Kenwood TH-235A Service Manual page 5

TH-235A/E/234
CIRCUIT DESCRIPTION
e IF amplifier
The first IF signal is amplified by Q20 and enters IC3 (FM
signal processing IC) where it is mixed with the second local
oscillator signal and so converted into the second IF signal.
Here, the second local oscillator signal is generated by tri-
pling the first local oscillator signal (12.8 MHz). The unwanted
frequency band of the second IF signal is eliminated by ce-
ramic filter CF1. The resultant signal is then amplified and
detected.
Item Rating
Center frequency of 6dB bandwidth(fo) | Within 450kHz+ 1.0kHz
6dB bandwidth £7.5kHz or more
50dB bandwidth £15kHz or less
Passband ripple 3dB or lass
Guaranteed attenuation 45dB or more
Insertion loss 6dB or less
Input/output impedance 1.5kQ
Table3 Ceramic filter (L72-0944-05)
characteristics (TX-RX unit CF1)
Ic3
Q215
MUTE
© AF amplifier
The frequency response characteristics of the audio sig-
nal output by the FM detector are corrected by the Q214 ac-
tive high-pass filter and de-emphasis circuit consisting of R299
and C281.
The audio signal is then passed through an AF variable
resistor and amplified by the power amplifier IC (IC202) to
obtain the desired output.
@ Squelch and mute circuit
The output signal which is detected by IC3 is input to the
microprocessor analog port (pin-100) as the DC current. The
input voltage to the microprocessor is A/D converted, and the
microprocessor controls the MUTE, AFCO, and AFC1, thus
controlling the audio signal. This port has hysteresis.
The microprocessor also controls the audio signal by con-
trolling the MUTE, AFCO and AFC1 during the CTCSS and
DTSS operation modes.
IC202 Q205
aon
Pam @ UH
Q203 AFB Q216
Fo ue
AVR "=
Q204
+
BEEP
AFCO
AFC1
Fig.3 AF amplifier, squelch and mute circuit
Conditions MUTE ;} AFCO AFC1
, During transmission L L L
During | Normal
When squetch is ON L
L
L
reception) operation [Wien squelch is OFF| H H H
MUTE : Muted when low.
AFCO : Muted when low.
AFC1 : Muted when low.
Table 4 Mute operating conditions