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ICOM IC-751A transceiver and GENERAL COVERAGE RECEIVER
INSTRUCTION MANUAL


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TABLE OF CONTENTeS

SECTION 1 FEATURES 1
SECTION 2 SPECIFICATIONS 2
SECTION 3 CONTROL FUNCTIONS 4
3 - 1 FRONT PANEL 4
3- 2 FREQUENCY DISPLAY 12
3-3 TOP PANEL 12
3 - 4 REAR PANEL 13
SECTION 4 INSTALLATION 15
4 - 1 PLANNING 15
4 - 2 MOUNTING THE TRANSCEIVER 15
4 - 3 ANTENNA 15
4-4 MICROPHONE 16
4-5 CW KEY 16
4 - 6 POWER SUPPLY 16
4-7 GROUND 17
SECTION 5 SYSTEM INTERCONNECTIONS .".". 18
5-1 AC POWER SUPPLY 18
5-2 EXTERNAL (LINEAR) AMPLIFIER 18
5 - 3 ANTENNA TUNER 19
5-4 RTTY TERMINAL UNIT 20
5 - 5 SSTV UNIT 21
5 - 6 ACCESSORY SOCKET CONNECTIONS 21
SECTION 6 GENERAL OPERATION 22
6 - 1 SETTINGS 22
6 - 2 BASIC OPERATION 22
6-3 SSB OPERATION 25
6-4 CW OPERATION 26
6-5 AM OPERATION 27
6-6 FM OPERATION 28
6 - 7 RTTY OPERATION 29
6 - 8 GENERAL COVERAGE RECEIVER 30
SECTION 7 FUNCTIONS OPERATION 32
7 - 1 VOX OPERATION 32
7 - 2 SPEECH COMPRESSOR OPERATION 32
7 - 3 NOISE BLANKER (NB) OPERATION 33
7-4 CW SEMI/FULL BREAK-IN OPERATION 33
7 - 5 SPLIT (DUPLEX) OPERATION 34
7-6 FM REPEATER OPERATION 34
7 - 7 PASSBAND TUNING (PBT) OPERATION 35
7 - 8 NOTCH FILTER OPERATION 36
7 - 9 FILTER SWITCH OPERATION 36
7 - 10 RITMTX OPERATION 36
7-11 MONITOR OPERATION 38
7-12 MICROPHONE UP/DOWN OPERATION 39
7-13 AGC OPERATION 39
7 - 14 PREAMP/ATT SWITCH OPERATION 39


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FOREWORD

Thank you very much for choosing this ICOM product.

The IC-751 A HF transceiver is a refined version of the popular IC-751.
Through intensive market research, ICOM has compiled ideas from Amateur Radio
operators worldwide and used this feedback in an effort to design the best
transceiver with those features actually required by today's active
participant
in the field of Amateur communications. This research program is evidence of
the basic ICOM philosophy
of providing a product Amateurs want.

This is it. The IC-751A is an all mode HF transceiver and general coverage
receiver developed using the
most current computer-based design techniques and HF engineering.

Please study this instruction manual thoroughly to learn all functions of the
IC-751 A and refer back to
the manual periodically as necessary. Feel free to contact any authorized
ICOM dealer should you have
questions regarding the operation or capabilities of the IC-751 A.

UNPACKING

  1. DC power cable 1
  2. Pin plugs 2
  3. External speaker plug 1
  4. Spare fuses (20 ampere) 2
  5. Spare fuses (3 ampere) 2
  6. CW key plug 1
  7. Microphone (HM-36) 1

 

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contents

SECTION 8 MEMORY AND SCANNING OPERATION 40
8 - 1 MEMORY CHANNEL SELECTION 40
8 - 2 PROGRAMMING MEMORY CHANNELS 40
8-3 MEMORY CLEARING 41
8 - 4 MEMORY SCAN 41
8 - 5 MODE SCAN 42
8 - 6 PROGRAMMED SCAN 42
8 - 7 SCAN RESUMPTION 42

