Icom IC-751A Service Manual page 22

signals from Q36 are detected at D94 and D95 and are then
current amplified by Q37 and applied to the COMP METER.
When the [COMP] SWITCH is OFF, output from IC10 is
applied directly to Q38 by D90.
4-2-5 CW, RTTY OSCILLATOR CIRCUITS
In CW or RTTY mode, Q3 and X1 oscillate the transmit
carrier signal. In CW mode, oscillation is controlled by the
external keys. In RTTY mode, the oscillating frequency is
controlled by C17 and C20.
The emitter resistor of Q3 is connected to the collector of Q4
to control oscillation. When Q5 or Q6 is activated, D4 is
turned ON by R17, R18 and R19. C15~C20 are then series
connected to X1 to oscillate 9.0106MHz (an RTTY marker
frequency or a CW oscillator frequency). When both Q5 and
Q6 are OFF, D4 is also OFF, thus capacitors C19 and C20 or
C17~C20 (which are controlled by S1, the Shift Selection
Switch on the MAIN UNIT) are cut from X1 to oscillate either
9.01077MHz (the 170Hz shift frequency) or 9.01145MHz (the
850Hz shift frequency), respectively.
Q5 is an inverter for changing the Mark and Shift polarities in
RTTY modes. Polarity is fixed by S2 on the MAIN UNIT, the
RTTY Polarity Switch. In CW mode, Q5 receives voltage
from D5, so S2 has no relation to the oscillation frequency
which is 9.0106MHz.
4-2-6 CWKEYING CIRCUIT
A keying signal from the [EXT KEY] JACK is applied to Q4
through a delay circuit which consists of IC1, R31, and C23
(delay time is approximately 6ms). Transmit and receive
switching time is approximately 20ms, but when using full
break-in operation RF signals are delayed for less than 6ms.
FM OSCILLATOR AND MODULATOR
CIRCUITS
An audio signal from the mic amplifier circuit is fed into
1C14(a), a limiter amplifier, through C175 and R279, the
pre-emphasis circuit. After being amplified at 1C14(a), an
audio signal is applied to 1C14(b), a splatter filter which
reduces distortion components. The signal is then applied to
the FM modulator circuit.
4-2-7
Output from IC14(b) is applied to the anode of D118 and
signals oscillated by Q47 and X4 are frequency modulated.
Output from a UT-30 optional tone encoder is amplified at
Q46 and is then applied to the anode of D118, the same as an
audio signal. ON and OFF switching in the UT-30 is
controlled by the FUNCTION LED voltage. when the
FUNCTION LED lights up, the UT-30 is activated.
Audio deviation is adjusted by R292 and subaudible tone
deviation is adjusted by R290. Output signals from the FM
local oscillator circuit are buffer amplified at Q2 and are then
applied to Q1, the same as in CW and RTTY modes.
4-2-8 BFO CIRCUIT
This circuit oscillates local signals for detection of SSB,
RTTY, and CW modes via 1C20 and for detection of the
monitor circuit via IC15. The circuit provides balanced
modulation with IC9.
The oscillation frequency should be as follows:
USB : 9.01300MHz
LSB, AM (Receive) 9.01000MHz
CW (Receive) 9.00990MHz
RTTY (Receive) 9.008475MHz
The circuit oscillates using X6 in USB mode and X5 with L33,
L34 and L35 connected in series in LSB, CW and RTTY
modes, respectively. The circuit stops oscillating when the
transceiver is operating in AM receiving and CW
transmitting as Q52 and Q53 provide no voltage to the
circuit.
4-2-9 | TRANSMITTER IF AMPLIFIER CIRCUITS
1. MAIN UNIT
9MHz IF frequencies from the oscillator circuit of each mode
pass through monolithic filter Fl1 to remove spurious
components. The signals are then fed into Q1. Gate 1 of Q1
receives ALC voltage and gate 2 receives control voltage
from the power control unit.
2. RF UNIT
IF signals from the MAIN UNIT pass through a low pass filter
circuit and are converted into 70.4515MHz signals by IC3, a
balanced mixer diode. 9.0115MHz noise components are
removed from the output of IC3 by a high pass filter
consisting of C64~C66. 61.44MHz noise components are
removed by aseries resonance circuit consisting of L30 and
C67. Signals are then amplified at Q11, a dual gate FET that
receives ALC voltage through D22, a T/R switching diode.
Amplified signals from Q11 are fed into Q12 and Q13, the
transmit mixer circuit, through a doubie tuning circuit
consisting of L32, L33, and C77. Spurious noise components
are then removed from the signal by the 2nd gate of Q12 and
Q13 which receive local frequencies with DC bias voltage
that eliminates signal spuriousness. Signals are then
converted to the desired frequency of 0.1~30MHz.
Converted signals pass through a low pass filter to have local
oscillator components eliminated. They are then fed to an
attenuator circuit consisting of pin diodes and to a receiver
preamplifier bypassing circuit consisting of D12 and D13. In
transmit mode, this circuit is provided with bias voltage from
D9. Signals then pass through one of nine bandpass filters
that are selected by a voltage from the LOGIC UNIT before
being fed through a high pass filter.
In receive mode, D47 is ON. In transmit mode, D47 turns
OFF since Q16 (ON) and Q18 (OFF) combine to reverse the
bias of the diode. Q16 and Q18 are bandpass filter switching
transistors that control other diodes such as D26, D28, D30,
D32, D34, D386, D38, and D40. Note that current flows
through L99 and L100.
In transmit mode, D45 turns ON and signals are wideband-
amplified by Q14 and output to the PA UNIT.
When a transverter is used, D46 turns ON and inputs a
converted receive signal to the transceiver or outputs a low
level transmit signal from the transceiver through the
transverter terminal.
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