Akai AP-D40

WORKS
Vili. EXPLANATION OF HOW THE SERVO CIRCUITRY
1. MODEL AP-D40/C (Refer to the schematic diagram.)
TRI
THRESHOLD LEVEL
(a) icl 2 0
{ i
|
I I
I !
| i
i '
| I
TRI
(b) COLLECTOR oO
TR2
(c) BASE fe)
TR3
(d) BASE
f $25Hz (33-1/3rpm)
f =32Hz (45 rpm)
TRS
THRESHOLD LEVEL
TRS
(e) BASE
A Z
TRS | | | |
(f) COLLECTOR Oo L
A
|
ae bs [Ys -——
TRIO
(h) BasE oo
The FG signal obtained from the motor block print
coil is amplified by IC1 and added to TR1’s base.
When the FG signal passes TR1’s threshold level. TR1
goes on and waveform (b) appears in the collector.
Differentiated by C4 and R7, the positive pulse of
the differential signal turns on TR2. As a result the
integral waveform of waveform (d), with C5’s current
curve, appears in TR3’s base. TR5 only goes on once
the threshold level of waveform (d) has been passed
and then waveform (f) appears in TR5’s collector.
The voltage of C6’s two terminals is as with the
waveform (g). This is because current flows in C6
during the period of waveform (f) only, regardless. .
This signal (waveform g) passes IC 1’s low pass filter,
makes a DC control signal and becomes TR10’s bias
voltage. As a result, collector current flows into TR10
and control current into the Hall elements H1 and
H2.
Fig. 11
When the speed is faster than fixed, the period of
FG signal shortens and the pulse width ¢ decreases,
Consequently waveform (g)’s voltage drops as does
the base bias of TR10. Then the current to the Hail .
elements is reduced, the motor torque is weakened
and the speed slows down, or in other words the
speed of revolution is maintained at a fixed rate.
When the speed slows, the period of FG signal
lengthens and th pulse width ¢ increases. Then the
current in the Hall elements is increases, and the
motor torque strengthened. In this way, the speed is
cotrolled at a fixed rate. Pitch control is the
changing of TR3’s emitter potential. by the changes
of TR4’s base bias. As a result, the threshold level of
waveform (d) is raised and the pulse width changed.
In this way, the Hall element current can be con-
trolled and the speed changed.
15