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Page 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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