Sony ST A6B Service Manual

Page 1

new circuit
operation

SONY INDUSTRIES

Hi-Fi Technical Support
47-47 Van Dam Street, Long Island City. New York 11101

PRINTED IN U.S.A.

Page 2

IF Bandwidth Control

The ST-AfiB offers a choice of IF
bandwidth, or degree of selectivity,
appropriate to the condition of
the received signal. Narrow selec-
tivity can be manually selected for
the highest possible S/N ratio, or
if a strong signal on an adjacent
channel is causing interference. 0n
the other hand, automatic selec-
tivity selection can be chosen; the
tuner itself will then select a
bandwidth appropriate to the signal
level.

Transistors Q201-Q204 are con-
nected as cascaded pairs of emitter
follower-common base amplifiers
(Fig. 1). This configuration is
akin to a differential amplifier
having only one input driven. These
amplifiers provide the extra gain
needed to overcome ceramic filter
losses, and improve limiting. An
interesting feature of this ampli-
fier block is that one bias-divider
string (resistors R206 and R207)
serves all four transistors.

Transistors Q203 and Q204 also
play a part in the dual-bandwidth
circuit. For normal selectivity
(wide bandwidth),signa1 is channel-
led directly from the collector of
Q203 to the input (pin 1) of IC201.
For narrow bandwidth, signal is
channelled through ceramic filters
CF203 and CF204. Bandwidth selec-
tion and switching is accomplished
by transistors Q205 and Q206, and
diodes D201 and D202.

Narrow bandwidth operation can be
selected either manually via the
SELECTIVITY switch or automatically
via the muting circuit. If the
SELECTIVITY switch is set to NARROW,
B+ is removed from resistor R241, so
transistor Q205 has no base bias.
Its collector-emitter impedance
therefore becomes an open circuit
and its collector voltage goes high.
This forward biases diode D201, con-
necting the narrow-band signal (from
CF204) to IC201, and turns on tran-
sistors Q206 and Q211. When satu-
rated on, the collector~emitter im-

pedance of Q206 is so low that it
shorts the direct signal (from Q203)
to ground. The resulting near-zero
collector voltage of Q206 kills the
bias on diode D202, thereby opening
the signal path from Q203 to IC201.
Similarly, when saturated on, the
resulting low collector-emitter im-
pedance of Q211 completes the ground
path for the NARROW lamp, turning it
on. The low collector voltage of
Q211 also turns off Q212 and the
NORMAL lamp.

When the SELECTIVITY switch is
set to AUTO, bandwidth selection
is matched to received signal lev-
el under the control of the muting
circuit. The muting control sig-
nal appearing at the variable con-
tact of pot RT401 is also applied
to transistor Q209 through resis-
tor R237. Transistors Q209 and
Q210 form a Schmitt trigger, so
when the muting voltage exceeds a
certain level, or falls below a
lower level, the trigger output
voltage will switch abruptly from
one state to the other. The dif-
ference in trigger levels (hystere-
sis) ensures that the tuner will not
rapidly switch back and forth be-
tween the two selectivity modes due
to minor signal-level variations.

At low signal levels, the voltage
delivered to QZOQ will be too low
to turn it on, hence the collector
voltage of Q210 will be low and the
conditions previously described for
narrow-band operation will exist.
However, if the signal is high
enough for low-noise wideband oper-
ation, the high muting voltage will
turn Q209 on and Q210 off. The re-
sulting high Q210 collector voltage
will bias Q205 into saturation,
shorting the output of filter CF204.
Transistor Q205's collector voltage
will also drop to near zero, killing
the bias on transistor Q206. This
last action removes the short from
the direct line, and the rise of
Q206's collector voltage causes di-
ode D202 to conduct and connect the
wideband signal path to IC201.

O