Kenwood TH-F6A Service Manual page 6

TH-F6A/F7E
IC706
M62364
FP
OSCILLATOR
TUNE
D20,24
FINE2
KV1566J
453KHz
2-2. Mixer local oscillator
2-2-1. First mixer
The local oscillator signal for the first mixer is supplied from
the VCO-PLL circuit. To offset according to modes, the PLL
switching frequency changes in FM/AM/CW, USB, LSB.
1st LOCAL (PLL Switching Frequency)
FM/AM
UPPER Per 5kHz FM-2.5kHz FM-4.5kHz FM-0.5kHz
LOWER Per 5kHz FM-2.5kHz FM-4.5kHz FM-0.5kHz
* PLL Switching Frequency changes with the modes.
2-2-2. Second mixer
The local oscillator signal of the second mixer uses
57.15MHz which is three times as high as the 19.05MHz crystal
oscillator (X3) output. This local oscillator signal is used to (i)
implement fine steps during fine tuning and (ii) offset during
SSB/CW reception.
2rd LOCAL (Oscillator Frequency)
FM/AM
UPPER 57.15MHz
LOWER 57.15MHz
* During fine tuning, it changes with 33.3 Hz step from PLL
Switching Frequency to the next Switching Frequency.
(i) Fine tuning function
When the fine tuning function is ON, the PLL comparison
frequency is 5 kHz (5kHz step). The "receive frequency of
10Hz steps as a set" is implemented by operating the local
oscillator signal of the second mixer in 33.3Hz steps in the
5kHz frequency range. The frequency is varied by 57.15
MHz +/- 2.5 kHz by changing the voltage applied to the
varicap (D29) installed in the local oscillator circuit.
(ii) Mode offset
The IF frequency is adjusted by 2 Hz in SSB mode so that
the demodulated signal passes through the center of the
4kHz ceramic filter (CF1). The frequency is adjusted by
varying the local oscillator frequency.
6
CIRCUIT DESCRIPTION
SMB
SQB
SW
FM
Q22,29
AM
UPA672T
TK10931V
SSB DET
Q23
IC4
LMX
TA4101F
2N
SSB/CW
SW
D25
DAN
235E
10.8MHz
Fig.6
CW USB
Table.1
CW(Fine Tuning) USB(Fine Tuning) LSB(Fine Tuning)
57.15MHz+2.5kHz
Oscillator Frequency
57.15MHz-2.5kHz
57.15MHz+2.5kHz
Oscillator Frequency
57.15MHz-2.5kHz
PLL Switching Frequency
Table.2
2-2-3. Third mixer
The third mixer (IC4) works in SSB/CW mode only. The
local oscillator signal of 450 kHz +/- 2 kHz is produced in SSB
IC5
mode to restore 2kHz correction (offset), and the signal of 450
kHz - 800 Hz is produced to generate an 800Hz beat frequency
CF1
in CW mode. A demodulation signal is produced in SSB, and
a 800Hz beat signal is produced in CW.
450KHz
3rd LOCAL (Oscillator Frequency)
CF3
UPPER
CF2
LOWER
* Perform Mode OFFSET
2-3. AGC circuit
The AGC is controlled by using the output from the RF AGC
built into the FM IC (IC5). The AGC is controlled by controlling
the bias current of the IF amplifier (Q26) and the forward
current of the pin diode (D32) for the attenuater. In non-FM
mode, the AM AGC circuit built-into the FM IC is also used.
LSB
3. Control
3-1. Reset and backup circuits
The CPU reset signal is generated with the CR time constant
by detecting a rising edge of the M4 line voltage with the
voltage detection IC (IC709). If the voltage supplied to the TH-
F6/TH-F7 decreases and the M4 line voltage falls below the
detection voltage of the voltage detection IC (IC710), the CPU
(IC705) detects it through the interrupt pin, backs up data in
the EEPROM (IC704), and shuts the power off.
3-2. Voltage detection processing
The voltages are measured through the A/D port of the CPU
(IC705) for processing. The battery voltage is supplied through
a resistor, and a warning sound is produced when an abnormal
power supply voltage (17.5 to 22.0 V) is applied to the battery
meter during transmission. The squelch voltage is input from
the IF IC, and a change in the noise voltage is detected to
control squelch. The S meter voltage is input from the IF IC to
control the S meter display. Thermistor voltage (temperature)
detection, remote control microphone key operation, VOX
voltage monitoring, and TONE/DCS decoding are performed
through the A/D port.
3-3. VOX
The signal output from the microphone amplifier (IC702) is
amplified by Q701, rectified/integrated by D709 to convert to
DC voltage, and monitored through the A/D port of the CPU
(IC705) to perform VOX processing.
3-4. Battery save
The CPU (IC705) controls Q728 through the SAVE port to
save battery power.
3-5. LED drive circuit
The CPU (IC705) controls Q709 to turn LEDs on to illuminate
the LCD and keys. The ON AIR/BUSY LED is directly controlled
through the open drain port of the CPU (IC705).
FM/AM CW USB
450kHz 450kHz+800Hz 450kHz+2kHz 450kHz-2kHz
450kHz 450kHz+800Hz 450kHz-2kHz 450kHz+2kHz
Table.3
LSB