Akai GX-635D Service Manual page 20

2) Explanation of the circuit operation (Refer to
F i g s . 6 , 7 )
The velocity signal obtained from the motor's
frequency generator is shaped into a waveform in
the period of "T" proportionate to the velocity as
in (a) on collector TR6. This signal is differen-
tiated and enters TR7 base. This makes TR7's
collector output as in (c) and turns ON TRl0
during the time of the negative pulse.
Meanwhile TR8 base is supplied with a waveform
like (d) and when it reaches the threshold level,
TR8 is turned on. A waveform (e) with pulse
width "11 " is obtained from the collector. During
the time "T" that TR28 is ON, TR9 base electric
potential decreases to turn ON.
Consequently, TRI I base electric potential
inèreases and TRI I is also turned ON for the time
" t r " .
When the period "T" varies with the velocity,
the pulse width "t1 " varies according to TRS's
threshold level. But when TRl0 is on, it is con-
stant due to the time constant. So that by using
the period that TR is ON as the standard pulse,
the pulse width "t2 " is constant regardless of the
velocity.
(g) and (h) show the ON and OFF condition of
TRl0 and TRll. When the electric charge that
was charged to C16 is discharged during the time
T R l I i s O N " t 1 " , a s i g n a l v o l t a g e o f " e " p r o p o r -
tionate to the velocity can be obtained. Next, with
"e" as its basis, Cl6 is charged
during the limited
time of the standard pulse width "t,
",
and by the
charging voltage "e" the charging is stopped.
The voltage is then held until the hext standard
pulse.
This voltage "e" is supplied to the TRl2 base and
a servo signal "E" is obtained. This servo signal
"E"
controls TR13 and drives the motor.
As shown above, since "tt " and "t2 " are small,
servo signal ripples are made small and a near
direct current servo signal is obtained. In addition,
the time constants of the smoothing circuit can
also be made small that phase lag factor can be
minimized.
22