Kenwood TK-701S Service Manual

4. CIRCUIT DESCRIPTION1. TX (Transmitter)The VHF signal fed from the PLL unit is apmlified ateach stage from Q1 (2SC2570A) through 04 (28C2630)and is fed to the triple seciton Low Pass Filter (LPF). Moreor less than 50W is then obtained at the antenna terminal.For harmonics, both 2nd f and 3rd f provide betterthan—70 dB.8~|10dBmTK-701:The signal detected at the 04 (2SC2630) output var-ies the applied voltage of Q3 (2SC2539) and Q2(2SC2538). This controls the antenna output from 20Wto 50W. This APC circuit is similar to that of the TK-801.If the final transistor temperature rises abnormally, thetemperature protection activates to reduce the thermistorTH 1 resistance. This reduces the standard voltage of theAPC circuit and that of the Q6 (2SB946) to reduce the out-put power.LPF-idBFig. 4-1 Transmitter Block Diagram2. RX (Receiver)Receiver input supplied through the antenna terminalpasses through the low pass filter and antenna switchingcircuit, and is initially VHF-filtered by BPF assembly (L20,21). The filtered signal is RF amplified by Q10:2SK241(GR)-1 and is further filtered by BPF L23 and L24.This is then input to the 1st mixer Q11: 2SK125-4. The1st oscillator injection signal is supplied from the PLL unitto the RXO input. This is multiplied by , filtered by BPF:L18,L19, and coupled to the 1st mixer O11: 2SK125-4.4-1The ist IF signal frequency (21.4 MHz) cutput fromthe 1st mixer is filtered by Monolithic Crystal filter (MCF)Z27 and Z28: 21F15B, and is post-amplified by Q12:2SK241(GR)-1. This is coupled to U13: MC3359P whichcontains the 2nd local oscillator, mixer, 2nd IF amp, qua-drature detector, noise amp and squelch switchThe 2nd local osc operates at 20.945 MHzto convertthe 1st IF from 21.4 MHz to the 2nd IF at 45§k Hz. This2nd IF signal is filtered by ceramic filter 231: C'W455D, islimiter amplified, and is then quadrature detected.