SECTION 9 OTHER FUNCTIONS 43
9 - 1 VSWR READINGS 43
9 - 2 WWV/WWVH RECEPTION 43

SECTION 10 OPTIONS INSTALLATION 44
10- 1 FILTERS 44
10-2 IC-PS35 INTERNAL POWER SUPPLY 46
10-3 IC-EX310 VOICE SYNTHESIZER UNIT 47
10-4 CR-64 HIGH-STABILITY CRYSTAL UNIT 47
10-5 UT-30 PROGRAMMABLE ENCODER UNIT 48

SECTION 11 CIRCUIT DESCRIPTION ... 49
11-1 RECEIVE CIRCUITS 49
11-2 TRANSMIT CIRCUITS 50
11-3 PLL CIRCUITS 51
11-4 LOGIC CIRCUITS 52

SECTION 12 MAINTENANCE AND ADJUSTMENTS 53
12-1 CLEANING 53
12-2 MAINTENANCE 53
12-3 ADJUSTMENTS 53
SECTION 13 INSIDE VIEWS 55
13 - 1 TOP VIEW 55
13 - 2 BOTTOM VIEW 56
13-3 RF UNIT 56
13 - 4 ADJUSTMENT LOCATIONS. 57
SECTION 14 BLOCK DIAGRAM 58
SECTION 15 OPTIONS 59
SECTION 16 SCHEMATIC DIAGRAM SEPARATE


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SECTION 1 FEATURES

COMPLETE HF RADIO

ALL BAND, ALL MODE
ALL SOLID-STATE

The IC-751 A covers all Amateur HF frequencies from 1.8MHz to
30MHz, including the three new bands of 10MHz, 18MHz and
24MHz. It also offers SSB, CW, AM, FM and RTTY operating
modes as standard features. All circuits in the IC-751 A, including
the driver and final power stages, are completely solid-state producing
a final output power of 100 watts.


GENERAL COVERAGE RECEIVER

FULL BREAK-IN FUNCTION
The IC-751 A features general coverage receive capability with a
tuning range from 100kHz to 30MHz. This wide range is accomplished
by means of up-conversion using a high side IF and a CPU
control system.

For CW operators, both semi break-in and full break-in are provided
for smooth, fast and natural CW conversations.

OUTSTANDING RECEIVER PERFORMANCE

ICOM'S DFM SYSTEM
The ICOM DFM (Direct Feed Mixer) feeds the incoming signals
directly into a high level first mixer developed by ICOM. This
advanced system produces a higher spurious response rejection ratio,
a higher receiver sensitivity and a wider dynamic range.


105dB DYNAMIC RANGE

PREAMP AND ATTENUATOR
INCLUDED
The IC-751 A has a 105dB dynamic range. Even with the PREAMP
switched ON, the dynamic range is approximately 100dB.

Both a 10dB preamplifier PLUS a 20dB attenuator are installed as
standard equipment. The preamplifier increases receiver sensitivity
while the attenuator provides added protection from intermodulation problems.


NEWLY DEVELOPED CPU VARIOUS SCANNING FUNCTIONS

Memory Scan allows monitoring of all memory channels or only
those storing a particular mode. Programmed Scan provides scanning
between any two programmed frequencies. Mode Scan automatically
monitors only memories containing frequencies with a
similar mode. In all cases, the Auto-stop function halts the scan
when a signal is received and the Resume function restarts the
scan when the frequency is clear.


32 MEMORIES
Thirty-two programmable memories are provided to store mode and
frequency, and the CPU is backed up by an internal lithium battery
to maintain the memories for up to ten years. Scanning of frequencies
or memories is possible from either the transceiver or the
HM-36 scanning microphone.


OPTIONS AVAILABLE IC-PS30

IC-2KL

The IC-PS30 external AC power supply was developed especially for ICOM base
radios and matches the style and size of the IC-751 A.
This power supply uses a recently designed switching regulator,
resulting in light weight and high efficiency.

The IC-2KL is a completely solid-state, HF broadband linear amplifier
with 500 watts output to give your signal the extra boost
needed for solid contacts when propagation is poor.


