HYT TC-268 Service Manual page 12

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)
ANT
D102
D103
C
APC
IC403
MCU
B
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 IF) signal. The first IF signal is then fed through two monolithic crystal filters (MCFs: XF200) to
st
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)
BPF RF AMP BPF
L208-L211 Q203 L214.L203.L204
TUNE
Fig. 2-b. Receiver configuration
CF200
MIXER MCF IF AMP
Q202 XF200 Q201
IF.MIX
IC200
TCXO multiply
1st Local OSC
(PLL)
10
TC-268/368 Service Manual
AF PA AMP
IC302
AF AMP
LPF.HPF SP
IC300
X3
WIDE/NARROW SW
Q1 Q303
C
B