Kenwood TH-F6A Service Manual

Also see for TH-F6A: Instruction manual Instruction manual Supplemental service manual Service manual Quick reference

Contents

TH-F6A/F7E6CIRCUIT DESCRIPTIONFINE2 D20,24KV1566JIC706M62364FPTUNE IC4TA4101FSSB DETSSB/CWQ22,29UPA672T IC5TK10931VSW FMAMSMBSQBQ23LMX2NOSCILLATOR453KHzSWCF1450KHzCF3CF210.8MHzD25DAN235E2-2. Mixer local oscillator2-2-1. First mixerThe local oscillator signal for the first mixer is supplied fromthe VCO-PLL circuit. To offset according to modes, the PLLswitching frequency changes in FM/AM/CW, USB, LSB.1st LOCAL (PLL Switching Frequency)Fig.6FM/AM CW USB LSBUPPER Per 5kHz FM-2.5kHz FM-4.5kHz FM-0.5kHzLOWER Per 5kHz FM-2.5kHz FM-4.5kHz FM-0.5kHz2-2-2. Second mixerThe local oscillator signal of the second mixer uses57.15MHz which is three times as high as the 19.05MHz crystaloscillator (X3) output. This local oscillator signal is used to (i)implement fine steps during fine tuning and (ii) offset duringSSB/CW reception.2rd LOCAL (Oscillator Frequency)(i) Fine tuning functionWhen the fine tuning function is ON, the PLL comparisonfrequency is 5 kHz (5kHz step). The "receive frequency of10Hz steps as a set" is implemented by operating the localoscillator signal of the second mixer in 33.3Hz steps in the5kHz frequency range. The frequency is varied by 57.15MHz +/- 2.5 kHz by changing the voltage applied to thevaricap (D29) installed in the local oscillator circuit.(ii) Mode offsetThe IF frequency is adjusted by 2 Hz in SSB mode so thatthe demodulated signal passes through the center of the4kHz ceramic filter (CF1). The frequency is adjusted byvarying the local oscillator frequency.FM/AM CW(Fine Tuning) USB(Fine Tuning) LSB(Fine Tuning)UPPER 57.15MHz 57.15MHz+2.5kHz57.15MHz-2.5kHzLOWER 57.15MHz 57.15MHz+2.5kHz57.15MHz-2.5kHz2-2-3. Third mixerThe third mixer (IC4) works in SSB/CW mode only. Thelocal oscillator signal of 450 kHz +/- 2 kHz is produced in SSBmode to restore 2kHz correction (offset), and the signal of 450kHz - 800 Hz is produced to generate an 800Hz beat frequencyin CW mode. A demodulation signal is produced in SSB, anda 800Hz beat signal is produced in CW.3rd LOCAL (Oscillator Frequency)FM/AM CW USB LSBUPPER 450kHz 450kHz+800Hz 450kHz+2kHz 450kHz-2kHzLOWER 450kHz 450kHz+800Hz 450kHz-2kHz 450kHz+2kHz2-3. AGC circuitThe AGC is controlled by using the output from the RF AGCbuilt into the FM IC (IC5). The AGC is controlled by controllingthe bias current of the IF amplifier (Q26) and the forwardcurrent of the pin diode (D32) for the attenuater. In non-FMmode, the AM AGC circuit built-into the FM IC is also used.3. Control3-1. Reset and backup circuitsThe CPU reset signal is generated with the CR time constantby detecting a rising edge of the M4 line voltage with thevoltage detection IC (IC709). If the voltage supplied to the TH-F6/TH-F7 decreases and the M4 line voltage falls below thedetection voltage of the voltage detection IC (IC710), the CPU(IC705) detects it through the interrupt pin, backs up data inthe EEPROM (IC704), and shuts the power off.3-2. Voltage detection processingThe voltages are measured through the A/D port of the CPU(IC705) for processing. The battery voltage is supplied througha resistor, and a warning sound is produced when an abnormalpower supply voltage (17.5 to 22.0 V) is applied to the batterymeter during transmission. The squelch voltage is input fromthe IF IC, and a change in the noise voltage is detected tocontrol squelch. The S meter voltage is input from the IF IC tocontrol the S meter display. Thermistor voltage (temperature)detection, remote control microphone key operation, VOXvoltage monitoring, and TONE/DCS decoding are performedthrough the A/D port.3-3. VOXThe signal output from the microphone amplifier (IC702) isamplified by Q701, rectified/integrated by D709 to convert toDC voltage, and monitored through the A/D port of the CPU(IC705) to perform VOX processing.3-4. Battery saveThe CPU (IC705) controls Q728 through the SAVE port tosave battery power.3-5. LED drive circuitThe CPU (IC705) controls Q709 to turn LEDs on to illuminatethe LCD and keys. The ON AIR/BUSY LED is directly controlledthrough the open drain port of the CPU (IC705).* During fine tuning, it changes with 33.3 Hz step from PLLSwitching Frequency to the next Switching Frequency.PLL Switching Frequency* Perform Mode OFFSETOscillator FrequencyOscillator FrequencyTable.2Table.1Table.3* PLL Switching Frequency changes with the modes.