ABELL A-82 UHF Service Manual
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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 (Q300) to create a 38.85MHz first intermediate
frequency (1st IF) signal. The first IF signal is then fed through two monolithic crystal filters (XF301) to
further remove spurious signals.
3
）
IF AMP
The first IF signal is amplified by Q302, and then enters IC300 (FM processing IC). The signal is
heterodyned again with a second local oscillator signal within IC300 to create a 450 KHz second IF
signal. The second IF signal is then fed through a 450 KHz ceramic filter (CF300) to further eliminate
unwanted signals before it is amplified and FM detected in IC300.
4
）
AF amplifier
The AF signal from the IF IC300 is amplified by IC401
（
C
）
, then passes through the high-pass filter
IC401
（
A
）
，
IC401
（
B
）
，
and low-pass filter IC401(D), and though R436, C430 remove embitter. The
signal passes through AF volume control circuit and enters the audio power amplifier IC2 , drives the
speaker.
5
）
Squelch
Part of the AF signal from IC300 enters IC300 again, and the noise is amplified and rectified by a filter
and an amplifier to produce a DC voltage corresponding to the noise level. The DC signal goes to the
analog port of the microprocessor(IC1).IC1 determines whether to output sounds from the speaker by
checking whether the input voltage is higher or lower than the preset value.
To output sounds from the speaker, IC1 sends a high signal to the MUTE and AFCC lines through Q4,
Q5, and Q404 and turns on the audio amplifier.(See the chart 6)
ANT SW
ANT
D301,D302
SP
L310,L312,L314,L315
BPF
AFC
TUNE
IC1
CPU
Q301
RF AMP
L307,L308,L309
BPF
Q300
MIXER
1ST LOCAL OSC
(PLL)
XF301
MCF
Q302
IF AMP
TCXO
IC300
IF,MIX,DET
CF300
X3
MUITLPLY
Q1
RF AMP
LPF,HPF
IC401
IC2
AF PA AMP
Q303
WIDE/NARROW SW
Chart 5. Receiving Circuit
D303,D306,308
D304,D304,D307
