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CONTROL UNIT
mcmianrnmnen S
.
5.3v o>
{DECK +B)
1C3
PAQOO0S8
=
AMP 12
"<1 AMP
SS
ate
PL AY CURRENT
MODE |
CURRENTAMP II
PLAY COMMAND
PLAY MOTOR
OVER SPEED
DETECTOR
Vsl=6.5v)
mh
Vs(*6.5v}
me AMP 21
9.17
FF/REW AND MS MODES
A constant speed servo is applied to the motor during FF
and REW modes. And by introducing a slight load gradient,
the motor
rpm
speed
is decreased
at a fixed rate as the
motor load is increased. Consequently, the tape travel speed
can be kept at a constant value irrespective of the winding
diameter on the take-up reel. The motor speed control in
this case is handled by a bridge servo, and for this purpose
the IC also includes a differential amplifier SERVO AMP
and
a CURRENT
AMP
for amplification of the SERVO
AMP
output,
Externally, the IC is connected
to a bridge
circuit with the motor armature coil resistance (Ra) form-
ing one
of the arms to enable the circuit to detect the
motor rpm speed. Constant current from the deck +B line is
passed to R112 to generate a reference voltage Vref across
that resistor.
The
bridge resistance values are set so that
R8 x Rb = Rax Re
If the motor rpm speed should happen to decrease, result-
ing in a motor counter electromotive voltage Ea lower than
the reference voltage Vref, a positive voltage is applied to
the servo amplifier input to amplify the output from that
amplifier. This output is inverted and amplified by CUR-
RENT AMP 22, resulting in the Q2 base current also being
amplified. The collector current is consequently amplified
..and the motor
is accelerated. The motor counter electro-
motive voltage Ea is thereby increased as the motor rpm
speed is increased. In other words, the motor finally settles
down at a constant rotational speed where Ea and Vref are
Practically equal.
FILTER Assy
eae
mes AMP 22
sc AMP
R107
R8
Fig. 39
a ,
In this arrangement,
however, there is no decrease in rpm
speed when the motor load is increased. For this reason, the
bridge balance is offset a little (that is, R8 x Rb is set toa
value lower than Ra x Rc) to enable error to be generated
in the
bridge
detector
voltage when
the motor
current
is increased (as a result of the increasing load).
In other words, the apparent motor counter electromotive
voltage Ea is increased in response to load increases, there-
by enabling
a decrease
in the
motor
rpm
speed
to be
achieved.
The
same
mechanism
takes
effect during MS
mode.
34
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