Sony CRF-230B Service Manual page 7

Mut ing Crcuit:
Whe: the receiver is detuned from a signal, the
signal deweases. Since less i-f signal is then rec-
tifiecl bydiodes D207 and D208, the negative out-
put voltze of these rectifiers can no longer back
the positie voltage applied to the base of transistor
Q206 though adjustable resistor VR901 (Fig. 1-3).
This allo's Q206 to conduct lowering its collector
voltage. Since the collector of Q206 is connected
to the bze of PNP transistor Q209, Q209 conducts
wherl its base voltage decreases (with respect to
ground), thereby causing transistor Q208 to con-
duct. Th collector voltage of Q208 then drops to
near grouid potential. The voltage at the collector
of Q208 is fed to the base of transistor Q609
through WUTING switch $203. Since this voltage
is so lowQ609 cannot conduct and complete the
emitter cicuit of transistor Q608. This prevents
Q608 fro: amplifying the detector output.
When tuned to a signal, the opposite actions
occur. IJ-! signal through capacitor C227 is rectified
into negatve d-c voltage by diodes D207 and D208.
This voltae cuts off transistor Q206 and eventually
turns on tansistor Q609, thereby enabling transistor
Q608 to amplify the detector output. The muting
level can be adjusted by potentiometer VR901.
CRF-230B
lower than the signal level. If the receiver tuning
is shifted within range B of Fig. 1-4, the difference
between the collector voltages of transistors Q211
and Q212 becomes large enough drops across re-
sistors R251 and R252, thus lowering the base
voltage of PNP transistor Q209 and results in its
conduction. As before, the conduction of Q209
begins a chain of events which prevents Q608 from
amplifying. If the tuning is shifted within range C
of Fig.14, the difference between the collector
voltages of transistors Q211 and Q212 is so small
that transistor Q210 is turned off within this range
as well as in range
A. Noise
components
caused
by detuning, however, are coupled to transistor
Q207 through capacitor C240 and resistor R247
from the detector output.
| |
C—mt— Bf Af Bet — 6
The mutiry level is usually set approximately 20dB Fig. 1-4 Discriminator characteristic
from
discriminator
trol
discriminator = I re
a0 D2i2 D214
G)
D210 R252 D215 D213
CH
"ay,
b2ii 9209 c240
Wy——=1 To AFC
0208 R250) R247 Ps a va Fo
(y' (Vv NYY
KO) C239 1) &
I Q2it Q212
> +B
to af S
+B circuit << Q6og
S203
MUTING
OFF (fv
p Q609
60n Us
=
Fig. 1-3