HYT TC-268, Service Manual

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TC-268/368 Service Manual
10
1-b) Front-end RF amplifier (TC-368)
An input signal from the antenna is applied to a RF amplifier (Q203) after passing through a
transmit/receive switch circuit (K102 and D103 are off) and a 3-pole LC filter. After the signal is
amplified (Q203), the signal is filtered by a band pass filter (a 3-pole LC filter) to eliminate unwanted
signals before it is passed to the first mixer. The voltage of varactor is provided by the MPU. (See Fig.
2-b)
B
C C
B
ANT
D102
D103
BPF
Q203
RF AMP
L208-L211
BPF
L214.L203.L204
MCF
XF200
IF AMP
Q201
MIXER
Q202
CF200
IF.MIX
IC200
AF AMP
LPF.HPF
IC300
AF PA AMP
IC302
SP
WIDE/NARROW SW
Q303
X3
multiply
Q1
TCXO
1st Local OSC
(PLL)
TUNE
APC
IC403
MCU
Fig. 2-b. Receiver configuration
2) First mixer
The signal from the RF amplifier is heterodyned with the first local oscillator signal from the PLL
frequency synthesizer circuit at the first mixer (Q202) to create a 38.85 MHz first intermediate frequency
(1
st
IF) signal. The first IF signal is then fed through two monolithic crystal filters (MCFs: XF200) to
further remove spurious signals.
3) IF amplifier
The first IF signal is amplified by Q201, and then enters IC 200 (IF processing IC). The signal is
heterodyned again with a second local oscillator signal within IC200 to create a 450kHz second IF
signal. The second IF signal is then fed through a 450kHz ceramic filter (CF200) to further eliminate
unwanted signals before it is amplified and FM detected in IC200.
4) AF amplifier
The recovered AF signal obtained from IC200 is amplified by IC300 (1/4), filtered by the IC300
low-pass filter (2/4) and IC300 high-pass filter (3/4) and (4/4), and de-emphasized by R303 and C306,
and then passed through a WIDE/NARROW switch (Q303). The processed AF signal passes through
an AF volume control and is amplified by an AF power amplifier (IC302) to a sufficient level to drive a
loud speaker.
5) Squelch
Part of the AF signal from the IC re-enters the FM IC, and the noise component is amplified and
rectified by a filter and an amplifier to produce a DC voltage corresponding to the noise level.
The DC signal from the FM IC goes to the analog port of the microprocessor (IC403). IC403 determines
whether to output sounds from the speaker, IC403 sends a high signal to the MUTE and AFCO lines and turns
IC302 on through Q302, Q304, Q305, Q306 and Q307. (See Fig.3)