HYT TC-700P, Руководство по обслуживанию

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TC-700P Service Manual
9
Figure 3 Diagram of RX Circuit
(1) RF AMP & BPF
After passing through the TX-RX switch circuit and bandpass filter (BPF) circuit, signals received by the
antenna are amplified by the RF amplifier (Q410). Then the amplified signals feed to the bandpass filter
to remove unwanted signals, allowing wanted signals to go to the first mixer only.
(2) The First Mixer
The signals output by RF AMP&BPF are mixed with the first LO signal output by the PLL frequency
synthesizer at the first mixer (Q415) to generate a 44.85MHz first IF signal, which will feed to a crystal
filter (XF401) to further remove spurious signals.
(3) IF Amplifier
After amplified at Q412, the first IF signal enters U403 (TA31136FN), where it is mixed with the second
LO signal (44.395MHz) to generate a 450KHz second IF signal. Then the second IF signal feeds to a
pair of ceramic filters (N: Z403; W: Z404), where unwanted signals are removed. Finally the signal
passes through the frequency discriminating circuit of U403 and is output as audio signal from Pin 9.
(4) Wide/Narrow Band Switch Circuit
When the WCON pin of U605 is at high level, the diode circuit is turned on, and the ceramic filter Z404
(W) is operating to remove unwanted signals. When the NCON pin of U605 is at high level, the diode
circuit is turned on, and the ceramic filter Z403 (N) is operating to remove unwanted signals.
(5) Audio Amplifier Circuit
After amplifier at U201, the audio signal output by U403 enters U203 for audio processing (the received
signaling is derived for decoding). Then the signal passes through AF_MUTE, SW601 (volume control),
Q615 (PA_MUTE). Finally the signal goes to the audio amplifier (U609) and is output to the speaker as