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Nagra III - Motor Service and Noise Reduction; Motor Dismantling and De-magnetisation

Nagra III
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Page
11
MOTOR
The
motor
of
the
Nagra
is
of
the
permanent
magnet
electrodynamic
type
and
resembles
a d I
Arsonval
galvanometer
with
central
magnet.
It
may
be
the
cause
of
a
number
of
difficulties
and
the
following
notes
are
intended
to
assist
in
such
cases:
If
the
recorder
refuses
to
start
occasionally,
even
when
one
presses
the
accelerator
button,
but
in
general
starts
when
one
turns
it
lightly
by
hand,
the
surface
of
the
com
mutator
should
be
examined
-
see
paragraph
5 .
If
the
recorder
runs
irregularly,
see
paragraph
5
and
subsequently
paragraph
6.
If
the
recorder
produces
a
flutter,
check
the
smoothness
of
rotation
of
the
motor.
There
must
be
neither
rough
spots
nor
perceptible
play.
If
the
motor
shows
speed
up
following
excessive
heating
of
the
power
transistor
of
the
servo
amplifier,
see
first
paragraph
6,
then
4
and
3,
and
finally
1.
1.
Dismantling
and
de-magnetisation
The
magnet
is
fixed
in
the
assembly
by
a
left
hand
threaded
set
screw.
The
winding
surrounds
it.
To
remove
the
rotor,
after
first
having
unscrewed
the
centre
screw
(normal
right
and
thread),
it
is
necessary
to
introduce
a
rod
3
mm
in
diameter
into
the
hole
in
the
rotor
between
two
wires.
Turning
the
rotor,
if
necessary,
one
can
reach
with
the
rod
one
of
the
two
notches
in
the
magnet
and
so
unscrew
it.
This
gives
access
to
the
central
ball
race
of
the
assembly
(type
EL4ZZ).
This
ball
race
must
be
of
very
high
accuracy
,
much
superior
to
those
of
normal
commercial
type.
In
case
of
need,
ask
for
replacements.
In
taking
out
the
rotor,
it
is
essential
(unless
you
have
a
magnetising
machine
of
considerable
power
such
as
we
have)
to
short
circuit
the
magnetic
circuit,
otherwise
the
magnet
will
lose
about
25
%
of
its
magnetism.
One
can
do
this
by
sliding
the
rotor
into
another
iron
tube
of
suitable
size
(internal
diameter
56
mm,
external
diameter
minimum
60
mm)
in
such
a
manner
that
the
magnetic
circuit
is
never
opened.
In
case
of
repairs
on
the
field,
one
obviously
cannot
take
this
precaution.
The
considerable
reserve
of
power
of
the
motor
ensures,
nevertheless
,
that
the
recorder
will
still
be
usable.
The
consumption
will
however
be
higher
and
rapid
rewinding
will
be
slower.
In
such
cases
it
is
necessary
to
return
the
rotor
to
us.
We
will
return
it
to
re
-
magnetised
and
in
a
short
circuiting
type
of
tube
from
which
it
can
be
slid
into
the
motor.
To
check
that
the
magnet
has
not
been
partially
de-magnetised
the
voltage
required
on
no
load
should
be
measured,
that
is,
without
tape
and
without
the
pinch
wheel
being
engaged,
at
15
i.
p.
s.
(600
r.
p.
m.)
A
motor
in
good
condition
requires
9 V ±
0.5
V.
A
motor
de-magnetised
by
dismantling
without
a
magnetic
short
circuit
will
need
only
7,5
volts.
Later
dismantlings
will
not
increase
the
amount
of
de-magnetisation.

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