1


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SECTION 2 SPECIFICATIONS

2-1 GENERAL

Number of semiconductors
Transistors 59
(Australia, France: 61)
FETs 23
Diodes 336
ICs (Includes CPU) 64

Frequency coverage
Ham Bands
1.8MHz - 2.0MHz
3.45MHz - 4.1MHz
6.95MHz - 7.5MHz
9.95MHz - 10.5MHz
13.95MHz - 14.5MHz
17.95MHz-18.5MHz
20.95MHz - 21.5MHz
24.45MHz - 25.1MHz
27.95MHz - 30.0MHz
General Coverage (Receive Only)
0.1MHz - 30.0MHz

Usable temperature range
-10C ~ +60C (+14F ~ +140F)

Frequency control
CPU based 10Hz step digital PLL synthesizer.
Independent transmit/receive frequency.

Frequency readout
6 digit 100Hz illuminated FIP.

Frequency stability
Less than +200Hz from 1 to 60 minutes after power ON.
Less than 30Hz after 1 hour at 25C.
Less than 350Hz in the range of 0C +50C.

Power supply requirements
13.8V DC 15% (negative ground), current drain 20A maximum at 200W input.
AC power supply is available for AC operation.

Current drain (at 13.8V DC)
Transmitting
Approx. 20.0A At 200 watts input
Receiving
At maximum audio output Approx. 1.8A
Squelched Approx. 1.5A

Antenna impedance
50 ohms unbalanced.

Weight
8.5kg

Dimensions
306(322)mm(W)x 115(120)mm(H) x 355(385)mm(D)
Bracketed values include projections.


2-2 TRANSMITTER

RF power
SSB (J3E) : 200 watts PEP input
CW (A1A) : 200 watts input
FM (F3E) : 200 watts input
RTTY (F1A) : 200 watts input
AM (A3E) : 50 watts output

Emission modes
SSB (J3E) Upper and Lower sideband
CW (A1A)
FM (F3E)
RTTY (F1A)
AM (A3E)

Harmonic emissions
More than 40dB below peak power output.

Spurious emissions
More than 60dB below peak power output.
(Guaranteed for transmissions within the Amateur bands.)

Carrier suppression
More than 40dB below peak power output.

Unwanted sideband
More than 55dB down with 1000Hz AF input.

Microphone
Impedance 600 ohms
Input level 12 millivolts typical
Dynamic or electret condenser

RIT variable range
9.9kHz


2


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2-3 RECEIVER

Receive system
Quadruple-conversion superheterodyne with continuous bandwidth control.

Receive modes
SSB, CW, RTTY, AM, FM

Intermediate frequencies

Sensitivity
(PREAMP ON)

Squelch sensitivity

Selectivity

Spurious and image response rejection

Audio output

Notch filter attenuation

RIT variable range

70.4515MHz
9.0115MHz
9.0106MHz
9.0100MHz
455kHz
9.0115MHz
9.0106MHz
9.0100MHz
Triple-conversion superheterodyne.
SSB (J3E) Upper and Lower sideband
CW (A1A)
FM (F3E)
RTTY (F1A)
AM (A3E)
1st: All modes
2nd: SSB
3rd:
4th:
CW, RTTY
FM, AM
All modes
SSB
CW, RTTY
AM
SSB, CW, RTTY
0.1 - 0.5MHz Less than 0.5uV for 10dB S/N
0.5- 1.6MHz Less than 1uV for 10dB S/N
1.6 - 30.0MHz Less than 0.15/iV for 10dBS/N
AM (NARROW FILTER selected)
0.1- 0.5MHz Less than 3uV for 10dB S/N
0.5 - 1.6MHz Less than 6uV for 10dB S/N
1.6 - 30.0MHz Less than 1uV for 10dB S/N
FM
28 - 30MHz Less than 0.3uV for 12dBSINAD
1.6 - 30MHz Less than 0.3uV
SSB, CW, RTTY (WIDE FILTER selected), AM (NARROW)
CW, RTTY
FM
AM (WIDE FILTER selected)
2.3kHz at -6dB points
3.8kHz at -60dB points
500Hz at -6dB points
1.3kHz at -60dB points
15kHz at -6dB points
30kHz at -50dB points
8kHz at -6dB points
18kHz at -50dB points
Image rejection More than 80dB
IF rejection More than 70dB
More than 2.6 watts at 10% distortion with 8 ohm load.
More than 45dB
+9.9kHz


3


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SECTION 3 CONTROL FUNCTIONS

3-1 FRONT PANEL

 

-4


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* POWER SWITCH
This is a push-lock switch which controls the input DC power to
the IC-751 A. When the IC-PS30 AC power supply is used, the
switch also acts as the AC power supply switch. Power is supplied to
the transceiver when the switch is pushed IN and locked. Power to
all circuits is cut (except to the PA unit when using a DC power
supply) when the switch is pushed again and released.

