Kenwood TH-28A Service Manual page 3

TH-28A/E
CIRCUIT DESCRIPTION
- IF amplifier
The first IF singal is amplified by Q16 and input to
IC1 (FM signal processing !C), where it is mixed with
the scecond local oscillator signal and so is converted
into the second IF signal.
The unwanted frequency band of the second IF sig-
nal is eliminated by a ceramic filter. The resulting signal
is then amplified and detected.
Item ; Rating
Center frequency of 6dB bandwidth (fo) | Within 455 +1.5kHz
6dB bandwidth +7.5kHz or more
40dB bandwidth +15kHz or less
Passband ripple 1.50B or less (within 455 +1.5kHz}
Guaranteed attenuation 27dB or more (t100kKHz)
Insertion loss 6dB or less
1.5kQ
Table 3 Ceramic filter (L72-0362-05)
(TX-RX unit CF1)}
Input/output impedance
+ AF amplifier
The frequency characteristics of the audio signal
output by the FM detector are corrected by the O12
ing of C29 and R43.
The audio signal is then passed through an AF vari-
able resistor and amplified by power amplifier C3 to
obtain the desired output.
+ Squelch and mute circuits
The output of the squelch circuit consisting of 1C1
and Q11 is output from SQ SW (Q9, 10) to pin 26 of the
microprocessor as the BUSY signal. The microproces-
sor controls the MUTE and AFC signals in accordance
with the BUSY input signal logic and other function
states, and so controls the audio signal.
The microprocessor also controls the MUTE and
AFC signals during the T. ALT and CTCSS and DTSS
operations, thus controling the audio signal.
RS
ct Ne W—< 5R L> iq SP
™ . are AF AMP |
— 5 ai2 we a
+
| AW 1 | te pra
NOISE AMP ™
Qi D2 ;
SQ VOL > | Ra3
N
oO
' il
Qg :
it
Q10 4, :
oye)
9 A
26 : Ic2
BUSY ;
oe AFG
Fig.3 AF amplifier, squelch, and mute circuits
Condition MUTE! AFC
Transmission L L
Reception Normal operation | Squelch on L L
Squenich off H H
T. ALT Standby L L
Receive (T. ALT) H L
MUTE : Muted when low
AFC : Muted when low
Table 4 Muting conditions