Sony TA-2000F - Page 10

TA-2000F
Stage/Control
REC OUT
Muting circuit
Q911, Q912
Function
sociated bias circuit, while Q909
(Q910) is forward biased by
R911. Asa result, Q909 (Q910)
is ON, shorting the output cir-
cuit to ground, and effectively
muting the output signals.
As Q908 is gradually turned ON
due to the slowly-increasing base
current flow, Q908 conducts
and cuts off Q909 (Q910), re-
moving the muting. Q907 is
employed to discharge C902
quickly when power switch is
turned off, preparing it for the
next muting operation.
This circuit is employed to mute
the REC OUT signals by means
of relay REL-1, preventing the
loud ‘‘pop”’ or click noises just
after turning the power switch
to ON. The base of Q911 is
connected to the collector cir-
cuit of Q912, while the base of
Q912 is connected to an RC
network (R903, C901) having a
long time constant. When you
first turn ON the power switch,
Q912 remains off due to the
long time constant of the associ-
ated bias circuit. This keeps
open the muting relay, discon-
necting the buffer amplifier’s
(Q01) output from the REC
OUT signal path. As Q912
is gradually turned ON due to
the slowly-increasing base cur-
rent flow, Q912 conducts and
turns on Q911, energizing the
muting relay to deliver the buf-
fer amplifier’s output to the
REC OUT terminal.
Power Supply Section
Two independent regulated pow-
Stage/Control
Voltage regulator
Q901, Q902, Q903
(Q904, Q905,
Q906)
Function
er supplies are employed to ob-
tain stable operation. One is
a low-voltage supply delivering
37 volts dc, and the other is a
high voltage supply delivering
150 volts dc. As both power
supplies have the same configu-
ration, only the low-voltage
regulator circuit is described
here. The high-voltage supply is
identical except for reference
numbers.
De output from bridge rectifier
D901 and D902 (D903 and
D904) is filtered by C18 (C17)
and applied to series regulator
Q901 and Q902 (Q904 and
Q905).
Q903 (Q906) compares a sample
of the output voltage picked
off across power supply adjust
control R926 (R920), with
reference voltage supplied by
zener diode D905 (D906).
A change in the output voltage
is detected at the base of Q903
(Q906) and therefore alters its
collector voltage. Since the
collector of Q903 is directly
coupled to the base of Q902
(Q905), the change in output
voltage alters the conduction
of Q901 and Q902 (Q904 and
Q905) by the amount necessary
to maintain the output voltage
constant.
An increase in output voltage
causes an increase in the imped-
ance (decrease in conduction)
of Q901 and Q902 (Q904 and
Q905), and vice-versa. The dc
output voltage supplied to the
preamplifier section is therefore
extremely stable.