Kenwood FM Transceiver TK-481 Service Manual

Also see for FM Transceiver TK-481: User manual Instruction manual Specifications Instruction manual Service manual

15TK-480/481CIRCUIT DESCRIPTION3-3. Audio amplifier circuitThe demodulated signal from IC9 is amplified by IC8 (2/2), high-pass filtered, low-pass filtered, high-pass filtered,band-eliminate filtered,and de-emphasized by IC12.The signal then goes through an AF amplifier IC3 (2/2), anelectronic volume control (IC4), and an AF switch (Q310 ison), and is routed to audio power amplifier (IC300), where itis amplified and output to the internal speaker.The audio mute signal (AM) from the microcomputer be-comes Low in the standby and Q304, Q305 which arepower supply circuit for IC300 turn off. Also, IC12 is set tothe power down mode according to data from microproces-sor, and the AF signal is muted. When the audio is output,AM becomes High to turn Q304, Q305 ON, and voltage issupplied to power terminal VP of IC300. Also, IC12 is can-celed out of the power down mode.The speaker is switched by the logic of speaker switch-ing terminal SSW on the universal connector. When SP-MICis not attached, the logic of SSW becomes High and SW(Q310) is turned ON, and the AF signal is input to both ampli-fiers of IC300.When SP-MIC is attached, SSW is connected to GND atinside of SP-MIC. For this reason, Q310 is turned OFF, andthe AF signal is input only to amplifier for EXT SP of IC300.Change of INT/EXT SP refer to Fig. 3.AM SSW VC1 VC2 SPH H H L INTH L L H EXTL H L L MUTEL L L L MUTEFig. 3 Audio amplifier circuitSW2VC18VC25VPQ305Q304IC300INT. SPEXT. SPQ301Q308AFAMSSWSB3-4. Squelch circuitThe output from IC9 enters FM IC again, then passedthrough a band-pass filter. The noise component outputfrom IC9 is amplified by Q19 and rectified by D3 to producea DC voltage corresponding to the noise level. The DC volt-age is sent to the analog port of the CPU (IC15). And IC9outputs a DC voltage (RSSI) corresponding to the input ofthe IF amplifier. The CPU reads the RSSI signal via pin 24.IC15 determines whether to output sounds from thespeaker by comparing the input voltage of pin 28 and pin 24with the preset value.Fig. 4 Squelch circuitFig. 5 Squelch and RSSI voltage vs ANT input levelIF AMPDETRSSI BPFAMPNOISEAMP DETQ19 D328IC15CPU24IC9 : FM IF IC712SQ voltageANT input levelSQ closeSQ openRSSI voltageANT input levelPresetvaluePresetvalue4. Transmitter System4-1. Microphone amplifierThe signal from the internal microphone goes throughthe mute switch (Q300).When the SP-MIC is not attached, the microphoneswitching terminal (MSW) on the universal connector be-comes High, and mute switch (Q300) is turned ON. Whenthe SP-MIC is attached, MSW is connected to GND at insideof SP-MIC. For this reason, Q300 is turned OFF, the internalmicrophone is muted, and only the input of the external mi-crophone is supplied to the microphone amplifier of the TX-RX unit.