* AUTOMATIC GAIN CONTROL SWITCH [AGC]
AGC FAST OFF SLOW
This switch changes the time constant of the AGC circuit. In the
SLOW position, the AGC voltage releases slowly for SSB reception.
In the FAST position, the AGC voltage releases quickly suitable for
receiving CW signals or signals with rapid fading.
In the OFF position, the AGC circuit and S-meter are disabled.
Also, the AGC circuit does not actuate in the FM mode.

*METER SWITCH
In the transmit mode, the front panel meter has six functions.

Vc
Indicates the collector voltage of the final transistors.
Ic
Indicates the collector current of the final transistors.
COMP
Indicates the compression level when the speech compressor is in use.
ALC
Indicates the ALC level. The ALC circuit begins to
function when the RF output power reaches a preset level.
Po
Indicates the approximate output power.
SWR
Indicates the SWR of the antenna system after the meter is referenced at "SET"
while the Po meter function is selected.

* TRANSMIT/RECEIVE (T/R) SWITCH
This switch is for manually changing between transmit and receive.
Set the switch to RECEIVE (down) to place the IC-751A in the
receive mode. Move the switch to TRANSMIT (up) to change to
the transmit mode. When using the PTT SWITCH on the microphone
or VOX operation, the T/R SWITCH must be at RECEIVE.

* MIC CONNECTOR
Connect a suitable microphone to this connector. The supplied HM-36
hand microphone or the optional SM-10 desk microphone may be used.
When using other microphones, refer to the diagram in SECTION 4-4.

*MIC INPUT
*+8V DC OUTPUT
* FREQ UP/DOWN
*AF OUTPUT
*GND (microphone ground)
*GND (PTT ground)
* PTT
* NC
(FRONT VIEW)


* PHONES JACK
This jack accepts a standard 1/4 inch plug from headphones with
an impedance of 4 - 16 ohms. Stereo headphones may be used
without modification.

* AF GAIN CONTROL
This control varies the audio output level in the receive mode.
Clockwise rotation increases the level.

*-RF GAIN
Increases the audio level.


5

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*RF GAIN CONTROL
This control varies the gain of the RF stage when the transceiver is
in the receive mode. Rotate the control fully clockwise for
maximum gain. When tuning in the SSB or CW mode, the S-meter
needle rises as the control is rotated counterclockwise and only
those signals stronger than the level indicated by the needle are heard.

*SQUELCH CONTROL
This control sets the squelch threshold level. To turn OFF the
squelch function, rotate this control completely counterclockwise.
To set the threshold level higher, rotate the control clockwise.
Adjust this control clockwise until the green RECEIVE indicator
just goes out while no signal is being received.

*TONE CONTROL
This control varies the receive audio tone. Adjust the control to
provide the clearest, most pleasing audio.

* MIC GAIN CONTROL
Adjust this control for a suitable modulation level while speaking
into the microphone in a normal voice. Rotate the control clockwise
to increase the gain.

* RF POWER CONTROL
This control varies the RF output power from 10 watts to maximum.
100W PEP ssb/cw/rtty
(AM : 50W)
Rotate the control clockwise to increase the output power. Use
the minimum power necessary for reliable communication.

* FUNCTION SWITCH
This switch activates the secondary switch functions on the front
panel as indicated by the reverse-image lettering (white letters on
a black background).

* FUNCTION INDICATOR
The FUNCTION INDICATOR lights after the FUNCTION
SWITCH is pushed, and indicates that the secondary switch functions
are activated.

