Nagra III - Speed Control Faults

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b) The motor starts occasionally but not always, or on turning by hand.
The commutator may be dirty, clean it but do not lubricate it.
c) The motor starts after pressing accelerator button and comes up to
speed, but does not start on its own. Fault in starter . Measure the
voltage ac ross the starting condenser.
d) The motor starts well, passes the normal sp e ed and then loses speed
and stops. This is a typical case where the servo amplifier and the
starter are functioning, but the discriminator or the tachometer head
is not working. Start the motor with the accelerator button with a
driving voltage of 10 to 15 volts. The tachometer head should develop
more than 8 millivolts. If the head is in good condition inject a signal
of 3 millivolts from a low frequency source corresponding to the speed
of working. This will permit tracing the signal through the tachometer
amplifier.
In case of trouble of this sort it is good to try straight away whether
operation is satisfactory on other speeds. It is at 3 3/4" per s e cond
that one has most difficulty because of a weak tachometer signal.
If operation is satisfactory at 3 3/4", but not at any other speed,
examin e carefully the wires joining the speed switch to the speed regulating
inductances etc .
e) The motor starts but does not reach normal speed. Disconnect the
collector of the transistor T. 4 which discharges the starter condenser
and so ascertain whether this transistor is dischargin g the condenser
too soon or whether the charge on the condenser is t oo sm a ll, that is to
say, whether the motor is too difficult to start. One can then determine
whether the motor is half open circuit and needs too much voltage or if the
supply is incapable of producing the necessary voltage or whether there is
untoward mechanical friction or a faulty servo amplifier. On replacing
the motor by a milliameter, there would be a starting current of at least
400 milliamps flowing, for more than one second. Measure the voltage
across the last transistor in the servo amplifier. If all the applied voltage
reaches the motor, without being able to reach its full speed .......... .
conclusion ob' Lous. On the other hand, if the disconnection of the
discharging transistor T. 4 makes starting possible, check whether the
stand off voltage is accurately applied and that the transistor T. 4 is in
good condition. At 3 3/4 i. p. s. the discharging signal being smaller the
stand off is not necessary. Also the tachometer amplifier does not
amplify the frequency 1000/3 sufficientl y for dipping.
f) The motor starts correctly but passes through normal speed and then
returns to normal. This shows that the starter discharger is not
functioning or functioning insufficiently. Its voltage should be measured
and compared with the schematic. (Check the diode).
2) Faults of Control
Typical Case: When the accelerator is depressed the motor speeds up but when
the button is released the speed of the motor does not fall to normal. This
fault is particularly obvious when there is no tape in the machine.