in

the

transistors.

This

trouble

will

arise

on

every

equipment

if

the

ambient

temperature

is

sufficiently

high.

It

is

abnormal

below

40

0

centi-

grade.

Two

possibilities

must

be

considered:

Cl)

The

motor

has

an

abnormally

high

consumption

which

heats

the

servo

amplifier.

(See

"Faults

in

Motors").

b)

One

or

more

transistors

of

the

servo

amplifier

have

deteriorated

(by

over-

heating,

for

exampl'e)

and

their

standing

current

has

becom

e

too

high.

Change

these

components,

being

careful

not

to

over-heat

when

soldering.

This

fault

will

disappear

when

one

short

circuits

to

earth

the

bases

of

the

suspected

transistors.

Each

transistor

base

is

connected

to

earth

by

a

resistance.

The

disconnection

of

such

resistances

is

sufficient

to

increase

the

fault.

A

resistance

of

this

type

is

also

contained

in

the

discriminator.

3)

Wow

and

Flutter

a)

Wow

at

the

frequency

of

rotation

of

the

capstan.

Possible

source:

Mechanical

or

electrical

braking

on

the

motor

(see

"faults

in

motors").

The

tachometer

wheel

is

eccentric

glVIng

a

signal

of

which

the

amplitude

varies

more

than

30%.

The

limiting

amplifier

can

change

this

amplitude

modulation

into

a

phase

modulation

of

the

square

wave.

Such

modulation

will

"mislead"

the

discriminator.

Tachometer

wheel

magnetised:

This

induces

into

the

tachometer

head

a

strong

signal

of

very

low

frequency

which

passes

through

the

tachometer

amplifier

and

upsets

the

discriminator.

Demagnetise

the

wheel.

When

one

has

sufficient

and

suitable

equipment,

this

type

of

fault

is

very

easy

to

localize.

It

is

necessary

to

find

out

first

whether

the

speed

stabiliser

is

not

able

to

compensate

for

a

fault

in

the

motor,

or

on

the

contrary,

if

it

is

the

stabiliser

which

provokes

the

wow,

having

been

upset

by

an

incorrect

voltage

from

the

tachometer

circuit.

b)

Irregular

Wow

The

most

likely

cause

is

slipping

of

the

tape.

Try

re-tensioning

the

pinch

wheel.

A

pressure

of

one

k.

gram

should

give

satisfactory

results;

Check

the

tension

of

the

tape

according

to

the

respective

instruction

sheet.

c)

Flutter

A

rapid

flutter

is

caused

by

a

break

in

the

servo-amplifier

feed-back

chain.

Too

much

feed-back

increases

the

wow

and

could

even

set

the

circuit

into

oscillation.

In

the

case

of

difficulty

with

a

recorder

of

the

first

series

modify

the

circuit

to

conform

with

the

new

schematic.

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