Circuit Description - Kenwood TH-F6A Service Manual

TH-F6A/F7E
1. A band receiver system
1-1. Receiver circuit
The A band can receive signals in two bands: VHF (220 MHz
band <K type only>) and UHF (K type: three bands). It uses
FM receive mode only, and uses double conversion with the
first IF of 59.85 MHz and the second IF of 450 kHz.
The first amplifier is divided into two bands: 137 MHz -
173.995 MHz (216 MHz - 260 MHz <K type only>) and 410 MHz
- 470 MHz. The incoming signal from the antenna passes
through a low-pass filter and a duplexer, and goes to an
independent amplifier (Q63, Q62) for each band.
1-1-1. VHF band frontend
The incoming signal from the antenna passes through a
VHF band low-pass filter and a duplexer, passes through a
band-pass filter where it is tuned with varicaps (D76, D77),
and goes to the first amplifier (Q63). Unwanted signal
components are eliminated by a two-pole band-pass filter
where it is tuned by varicaps (D72, D75, D81, D83), and the
signal goes to the common mixer (Q45) for the A band. (K
type only: For 200MHz band reception, D74 is turned off with
a control signal to improve band-pass filter coupling and
ensure pass bandwidth.)
ANT
BPF LPF
1-1-2. UHF band frontend
The incoming signal from the antenna passes through a
UHF band low-pass filter and enters the first amplifier (Q62)
common to both A and B bands. The amplified signal is
distributed by L distributors (L95, L96) and goes to the LC filter
module (L92). Unwanted signal components are eliminated
by the filter, and the resulting signal passes through another
band-pass filter and enters the mixer (Q45).
ANT
LPF
BPF
4

CIRCUIT DESCRIPTION

BPF RFAMP BPF MIX
Q45
Q63
MT6C
3SK320
04AE
D76
D72,D75
D77
D81,D83
LO
APC&VTUNEA
MPU
Fig.1
LC FILTER
HPF RFAMP MODULE
Q62 L95
3SK320 L96
BPF MIX
1st IF
Q45
59.85MHz
MT6C
04AE
LO
Fig.2
1-1-3. Circuits following IF
The signal heterodowned to the first IF of 59.85 MHz by the
mixer (Q45) passes through a 15kHz MCF (XF2) and unwanted
signal components are eliminated. The resulting signal is
amplified by the IF amplifier (Q43) and goes to the FM IC (IC7).
The FM IC heterodowns it to the second IF of 450 kHz. Then,
the signal passes through a 15kHz external ceramic filter (CF4)
and goes to the FM IC again. The signal amplified by the IF
amplifier built into the IC is demodulated by the quadrature
FM demodulation circuit using a discriminator (CD1) and
converted into an audio signal and output.
The FM modulation signal output from the FM IC passes
through a low-pass filter consisting of a resistor and a capacitor
and is output to the control section. The demodulation signal
input to the control section goes to the electronic volume
(IC706, pin 13) to adjust the audio balance with the B band.
The output signal is amplified by the operational amplifier
(IC717), passes through an active filter consisting of Q719 and
Q717, and goes to the audio amplifier (IC707). The signal
amplified to a higher power by the audio amplifier becomes
the final output signal from the set and output through the
internal speaker or speaker output pin (J701).
MIX MCF
Q45
MT6C
04AE
XF2
1st IF
59.85MHz
LO
TCXO
19.8MHZ
X1
1-2. Mixer local oscillator
1-2-1. First mixer
The local oscillator signal for the first mixer is supplied from
the VCO-PLL circuit.
1-2-2. Second mixer
The local oscillator signal for the second mixer is a 59.4
MHz signal that is produced by multiplying the 19.8MHz TCXO
(X1) oscillator output with a multiplier (Q44).
2. B band receiver system
2-1. Receiver circuit
The B band has a broadband receiver circuit configuration
and implements broadband reception of 100 kHz to 1.3 GHz.
FM/AM/SSB/CW reception is possible in the range 100 kHz to
L92
29.7 MHz, and FM/wide FM/AM/SSB/CW reception is possible
in the range 29.7 MHz to 1.3 GHz. The FM/AM receiver circuit
uses double conversion with the first IF of 57.6 MHz and the
second IF of 450 kHz. The SSB/CW receiver circuit uses triple
conversion with the first IF of 57.6 MHz and the second IF of
450 kHz. The wide FM receiver circuit uses single conversion
with the IF of 10.8 MHz.
IFAMP
LPF
Q43
2SC
IC7
5108(Y)
X3
4.50KHz
Q44
2SC
4915
CF4
AUDIO
AMP DE EMPHASIS
AMP
Q719
IC717 IC707
Q717
Fig.3
VR
IC700
BAF
J701
SP