Nagra III - Electrostatic Discharge and Motor Noise

Page 15
residuals during recording or play-back wlthout signal on til e other two speeds.
The l eve l in general will be b ettel' thall - 80 db.
Various forms of break-through are po ssible:
On play-ba ck induction between the motor and the replay head, or equally,
induction be 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 by 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 or finall y, 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 era tion, at
irregular intervals. They are caus ed by an electrostatic char ge on the belt.
A little brush is fitted to reduce them. This brush should almost to 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 permallo y C are fitted to eliminate this ancJ they should
not be disturbed. It is norm al that a certain l eve l of pa rasitic noi SE'S should
be obser va ble 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 by 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, beha ves as a ver y small spark-t ype emitter. The
consequent wa ve 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 cke d by ferrite filters in the c:o nnt · ct ing
leads.