HYT TM-800K, Service Manual

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TM-800K Service Manual
18
Transmitter Circuit
Figure 2 Transmitter Section Configuration
MIC Circuit and Modulation Circuit
The AF signal from MIC is amplified by IC104 after passing through the MIC control switch (Q507). The
resulting signal is then amplified by IC106 and pre-emphasized, encoded. It is passed to IC121 (electronic
switch) before reaching IC125. The signalling is inputted into IC125 and enters VCO for modulation.
Driver and Final-Stage Power Amplifier Circuit
TX-RF signal is outputted from Q703 in VCO circuit and amplified by Q126, Q127 and Q128. The
amplified signal is then fed to IC102 (Power Module) and passes through LPF before reaching the
antenna terminal.
APC
The APC is used to keep the power output at a constant preset value. D105 and D106 transform the
signal from detector into DC voltage which is then compared with the reference voltage from CPU in
IC118 and outputted as DC control voltage. The DC control voltage controls the output power by
controlling the grid of IC102.
PLL Circuit
PLL circuit generates the first local oscillator signal for reception and the RF signal for transmission.
PLL circuit consists of TX frequency oscillator (Q701), RX frequency oscillator (Q702), buffer amplifier
(Q703), RF amplifier (Q124), PLL IC (IC801), LPF (Q804, Q805) and TX/RX VCO control switch
(Q704, Q706).
In transmit mode, IC120 transmits the frequency data to PLL IC. Q704 is turned on to activate TX VCO.