Abell A-80, Руководство по обслуживанию

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A-80 UHF SERVICE MANUAL
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1
）
Front-end RF amplifier
An incoming signal from the antenna is applied to an RF amplifier (Q314) after passing through
a transmit/receive switch circuit (D301 and D302 are off) and a band pass filter. After the signal is
amplified (Q314), the signal is filtered through a band pass filter (D301 and D302) to eliminate the
unwanted signal before it is passed to the first mixer.
The voltage of the varactor diodes on the band pass filters are controlled by the CPU (ICI) center
frequency.
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 (Q301) to create a 45.05MHz first intermediate
frequency (1st IF) signal. The first if signal is then fed through two monolithic crystals filters (XF300
and 301) to further remove spurious signals.
3
）
IF amplifier
The first IF signal is amplified by Q201, and then enters IC300. The signal is heterodyned again
with a second local oscillator signal within IC300 to create a 450MHz second IF signal. The second IF
signal is then fed through a 450MHz ceramic filter (CF300) to further eliminate unwanted signals
before it is amplified and FM detected in IC300.
4
）
AF Amplifier
The recovered AF signal obtained from IC300 is amplified by IC401D, filtered by the high-pass
filter IC401D to eliminate signals under 300Hz. And de-emphasized by Q405 and Q406. The processed
AF signal passes through an AF volume control and is amplified to a sufficient level to drive a loud
speaker by an AF power amplifier (IC2).
5
）
Squelch
Part of the AF signal from IC300 enters IC300 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 the FM IC 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 AFCC lines and turns IC2 on
through Q4, Q5, Q404, Q7 and Q6.
6
）
Receive signaling
300Hz and higher audio frequencies of the output signal from IF IC are cut by a low-pass filter
(IC404C and IC404B). The resulting signal enters the microprocessor (IC1). IC1 determines whether
the QT or DQT matches the preset value, and controls the CPU and AFCC and the speaker output
sounds according to the squelch results.
3．PLL frequency synthesizer