Akai GXC-750D

5. FAST FORWARD AND REWIND SPEED
CONTROL CIRCUIT
1) The reel motors of this deck are DC motors which
at a non-load condition rotate at a high speed，
Consequently，when Ғаві Forward or Rewind is
effected, there is a possibility of tape damage due
to gradual build-up of inertia and increased revolu-
tions. The purpose of this circuit is to control the
voltage supply to the take-up reel in order to
prevent increased motor revolutions.
2) When the deck is set to Fast Forward mode, TR13
is turned ON and the take-up motor begins to
rotate. When the supply reel motor is not rotating,
because bias is not supplied to the base of TR9,
the resistance between TR3 collector and emitter
is infinite, and a fixed bias is supplied to TR12
through. R35 and C13, a fixed DC voltage is
supplied to the take-up reel motor, and there is a
gradual build-up of inertia and gradual increase in
the motor revolutions. However, at Fast Forward
mode, the supply reel motor functions as a
generator. Consequently, the electromotive force
generated by the supply reel motor passes D13,
R30 and R31 and becomes TRO base bias, and the
resistance between TR9 collector and emitter
varies proportionately according to the extent of
the generator’s electromotive force, That is to say,
the variation in the resistance between TR3 collec-
tor and emitter influences the bias supplied to the
ТК12 base. 4
For example, let's say that the take-up reel begins
to rotate at a very high speed. This causes a
counterbalanced electromotive force to be gener-
ated by the supply reel motor and this generated
voltage decreases the resistance between TR9
collector and emitter and lowers the TR12 bias.
Then the resistance between TR12 collector and
emitter is increased, the supply voltage to the
take-up reel is decreased, and finally the motor
revolution is decreased. Thus, the take-up speed
always corresponds with the supply reel motor
speed to prevent high speed motor revolutions.
3) In case by some irregularity, the collector current
of TR12 is increased to cause high speed rotation,
R43 terminal voltage is increased, bias is supplied
to ТЕ base, the resistance between ТК11
collector and emitter is decreased, ТК12 bias is
lowered, and the collector current is decreased.
4) Speed control also functions in exactly the same
way at Rewind mode. However, in this case, the
right hand side reel motor functions as a generator,
` and left hand side reel motor revolutions are
controlled by means of supply voltage control.
Therefore, rewind speed is controlled in the same
way as at fast forward.
А
6. AUTOMATIC SHUT-OFF MECHANISM
CIRCUIT
1) This circuit is for the purpose of effecting auto-
matic shut-off when tape travel has stopped after
play, recording, fast forward, or rewind mode.
2) During tape travel TR25 performs an ON € OFF
switching operation because of the pulse generat-
ing from a rotating rotary magnet within SW9.
Also during tape travel, charge and discharge
current alternately flows to C36 due to the TR25
switching operation that TR26 also performs an
ON + OFF switching operation. From the TR26
switching operation, charge and discharge current
alternately flows. to C39. Only when C39 is
charged, TR28 base electric potential increases
and TR28 is turned ON. This is because C40 is
charged through D70. On the other hand, when
C39 is discharged, C40 discharge current flows
through R130 > R118 ~ ground and TR28 base
electric potential gradually decreases. But before
TR28 is turned OFF, C40 is charged by the
charging current from C39 that TR28 ON condi-
tion is maintained. Consequently, TR29 is in OFF
condition because TR29 base is ground potential.
3) When tape travel stops, C39 charge and discharge
current stops, C40 discharge ends, and TR28 turns
OFF. Then, the TR29 base electric potential
increases to turn TR29 ON, and through D74
which is connected to TR29 collector, Ф side of
the STOP switch is grounded to effect STOP
mode.
4) Also, this circuit functions only when PLAY,
FAST FORWARD, or REWIND circuit is in
operation. When these are not in operation, direct
current voltage is supplied to C40 through R117
and D71, TR28 is turned ON, and TR29 OFF
condition is maintained.
When PLAY mode is in operation, low level IC4
output terminal (6) causes D71 anode electric
potential to be decreased to near ground potential
through D48. And when FAST FORWARD OR
REWIND is in operation, because TR18 is ON,
D71 anode electric potential is decreased to near
ground potential through D44 and TR18.
Consequently, TR29 is turned ON by pulse
electric current from SW901 only when PLAY,
FAST FORWARD, REWIND circuit is in oper-
ation.