Transmitter System - Kenwood TR-751A Service Manual

TR-751A/E
CIRCUIT DESCRIPTION
TRANSMITTER SYSTEM
© General
The transmitter system operates as a single conversion
system,
Audio signals from the microphone are amplified by a
low-noise transistor (Q25) and applied to the SSB or FM
circuits which provide approx. 26dB gain
In SSB, the amplified signal passes through the SSB
microphone gain control, and is amplified by (Q27) and
applied to the balanced modulator (IC4). The balanced
modulator consists of an IC that provides stable carrier
suppression without being influenced by changes in tem-
perature. Signals from the microphone amplifier are mixed
with the carrier to produce a DSB signal. The DSB signal is
applied to the SSB filter (with a center frequency of
10.695MHz) to produce the SSB signal.
In FM, the signal amplified by the SSB/FM common micro-
phone amplifier (Q25) is applied through the buffer ampli-
fier (Q24) and FM microphone gain control, and then to
the pre-emphasis circuit, amplified by (1C1), and limited by
Op amplifier (1C1). High frequency components are then
removed from the signal by a 18dB/oct splatter filter,
and the signal is sent to the FM modulation circuit.
In the FM modulation circuit, signals from the 10.695
MHz crystal oscillator circuit are directly modulated by
varactor diode (D21), variable capacitor. This direct fre-
quency modulation enables a flat transmitter frequency
response to be obtained from low frequencies to high
frequencies. If the frequency deviation becomes excessive,
the deviation level will vary from the upper to the lower
portion of the signal. The TR-751A/E is designed so that
the upper and lower portions of the signal are balanced
even with maximum frequency deviation.
SSB/FM switching is performed by diode switching cir-
cuit according to the selected mode, and then amplified
by the transmitter IF circuit. This IF amplifier circuit con-
sists of a dual-gate MOS
FET, whose second gate is pro-
vided with ALC voltage to control the transmitter output.
© CW circuit description
In CW, the balanced modulator is unbalanced by AGC
signal to allow the carrier to pass.
CW keying is performed by switching the balanced
mixer in the drive circuit and a bias voltage is applied to the
first gate of amplifier. Fig. 5 shows the keying waveform.
The leading and trailing edges are smoothed to prevent key
clicks.
To facilitate CW communications, the CW circuit con-
tains CW semi break-in and side tone circuits.
The CW semi break-in circuit is a Schmitt circuit consist-
ing of transistors (Q1—Q4) or the break-in sub assembly.
The delay time can be adjusted with VR7.
The side tone circuit operates whenever the key is
closed. The side tone circuit operates in modes other than
CW, so key adjustment and morse code practice can be
performed. Transistor (Q9) is used as the oscillator. Signals
from the side tone circuit are amplified by the audio amp-
lifier (1C1). The output frequency of approx. 800Hz
can be adjusted with potentiometer (VR4).
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Fig. 5 CW waveform