Section 7._ Circuit Description - Kenwood TS-120S Operating Manual

The signal from the antenna is fed to the 8.83 MH? IF trap.
jouit. This signal is stepged up about 10 dB and
impedance-converted by a wide-band transformer before it
is applied to the bandpass filter (BPF). The 8PF is common.
to transmission and reception, eliminates the need for @
preselector, and makes the RF section compact in design,
The signal from the BPF is fed to the dual-gate MOS FET
wideband RF amplifiers consisting of a 3SK74 (Q1) and a
28C1815 (Q2), where the 2 MHz to 35 MHz signal is
amplified about 20 dB
The RF amplifier output is through a wide-band transformer
to the input balanced transformer of the balanced mixer {two
3SK745, G3 and Q4), where it is mixed with the VCO output
from the PLL and converted to the 8.83 MHz IF. This signal
is applied to the IF unit through the ceramic f i l t e r s , NB gate
circuit and the crystal filter. The NB circuit is controlled by
the NB switch on the front panel
The signal. passing through the crystal filter. is amplified
about 90 dB by the three-stage 3SK74 MOS FET IF
amplifier (1, Q2, and Q3) and is demodulated into audio by
the four-diode ring detector,
From the final IF stage, the signal also passes through a
butfer amplifier and is fed to the AGC circuit where it is
detected and amplified. Receiver gain is controlled by this
AGC voltage applied to the second gates of the RF and IF
amplifiers, with the time constant determined by R38 (2.2
(M2) and C40 (1 yf). Input levels of 2 dB and 34 dB are in-
dicated as $1 and $9 on the S-meter. The AF signal is
amplified by Q1, a 2SC2240 (GR). gain controlled, and
further amplified by an HA1366W (Q7. the power-amplifier
IC, to drive the speaker. This signal, which is transistor-
coupled with Q14, a 2SC18151¥1, and Q15, a 2SA10151¥),
is sampled for ANTI-VOX control, so the VOX circuit is not
'wiped by the speaker output. This new system is a depar-
ture from the conventional transformer-coupled sampling
systems,
Fitren
"SECTION 7._ CIRCUIT DESCRIPTION
TRANSMITTING CIRCUIT
The microphone signal is amplified by transsitors Q18, a
2SC22401GR). O10. and O11. each a 2SC18151¥), and is
fed to the four-diode balanced modulator (BM) circuit
Microphone impedance is 500 { to 60 ki
The 8 83 MHz DSB signal from the 8M is amplified about
10 dB by Q12, a 2SK19 FET. and is fed to the IF unit where
the unwanted sideband is removed by the crystal filter to
produce an SSB signal, The gate of Q12 «s also controlled
by the protection voltage which is developed when the
'transmitter output locks into an incorrect load, continuously
reducing output power. The SSB signal from the crystal
filter is amplified about 30 dB by the 3SK74 IF amplifier
(Q1). and is fed to the transmit 3SK74 MOS FET balanced
mixer (Q5 and Q6). where it is mixed with the VCO output
'and converted to the final transmit frequency.
Unwanted spurious components are eliminated by the
transmit/receive BPF circuit, and the signal is wideband-
amplified by Q7. G8 (2SC1815). and Q9 (2SC2086).
In the final unit, the signal is amplified by the 2SC2078
driver (Q1), and by the 2SC2509 push-pull power amplifiers
{02 and Q3) TS-120V. and plus 28C2290 push-pull power
amplifiers (Q4 and Q5) (TS-120S). The signal then passes
through an AF filter and is fed to the antenna
Antenna output is toroid-sampled to detect the forward and
reflected power. The forward power is used for ALC and the
reflected power for protection. Forward Power is fed to the
second gate of the 3SK74 transmit/receive IF amplifier
(1), with 9 time constant determined by R7 (1 M2} and
C40 0.47 uF). For CW operation, block bias keying controls.
the base circuit of the 241018 switching transistor (10)
im the RF circuit. Q10 controls the first and second gate
voltages of the transmit mixer (Q5 andQ6) and the base
voltage of the predriver (Q9).
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