*MODE SWITCHES
AM: Push for AM operation.

FM:
Push the FUNCTION SWITCH first, then push the AM SWITCH to select the FM mode.
CW:
Push for CW operation with the standard filters.


-6-

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CW NARROW (NAR):
Push the FUNCTION SWITCH first, then push the CW SWITCH
for operation with the 455kHz narrow CW filter. There is no audio
output from the speaker if the optional filter is not installed.

SSB:
Push to automatically select upper sideband (USB) operation on the 10MHz band
and above, and lower sideband (LSB) operation on the 7MHz band and below.
In the GENERAL COVERAGE mode, USB is selected from 9MHz upwards, and LSB is
selected below 9MHz.

SSB REVERSE:
Push the FUNCTION SWITCH first, then push the SSB SWITCH to select LSB on the
10MHz band and above, and USB on the 7MHz band and below. In the GENERAL
COVERAGE mode, LSB is selected from 9MHz upwards, and USB is selected
below 9MHz.

RTTY:
Push for RTTY operation.

 

RTTY NARROW (NAR):
Push the FUNCTION SWITCH first, then push the RTTY SWITCH for RTTY operation
using the optional 455kHz narrow CW filter. There is no audio output
from the speaker if the optional filter is not installed.

*HAM BAND/GENERAL COVERAGE SWITCH [HAM/GENE]
This switch changes the IC-751 A between the Amateur Band and
General Coverage modes. In the Amateur Band mode, the transceiver
functions on any of the HF amateur bands from 1.8 to
28MHz. In the General Coverage mode, the receiver functions on all
frequencies from 0.1 to 30MHz. The IC-751 A does not transmit
while in the General Coverage mode.

* SPEECH SWITCH
This switch activates the optional IC-EX310 speech synthesizer
which announces the displayed frequency in English.


HAM/GENE SPEECH MODE-S SCAN


)MODE SCAN SWITCH [MODES]

HAM/GENE SPEECH MODE-S SCAN
This switch engages the selective mode function. Only memory
channels with the same operating mode as is displayed just prior to
pushing this switch are selected when the TUNING CONTROL or
the scan function is used. See SECTION 8 for a full description of
the IC-751 A scanning systems.

)SCAN START/STOP SWITCH [SCAN]

HAM/GENE SPEECH MODE-S SCAN

This switch starts and stops all scan functions. When the scan
restarts, it begins from the frequency on which the scan halted while
in the programmed scan mode, or from the highest memory channel while in the
memory mode. See SECTION 8 for a full description of
the IC-751 A scanning systems.


The "SCAN" indicator lights when any scan
function is activated.

* TUNING CONTROL
Rotate this control clockwise to increase, or counterclockwise to
decrease the frequency or memory channel number. The frequency
actually changes in 10Hz steps in all operating modes at all times,
however, the tuning rate is 10Hz if the control is rotated slowly, but
jumps to 50Hz if the control is rotated more rapidly. This allows
both fine and coarse tuning of your operating frequency without
removing your hand from the TUNING CONTROL. Refer to the
TUNING SPEED SWITCH description below.


- 7

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This switch electronically locks the IC-751 A on the currently displayed
frequency and deactivates the TUNING CONTROL. This
function is useful while rag chewing or for mobile operation after
the IC-751 A is set to the desired frequency. Disengage the dial lock
by pushing and releasing the switch.


* TUNING SPEED SWITCH [TS]

-- DFS SWITCH
-- BAND SWITCH
Push this switch to increase the tuning rate to 1kHz while in any
operating mode. The 100Hz digit on the frequency display clears to
"0" simultaneously. Push and release the switch again to return to
the normal tuning rate. Refer to TUNING CONTROL description above.

* TS INDICATOR
Illuminates when the TS SWITCH function is activated.

* DIAL FUNCTION SELECT SWITCH [DFS]
While using the VFO mode, push this switch IN to lock the displayed
VFO operating frequency, and rotate the TUNING CONTROL to
change the displayed memory channel number.
While using the MEMORY CHANNEL mode, push this switch IN
to lock the displayed memory channel, and rotate the TUNING
CONTROL to change the displayed frequency.

