Kenwood TM-271A Service Manual

Also see for TM-271A: Instruction manual Instruction manual Service manual

Contents

TM-271A/271E4■ Unlock CircuitDuring reception, the 8RC signal goes high, the 8TC signalgoes low, and Q34 turns on. Q33 turns on and a voltage isapplied to 8R. During transmission, the 8RC signal goes low,the 8TC signal goes high and Q36 turns on. Q35 turns on anda voltage is applied to 8T.The CPU monitors the PLL (IC401) LD signal directly.When the PLL is unlocked during transmission, the PLL LDsignal goes low. The CPU detects this signal and makes the8TC signal low. When the 8TC signal goes low, no voltage isapplied to 8T, and no signal is transmitted. (See Fig. 7.)IC101CPUQ34SWQ33SWIC401PLLQ36SWQ35SWLD8RC8C8R 8T8TCPLL lock: LD “H”Fig. 7 Unlock circuitTransmitter System■ OutlineThe transmitter circuit produces and amplifies the desiredfrequency directly. It FM-modulates the carrier signal bymeans of a varicap diode.■ Power Amplifier CircuitThe transmit output signal from the VCO passes throughthe transmission/reception selection diode (D409) and ampli-fied by Q501, Q502 and Q503. The amplified signal goes tothe final amplifier (Q504) through a low-pass filter. The low-pass filter removes unwanted high-frequency harmonic com-ponents, and the resulting signal is transmitted through theantenna terminal. (See Fig. 8.)■ APC CircuitThe automatic transmission power control (APC) circuitdetects part of a final amplifier output with a diode (D606,D607) and applies a voltage to IC501. IC501 compares theAPC control voltage (PC) generated by the D/A converter(IC161) and DC amplifier (IC203) with the detection outputvoltage. IC501 generates the voltage to control Q503 andQ504 and stabilizes transmission output.The APC circuit is configured to protect over current ofQ503 and Q504 due to fluctuations of the load at the antennaend and to stabilize transmission output at voltage and tem-perature variations. (See Fig. 9.)RFAMPQ501DRIVEAMPQ503FINALAMPQ504PREDRIVEAMPQ502DCAMPIC203ANTSWD602,D603D605LPFANTPOWERDETD606D607IC501 VR601APCCONTROLD409IC161(PC/TVO)(PC)Q411RF AMPQ501RF AMPQ502PREDRIVE AMPQ503ANTDRIVEAMPQ504FINALAMPIC4021/2DIVIDERIC161 Q406D/ACONVERTERD/ACONVERTERD/ACONVERTERX401TCXO16.8MHzVCOIC401PLLQ410BUFFERQ404RF AMPIC203BUFFERIC101IC161MIC KEYINPUT CPUIC161IC203SUMAMPIC202IC201MICMIC/IDC SPLATTERFILTERFig. 9 APC circuitFig. 8 Transmitter systemCIRCUIT DESCRIPTION