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Page 15
residuals
during
recording or
play-back
wlthout signal
o
n til
e
other
two
speeds.
The l
eve
l in
general will be
b
ettel'
thall - 80
db.
Various forms of
break-through
are
p
o
ssible:
On
play-back
induction
between
the
motor and the
replay head,
or equally,
induction b
e
tween the tachometer head and the
wiring
to the
replay
head.
A re-routing of these
wires may reduce the trouble.
One
can
diagnose thLs
trouble
by
short circ1Jiting the
wires to
the head
on the
H.
F. filter
on
the
chassis. A similar
fault
can be
produced
b
y
damage to the
discriminator
filter or
servo amplifier.
It
is normal that there
should
be some break
through when the
apparatus
is
working
with
the chassis
opened.
On
record, the residual
signal
may
come through
induction into the
microphone preamplifier
and
cable
or by
coupling to the input
connection
of the power amplifier and
cable
o
r
finally,
through
the
power leads
or the
earth
leads. These residuals
are adjusted
to
be
negligible before delivery
of
the recorder. Should
they
a
ppear, it is likely
that a
decoupling has
deteriorated,
or a change
in
earthing, or a misplacem ent of
the
connections.
d)
Electrostatic Discharge
This
consists of short sharp cracklings when the motor
is in op eration, at
irregular
intervals. They are
caus
e
d
by
an
electrostatic char
g
e on
the
belt.
A little brush is fitted to reduce
them.
This brush should
almost
t
o
uch
the
belt.
In
cases where
these
parasitic
noises
occur,
verify that this
brush has
not
been
displaced.
e)
Parasitic Noises from the Motor
These
are caused
by
commutation
of
the
motor.
Th
s
e
are easily
icJentifiabJe in
that
they are related to
the rotation of the motor.
Th
ey
can be recJucec! to
U
,en".!
extent
at
the
source
by
first che
cki
ng
that
the
commutator is in
guod
cll'zlr1
order
and
that
the brushes
make good contact,
and then
by
rernoving
a
ll
possible
paths of
transmission
between
the
motor ancJ
the amplifiers.
There
are various
routes:
Magnetic induction.
The
rotor carries a current affected
by
rarasltics
and, therefore,
generates a
magnetic
field containing
parasitic
com poncnts.
Various
screens of
permalloy
C
are
fitted to
eliminate
this
ancJ they should
not be disturbed.
It
is norm
al
that
a
certain l
eve
l of p
a
rasitic
noi
SE'S
should
be observable when
the recorder
is
open
because part
of
the
scrcpning
is
on the
bottom
of the
case.
Electrical
coupling at
low
frequency.
The motor
current
has
a
parasitic
component
which
is
by-passed b
y
smoothing condensers.
The
mvthod of
connections
to
earth
has been
very carefully
d
esigne
d
and any modification
can
pro
vo
ke
the occurence
of parasitic noise.
Electrical coupling
at
high frequency. The
co
mmutator,
as
with
all
contact
brakers,
behaves
as
a
very
small
spark-type
emitter. The
consequent
wave
trains which
are generated are very
easily propagat ed
and
then rectified by
a semiconductor, and
there ar
e
a
lot
in
thE:' Nagra.
This method
of
coupling is blo cked
by ferrite filters in
the
c:o nnt·ct
ing
leads.
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