HYT TM-800, Service Manual

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TM-800 SERVICE MANUAL
77
Circuit Description
1. Frequency Configuration
The receiver utilizes double conversion superheterodyne. The first IF is 49.95MHz and the second is
450KHz. The first local oscillator signal is supplied by PLL circuit. The second local oscillator signal
(50.4MHz) is generated from the frequency tripling of TCXO (16.8MHz).
The PLL circuit also generates the frequencies needed in the transmitter (See Fig.2).
Frequency Range: 136 MHz—174MHz
2. Receiver Circuit
The receiver section configuration is shown as Fig. 1.
Figure 1 Receiver Section Configuration
2.1 RF AMP BPF
It consists of BPF (D603, D602, D601) and RF amplifier (Q601). The range of bandpass frequency is
from 136MHz to 174MHz. The signal is filtered by the RF Amp BPF to eliminate unwanted signals
before going to the first mixer.
2.2 The First Mixer
The signal from RF AMP BPF is mixed with the first local oscillator signal from PLL circuit in the
double-balance mixer (Q138, Q144, Q145, Q146) to generate a 49.95MHz first IF signal. The first IF
signal is then fed through two crystal filters (N: XF101, XF103; W: XF102, XF104) to further remove
spurious signals.