Fostex A-8 - Page 17

Q26 and Q36.
Now, the takeup reel motor torque is adjusted to 320g.cm ±50g.cm by pot R129
(22KQ) , and the supply reel motor torque to 220g.cm ±50g.cm by pot R161 (22KQ)
which adjustments result in potentials of 14.5 'V 15V at pin #12 of U12-14, and
13.5 14V at pin #3 of U12-1.
U12 — 14, Q30, Q31 and U12-1, Q37, Q38 are constant current output circuits and
as the potential across R136 and R167 changes in accordance to the motor current,
these potentials are fed back to the constant current circuits of U12-14 and U12-1.
The two circuits of UI 2-8, Q24 and UI 2-7, Q34 are provided to prevent a sudden
high tape tension by gradually raising the voltage to the takeup reel motor at
F.FWD or REWIND modes.
Taking for example the circuit of U12-8 and Q24, when power is switched on,
the potential from the dividing resistors R131 and R132+R136 for +24V DC charges
C30 via U12-8.
The integration circuits of Q2, C35 and C49, R87 are the boost circuits to
prevent tape slacking at start up in the PLAY mode.
When an "H" level is applied to C49 from U10-4, an "H" level is applied to
Q2 for a certain length of time during which Q2 switches on. This in turn, applies
an "L" level to pins #13 and #2, respectively, of U12-14 and U12-1, thus raising
the amplifier gain at each output which results in increase of motor currents and
consequently, rise in motor torque. Then, when Q2 switches off after elapse of a
certain length of time , C35 starts to charge , the voltages applied to pins #13 and
#2, respectively, of U12-14 and U12-1 gradually rises, and each amplifier gain
drops to their normal level.
Then, if the F.FWD button is depressed, the "L" level from U9-22 is inverted
to an "H" level and applied to the base of Q22 which switches on both Q22 and Q23.
At switching on of Q23, the +24V DC passing through R116 and R119 further
charges C30, and the voltage divided by R116 and R119 is applied to U12-14.
As the potential applied to U12-14 rises slowly in step with the charging rate
of C30, the motor current also rises gradually, resulting in a smooth rise in tor
que of the takeup reel.
When the transport is put in the REWIND mode from F.FWD, the "L" level from
U9-23 is inverted to switch on Q32 and Q33, but at the same time switch off Q22 and
Q23. At switch on of Q33, the +24V DC flows through R148 and D50 to switch on Q24
which quickly discharges C30 down to the emitter potential of Q24, or in other
words, the output potential of U12-8.
C30 is also discharged when the transport is put in the STOP mode from F.FWD.
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