* DFS INDICATOR
Illuminates when the DFS SWITCH function is activated.

* BAND SELECT SWITCH [BAND]
Push this switch and rotate the TUNING CONTROL in order to
change your band of operation. In the HAM BAND mode, the
IC-751 A jumps to a factory set initialization frequency on each of
the amateur bands as the TUNING CONTROL is rotated. In the
GENERAL COVERAGE mode, the operating frequency is changed
in 1MHz steps.

* BAND SELECT INDICATOR
Illuminates when the BAND SELECT function is activated.

 

 

INCREMENTAL TUNING CONTROL [RIT/XIT)
This control shifts the receive or transmit frequency by up to
9.9kHz to either side of the frequency indicated on the main frequency
display. Refer to the RIT and ATX. SWITCH descriptions below.
Rotating this control clockwise (+ direction) raises the receive or
transmit frequency and counterclockwise (-- direction) lowers the
frequency. The shifted amount and direction is indicated with
smaller numerals to the right of the main frequency display.

When both the RIT and ATX circuits are ON, both the receive and
transmit frequencies may be shifted together to either side of the
displayed frequency.

* NOTCH FILTER CONTROL

PBT-©-NOTCH
This control shifts the notch filter frequency when the NOTCH
SWITCH is ON. Adjust the control to minimize interference. Refer
to the description for the NOTCH SWITCH below and see SECTION
7 - 8 for more information.

-8-


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* PASSBAND TUNING CONTROL
This control allows continuous tuning of the receiver selectivity
when using the SSB, CW or RTTY mode. Besides reducing interference
on the desired signal, PASSBAND TUNING may also
improve the receive audio. The normal position is in the center
(12 o'clock) which sets filter bandwidth at 2.3kHz on SSB. Refer to
SECTION 7 - 7 for more information.

* rit
This switch turns the variable receive frequency circuit ON and OFF.
When the circuit is activated, the letters "RIT" on the front panel
illuminate and the amount of shift is indicated. The amount and
direction of frequency shift is retained in memory whether the
circuit is ON or OFF until erased with the RIT/ATX CLEAR
SWITCH. Refer to the description for the INCREMENTAL TUNING
CONTROL and also SECTION 7 - 10 for more information.

* ATX SWITCH

 

This switch turns the variable transmit frequency circuit ON and
OFF. When the circuit is activated, the letters "ITX" on the front
panel illuminate and the amount of shift is indicated. The amount
and direction of frequency shift is retained in memory whether the
circuit is ON or OFF until erased with the RITMTX CLEAR
SWITCH. Refer to the description for the INCREMENTAL TUNING
CONTROL and also SECTION 7 - 10 for more information.

RIT/ZITX CLEAR SWITCH
This switch clears the memory which contains the frequency shift
information as set with the INCREMENTAL TUNING CONTROL
and resets the small incremental tuning display to "0.0".
Push the FUNCTION SWITCH first, then this switch in order to add
the shifted frequency as shown on the incremental tuning display to
the frequency on the main frequency display. The incremental
tuning display resets to "0.0". Refer to SECTION 7 - 10 for more
information.

* FILTER SWITCH
This switch selects a different combination of the second IF (9MHz) filter
and the third IF (455kHz) filter to vary the overall selectivity
as shown in the following chart.


FILTER

FILTER SWITCH
SSB
CW/RTTY
CW/RTTY
NARROW
AM
FM
OUT
2.3kHz
500Hz
500/250HZ
8kHz
15kHz
IN
2.6kHz
2.3kHz
500/250Hz
3kHz
15kHz

Refer to SECTION 7 - 9 for more information.

* notch switch
This switch controls the notch filter circuit.
Notch Filter OFF: Switch OUT
Notch Filter ON: Switch IN

* VFO A/B SWITCH [A/B]
This switch selects either VFO A or VFO B for tuning purposes.

* VFO EQUALIZING SWITCH [A=B]
This switch instantly matches the frequency and mode of operation
of the two VFOs. The display does not change when this switch is
pushed, however confirmation of the equalizing process is possible
by pushing the A/B SWITCH to check the frequency and mode of the opposite VFO.


