Akai GX-2800D-SS

Vill.
SERVO
MOTOR
OPERATING
PRINCIPLES
DISCRIMINATOR !
Fee
ee
ey
ee
eae
MOTOR
:
Fig.
34
SERVO
COIL
G
(Gap):
Adjust
to
obtain
a
detector
head
terminal
voltage
of
3
+0.5
mV
at
7-1/2
ips.
OUTPUT VOLTAGE €o(V)
DISCRIMINATOR
rpm.
FREQUENCY (Hz)
re
Fig. 35
1.
The
detector
head
core
is
comprised
of
a
permanent
magnet,
and
the
gear
installated
on
the
motor,
by
magnet,
and
the
gear
installed
on
the
motor,
by
the
magnetic
flux
of
the
detector
head
core.
Ac-
cordingly,
the
detector
head
coil
works
in
the
same
way
and
generates
A.C.
Voltage
(becomes
the
detec-
tor
signal
described
below).
When
this
detector
signal
voltage
(ej)
becomes
detector
signal
frequency
(fo),
this
adjusts
the
capstan
motor
revolutions
proportionately.
(See
Figs.
34
&
35)
2.
When
the
detector
signal
voltage
generated
from
the
detector
head
is
about
3
mV
(at
7-1/2
ips),
because
the
level
is
low,
the
perpendicular
(up
and
down)
waveform
is
amplified
by
the
A.C.
Amplifier
until
the
waveform
is
clipped.
(See
Fig.
34)
at FWD or REV
= Starting Time
7-1/2 ips 110V. 150V
; Capstan Motor
Capstan Motor | Supply Voltage
Supply Voltage
Chart 3
Voltage and Frequency
Generated at the
Detector Coil Speed
iar 040Hz ~ $20 r.p.m. 1,420 r.p.m.
1.5mV+0.25mV 520Hz
3. Discriminator Coil L1 (VI2031SC-01) and C210
(0.051/50) at 7-1/2 ips and C211 (0.27/100) at
3-3/4 ips constitute the resonance circuit, and this
resonance frequency becomes f,. Because the detec-
tor signal frequency generated at the detector coil
differs according to capstan motor revolutions, the
capacity of the discriminator resonance condenser
changes and the resonance frequency changes at
the different tape speeds of 7-1/2 and 3-3/4 ips.
4. When the discriminator input frequency and the
resonance frequency f; are simultaneous, the D.C.
signal to be supplied to the next stage D.C. Amplifier
is not generated. Consequently, when the capstan
motor rotates at normal speed, a higher than reso-
nance frequency f,, detector signal. Signal frequency
f. is established.
. As shown in Chart 3,-when electric current is not
‘flowing to the capstan motor servo coil, the capstan
motor revolutions are far faster than normal re-
volutions. Consequently, in order to maintain normal
revolutions, an electro-magnetic field is generated
at the servo coil by means of collector current
flowing to TR-209 (25D234), and this serves as an
electro-magnetic brake. This electro-magnetic brake
and the load torque balances the capstan motor
torque and normal revolutions are maintained.
* Uncontrolled
Capstan Motor
Speed
Controlled
Capstan Motor
260 r.p.m. 1,380 r.p.m.
rm...
26