Circuit Description .000 - Kenwood TS-120S Service Manual

CIRCUIT DESCRIPTION
RECEIVING CIRCUIT
The signal from the antenna is fed to the 8.83 MHz IF trap
circuit. This signal is stepped up about 10 dB and im-
pedance-converted by a wide-band transformer before it is
applied to the bandpass filter (BPF). The BPF is common to
transmission and reception, eliminates the need for a
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
25C1815 (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
3SK74s, Q3 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 filters, 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
(Q1, 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
buffer 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
(2.2MQ) and C40 (1puF). Input levels of 2 dB and 34 dB are
indicated as S1 and S9 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 to drive the speaker. This signal, which is tran-
sistor-coupled with O14, a 2SC1815(Y), and Q15, a
2SA1015(Y), is sampled for ANTI-VOX control, so the VOX
circuit is not tripped by the speaker output. This new system
is a departure from the conventional transformer-coupled
sampling systems.
TRANSMITTING CIRCUIT
The microphone signal is amplified by transistors Q18, a
2SC2240(GR), Q10, and Q11, each a 2SC1815(Y), and is
fed to the four-diode balanced modulator (BM) circuit.
Microphone impedance is 5000 to 5OkQ.
The 8.83 MHz DSB signal from the BM 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 is also controlled by
the protection voltage which is developed when the
transmitter output looks 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 con-
verted to the final transmit frequency.
Unwanted spurious components are eliminated by the
transmit/receive BPF circuit, and the signal is
wideband-amplified by Q7, Q8 (2SC1815), and Q9
(2SC2086).
In the final unit, the signal is amplified by the 2SC2075
driver (Q1), and by the 2SC2509 push-pull power amplifiers
(Q2 and Q3). Then the signal is amplified by the 25C2290
push-pull power amplifiers (Q4, 5). The signal then passes
through an RF 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 (Q1),
with a time constant determined by R4 (1.5 MQ) and C25
{0.47 wF). For CW operation, block bias keying controls the
base circuit of the 2SA1015 switching transistor (Q.10) in the
RF circuit. Q10 controls the first and second gate voltages of
the transmit mixer (O5 and Q6) and the base voltage of the
predriver (Q9).