9

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* DSPLIT (DUPLEX) SWITCH
This switch determines whether one VFO is used for both transmitting and
receiving, or whether both VFO A and VFO B are used.
Single VFO operation : SPLIT SWITCH OUT.
Dual VFO operation : SPLIT SWITCH IN.
Refer to SECTION 7 - 5 for more information.

* VFO/MEMORY SWITCH [VFO/M]
This switch selects either the VFO or MEMORY CHANNEL mode for tuning purposes.

* MEMORY WRITE SWITCH [WRITE]
This switch stores the displayed operating frequency and mode
information in the memory channel indicated on the frequency
display. Memory writing is possible in either the VFO or MEMORY
CHANNEL mode. Refer to SECTION 8 - 2 for information on programming
the memory channels.

* FREQUENCY TRANSFER SWITCH [M - VFO]
This switch operates differently depending on which mode the IC-751A is in.
In the VFO mode, the frequency and mode stored in the memory
channel displayed transfer to the selected VFO.
In the MEMORY CHANNEL mode, the displayed frequency and
mode transfer to the VFO used immediately prior to changing to
the MEMORY CHANNEL mode.

NOTE: When in the MEMORY CHANNEL mode, it is the
displayed frequency which transfers to the VFO. This is
not necessarily the MEMORY CHANNEL frequency
since the TUNING CONTROL and DFS SWITCH may
have been used to shift the operating frequency. Information
stored in the memory channel remains unchanged
after using the M / VFO function.

* FREQUENCY DISPLAY
Refer to SECTION 3 - 2 for a description of the frequency display.

*TRANSMIT INDICATOR
This indicator illuminates when the IC-751 A is in the transmit mode.

*RECEIVE INDICATOR
This indicator illuminates when the IC-751 A is in the receive mode.

*NARROW FILTER INDICATOR
This indicator illuminates when the narrow CW or RTTY filter is
selected. If the filter is not installed, the indicator still lights when
the narrow positions are selected, however the audio output is muted.


10

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* MULTIFUNCTION METER
In the receive mode, the IC-751 A front panel meter acts as an
S-meter (signal strength meter). In the transmit mode, the meter
has six different functions as stated in Item METER SWITCH.

* NOISE BLANKER TIMING SWITCH
[NB WIDE]
vox
COMP NBWIDE
The blanking time of the noise blanker circuit may be set for
NORMAL or WIDE with this switch. Use this switch in conjunction
with the NOISE BLANKER LEVEL CONTROL described in Item


GAIN--VOX--DELAY NB LEVEL

OFF
KEY SPEED FULL

*SPEECH COMPRESSOR SWITCH [COMP]
This switch turns the RF speech compressor circuit ON and OFF.
The circuit provides greater talk power by improving the intelligibility
of the transmitted signal over long distances.

*VOX SWITCH
This switch turns the VOX circuit ON and OFF. The automatic
T/R switching VOX circuit functions in the PHONE and CW modes
when the switch is IN. In CW, semi break-in or full break-in operation
is possible.

SEMI BREAK-IN
Semi break-in operation refers to automatic switching of the
transceiver between the transmit and receive modes in unison
with the operator's speech or the closing and opening of the CW
telegraph key.

FULL BREAK-IN OPERATION
Full break-in operation, usable only with CW, is the same except
the transceiver switches even more rapidly after each element of
an individual letter is sent. Full break-in operation closely
simulates full duplex communication.

*VOX GAIN CONTROL

GAIN--VOX--DELAY

Increases
This control increases and decreases the sensitivity of the VOX
circuit. When operating in the CW mode, iambic keyer operation is
possible.

When operating in the SSB, AM or FM mode, adjust the control so
the VOX circuit activates with a normal speech level.

When operating in the CW mode, this control turns the electronic
keyer unit ON and OFF, plus varies the speed (5-45 wpm) of the
unit. When the control is OFF, manual CW operation is possible.

* VOX DELAY CONTROL (VOX time constant)
This control changes the transmit to receive switching time. Adjust
it so transmit to receive switching occurs during pauses in your
speech or CW transmission.

