Icom IC-751A Service Manual page 23

RF UNIT BLOCK DIAGRAM
TX MIX
| TX BUFF
<30MHz
ACC 18 MHz
TRV8
22~30
MHz
15~22
MHz
W~15
MHz
18 TX AMP
2802053
8~11
MHz
6~8
MHz
4~6
MHz
2~3
16~2
MHz
0.5~16
MHz
PRE AMP
2SK125x2
R PRE 280945
ON x2
28A1015
>70MHz ND487C1 <9MHz
© iF
9.0115MHz
2nd LO
61.44MHz
Fig. 6
3. PA UNIT
RF signals input from the RF UNIT through P1 are amplified
by Q1, aclass A amplifier. Output from Q1 is converted to
balanced output by L1 and amplified by Q2 and Q3, class AB
push-pull amplifiers. Negative feedback circuits inserted
between the collector and base of Q2 and Q3 provide wide
frequency characteristics. The idling current of Q2 and Q3 is
controlled by the junction voltage of D1. The current is set at
about 100mA by R27. R30 prevents the adjustment point
from deviating due to variations in the characteristics of D1.
Output from Q2 and Q3 is fed into the impedance matching
section of L4 and is amplified by Q4 and Q5, class AB
push-pull amplifiers which provide 100W output power.
A portion of the output power from Q4 and Q5 is applied to
the bases of these transistors through a negative feedback
transformer (L9) which provides stability and broadband
characteristics in the frequency range 1.8~30MHz. R23
adjusts the idling current to approximately 600mA.
Output from Q4 and Q5 is then fed to L10 for impedance
conversion and output to the FILTER UNIT from P2.
4. FILTER, CONNECTOR UNITS
RF output from the PA UNIT is fed into J1 on the FILTER
UNIT to eliminate harmonic components. Filtered output
signals pass through the SWR detecting transformer (L18) to
the CONNECTOR UNIT via P2.
The forward wave component detected by L18 is rectified by
D1, filtered by C38, divided by R2 and R4, and fed into J7 on
the MAIN UNIT. A reflected wave component is also
detected by L18 andis rectified by D2, processed by C39, R3,
and R85, and is finally sent to the MAIN UNIT.
RF output fed to the CONNECTOR UNIT passes through a
diode switch (D4) to the ANTENNA CONNECTOR.
BIAS
CONT 3~4
| MHz
01~0.5
MHz
4-2-10 PBT OSCILLATOR CIRCUIT
This oscillator circuit oscillates local frequency signals for
mixers 1C10 and IC11 which are located at both the input and
output terminals of the 455kHz filter. In the receive mode, the
local signal frequency is changed by the [PBT] CONTROL
and the center frequency of the 455kHz filter is changed
likewise. Therefore, passband tuning functions with a 9MHz
filter.
In AM and FM modes, the center frequency of the 9M filter
is 9.0100MHz. In CW and RTTY-Narrow modes, the center
frequency is 9.0106MHz. In SSB mode the center frequency
is 9.0115MHz. Oscillator frequency variation causes
frequency differences between SSB and the other modes.
Oscillator frequency is changed by X7, L36, and varactor
diode D126 which is connected to X7 in series. The voltage
from the [PBT] CONTROL is between 0 and 8V and the
voltage passes to the cathode of D126 through Q58, R332,
and R336. —5V are applied to the anode of D126.
The oscillator frequency of this circuit is 9.4665MHz at the
center position of the [PBT] CONTROL. The frequency
varies up to +1.7kHz with [PBT] CONTROL rotation. In AM,
FM, CW, and RTTY-Narrow modes, R333 and R334 are
added to the oscillator circuit by Q56 and Q57. The
oscillation frequency for each mode is shifted to 9.4650MHz
or 9.4656MHz. In transmit mode the oscillator frequency is
fixed by applying a voltage via R337 and D129.
4-2-11 CW SIDETONE CIRCUIT
The CW sidetone circuit consists mainly of Q72 and employs
a phase shift oscillator and oscillator frequency of
approximately 700Hz. This oscillation is controlled by
control voltage from the CW keying circuit through D152.
Output signals in this circuit are fed into a monitor
preamplifier circuit.