Kenwood TK-250 Service Manual page 17

TK-250
CIRCUIT DESCRIPTION
3.3 IF amplifier
The first IF signal is amplified by 010, and then enterd IC2
(FM IC). The signal is heterodyned again with a second local
oscillator signal within IC2 to become a 455kHz second IF
signa!. The second IF signal is fed through a 455kHz ce-
ramic filter to futher eliminate undesired signals before it is
amplified and FM detected in IC2.
3.4 Audio amplifier
The recovered audio signal obtained from IC2 is amplified
by IC208 (1/2)(contral board), low-pass filtered by IC205 (2/
2), high-pass filtered by IC205 (1/2) and band-eliminate fil-
tered by IC211 (1/2). The audio signal is then passed through
an audio frequency switch (0204) and de-emphasized by
IC211 (2/2). The processed audio signal passes through an
audio volume control and is amplified to a sufficient level to
drive a loud speaker by an audio power amplifier {lC222).
3.5 Squelch and mute
The output signal from the squelch circuit, which consists
of IC208 (2/2) and 0203, is applied to the microprocessor.
The microprocessor passes information to the shift registor
(IC213) and it contrals the mute contrallines (AF MUTE and
AC) according to the input signal (noise pulse) and the mi-
croprocessor task condition.
Item
Rating
Nominal center frequency
455kHz
6dB band width
±6.0kHz or more
40dB band width
±12.5kHz or more
Ripple
1.5dB or less (455kHZ±4.0kHz)
Guaranteed attenuation
27dB or more within fo±100kHz
Insertion loss
6dB or less
Terminal impedance
1.5kQ
CF1,2: L72-0927-05( E)
Item
Rating
Nominal center frequency
455kHz
6dB band width
±4.5kHz or more
40dB band width
±11.0kHz or more
Ripple
2.0dB or less (455kHz±3.0kHz)
Guaranteed attenuation
25dB or more within fo±100kHz
Insertion loss
6dB or less
Terminal impedance
1.5kQ .
CF1,2 : L72-0922-05( T )
IC208(1/2) IC208(2/2)
Q203
Item
Rating
455kHz
±7.5kHz or more ±15.0kHz or more
2.0dB or less (455kHz±5kHz)
25dB or more within fo±100kHz
6dB or less 1.5kQ
from
DET
IC213
20
CPU
IC221
CF1,2: L72-0906-05 (ExceptT, E)
AF MUTE
AC
OUTPUT
EXPANDER
ANT
Fig. 2 Receiving system
Fig. 3 Squelch and Mute
DET
17