HYT TC-500, Руководство по обслуживанию, Страница: 11 / 77

TC-500 Service Manual
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synthesizer in the first mixer (Q23) to generate a 21.4MHz first IF signal. The first IF signal is then
fed through two monolithic crystal filters (XF1) to remove spurious signals from adjacent channels.
3. IF Amplifier
The first IF signal is amplified in Q22 and then enters the IF process chip IC1. The signal is mixed
with the second local oscillator signal in IC1 to create a 455KHz second IF signal. The second IF
signal is then fed to a 455KHz ceramics filter (CF1) to eliminate unwanted signals before it is
amplified and FM detected in IC1.
4. Audio Amplifier
The demodulated audio signal obtained from IC1 is amplified in IC7 (1/4), and then filtered by low
pass filter Q19 and high pass filter Q20, and then de-emphasized by R130 and C156. The resulting
audio signal passes through a volume control circuit and then is amplified to a sufficient level to
drive the speaker by audio power amplifier (IC8).
5. Squelch
Part of the audio signal from IC1 enters IC1 again, 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 IC1 goes to the analog port of the microprocessor (IC11). IC11 determines whether
to output sounds from the speaker by detecting whether the input voltage is higher or lower than
preset value.
To output sounds from the speaker, IC11 sends a high level signal to the MUTE and AFCO lines and
turns IC8 on through Q12.
DET
FM IF IC1
IF AMP
HPF
AMP
IC7
AF
LPF HPF
DET
2 39 40 1
SW
Q12
SW
AF/PF AMP
IC7
LPF
IC11
MCU
B
U
S
Y
M
U
T
E
A
F
C
O
T
I
IC8
SP
Figure 3 Audio amplifier and squelch circuit
6. Receive signaling
CTCSS/CDCSS
Audio frequencies over 300Hz of the output signal from IC1 is cut off by a low-pass filter. The
resulting signal enters the microprocessor IC11. IC11 determines whether the CTCSS/CDCSS