Denon DP-75 - Theory of Operation; Servo Circuit Operation

THEORY
OF
OPERATION
KU-350 E
"SERVO CONTROL UNIT
1" ;
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we it Limiter. - 1. Forword es
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Referens
Bie NS UR Se Ae oe oe Be 9 ae. Osciiletion
: IC 2, TRI
; | Limiter
ne i Amp.
boca |
IC 1
33,45, STOP :
| * 108, TR7.
| Rotation — :
a HTT
Levee
.-TRE6
|
—
:
:
a
er
a
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oe
-
Strobe
|.
oo
eee
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an
-
-
Scope
'
.
Ic2 TRIG, 15,18 _
Forward AM-
Modulator
* “| Reference
: Voltage - : oS oe e 2 | 4
; :
| Comparator
Reverse AM-
Modulator |
1
Ic2 “-. TRIG,I7,19
0 ~O————+ POWER |
—j Transformer
ue Tae: SOURCE
Fig.
1.
Block
Diagram
of
DP-75.—
Block
diagram
of
the
phono
motor
servo
circuit
is
shown
in
Fig.
1.
_
|
Pe
The
speed
detection
magnetic
head
encapsulates
two
sets
of
detection
windings
with
a
phase
difference
of
90
de-
grees.
This
head
detects
direction
of
platter
rotation
as
well
as
its
speed,
a
The
output
waveform
detected
by
the
magnetic
head
is
shaped
into
rectangular
waves
by
the
limiter
circuits
(IC
2
and
IC
3
on
KU-350).
The
phase
difference
of
these
rec-
tangular
waves
gives
signal
if
the
direction
of
platter
is
forward
or
reverse.
©
sees
At
the
same
time,
the
output
frequency
(555.5Hz
at
33
rpm,
750Hz
at
45
rpm)
of
one
of
the
limiter
circuit
(IC
2,
TR
1
on
KU-350)
is
fed
to
the
servo
control
IC
(IC
1
on
~KU-351).
On
one
branch
inside
the
1C.1
(on
KU-351),
the
frequency
‘signal
‘s
converted
into
a
voltage
inversely
proportional
to
the
speed
(frequency)
by
the
F/V
converter,
On
the
other
branch,
the
input
frequency
is
divided
therein
to
the
same
frequency
as
the
reference
frequency
(138.9
Hz
at
33
rpm,
187.5Hz
at
45
rpm)
which
is
also
diviced
from
the
9MHz
quartz
crystal
oscillator
so
that
the
phase
relation
can
be
compared.
The
phase
difference
appears
at
the
output
in
terms
of
voltage
difference.
Fis
_ The output voltages of the F/V converter and the phase
comparator are mixed and then compared with a reference
voltage to modulate the amplitude of the motor drive
circuits of either forward or reverse direction. |
The change-over of the speed ‘is performed by changing the
dividing denominator of the reference frequency and
simultaneously changing the proportion of the F/V con-
verter. oe
_ The strobe scope is: flashed by 1/2 the frequency of that
__ of the comparison reference. : es
The
limiter
output
(IC
:2,
TR
1
on
KU-350)
further.
detects
if
the
platter
is
rotating
or
not,
feeding
the
signal
into
the
rotation
detection
circuit
(TR
8,
IC
5
on
KU-350).
Com-
bined
with
this
rotation
detection
circuit,
when
the
start
button,
33
or
45
is
pressed,
the
power
memory
circuit
(IC
5
flip-flop,
TR
7
on
KU-350)
holds
the
power
relay
RL
11
on
PS-147)
until:
the
rotation
becomes
almost
still
after
the
stop
button
is
pressed,
)