* NOISE BLANKER LEVEL CONTROL [NB LEVEL]
This control varies the threshold level of the noise blanker. Adjust
to remove the noise without adding distortion to the received signal.


11

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3-2 FREQUENCY DISPLAY

 

* MODE INDICATOR
This area of the display shows the operating mode currently selected.
The modes available are FM, AM, CW, USB, LSB or RTTY.

* GENERAL COVERAGE INDICATOR
"GENE" lights when the IC-751A is in general coverage mode.
When "GENE" is not lit, the transceiver is in the HAM BAND mode.

* FREQUENCY READOUT
This readout shows the operating frequency using 6 digits and
100Hz resolution.

* SPLIT INDICATOR
"SPT" lights when separate VFOs are used for the transmit and
receive frequencies. This allows operation with a transmit frequency
which is different from the receive frequency. When "SPT" is not
illuminated, a single VFO is used for both transmit and receive.

* SCAN INDICATOR
"SCAN" lights whenever a scan function is selected.

* VFO INDICATOR
"VFO A" or "VFO B" lights to indicate which VFO is currently selected.

* MEMORY INDICATOR
"MEMO" lights when the memory mode is selected. The selected
memory channel number from 01 to 32 is also shown.

* SHIFT FREQUENCY INDICATOR
"RIT" or "ATX" lights when the receive or transmit incremental
tuning function is activated, respectively. The direction and
quantity of the shift is also shown. Both functions may be activated
simultaneously if desired. Refer to SECTION 7-10 for more information.


3-3
TOP PANEL

12


.
*PREAMP/ATT (Attenuator) SWITCH
This switch selects the RF preamplifier to amplify weak receive
signals or the RF attenuator to prevent overloading of the receiver.
Refer to SECTION 7 - 14 for more information.

*MONITOR/MARKER SWITCH
This switch turns the MONITOR or MARKER circuit ON or OFF.

*MONITOR LEVEL CONTROL
This control changes the audio level of the CW sidetone or SSB
monitor circuit when the MONITOR SWITCH on the top panel is
turned ON. Adjust the control for the desired monitor volume.

*MARKER CALIBRATOR
A calibration marker is generated every 10kHz when the circuit is
activated.
This control varies the marker frequency. Use an accurate standard
frequency source such as radio station WWV/WWVH to calibrate the
marker generator. See SECTION 12 - 3 (2) for more information.

*ANTI-VOX CONTROL
When using the VOX function, sound from the speaker may cause
unwanted switching of the T/R relay. Adjust this control in conjunction
with the VOX GAIN CONTROL so the T/R relay switches
only from the operator's voice and not from the speaker audio.


3-4 REAR PANEL

* T/R CONTROL JACK [RELAY]
An output signal from this jack controls the transmit/receive switching
of an external amplifier or transverter.
NOTE: DO NOT attempt to switch greater than 50 volts, 0.5 ampere.

* ANTENNA CONNECTOR
Connect a 50 ohm impedance antenna to this connector. The connector
mates with a PL-259 plug.

*RECEIVE ANTENNA OUTPUT [RECEIVE ANT OUT]
The receive signals from the ANTENNA CONNECTOR pass through the transmit/receive
antenna switching circuit to this jack. Normally, the receiver IN and OUT jacks
are connected together with a jumper. The ANTENNA OUTPUT JACK is useful
when operating with a separate receiver or an external preamplifier.

*EXTERNAL ALC JACK [ALC]
This jack accepts an automatic level control signal from an external amplifier
or transverter. The ALC voltage must be in the range of 0 volts to --4 volts.


13


.


* RECEIVER INPUT [RECEIVE ANT IN]
This is an input jack which connects to the receiver RF stage.

*TRANSVERTER JACK [X-VERTER]
Connect a suitable transverter to this jack for operation on VHF/ UHF
frequencies. The output is approximately 30mV.

* SPARE JACK
This jack is not used. It may be useful for connecting a personal accessory.

* GROUND TERMINAL
To prevent electrical shocks, TV I, BCI and other problems, be
sure to ground the equipment through the GROUND TERMINAL.
For best results, use the heaviest gauge wire or strap
available and make the connection as short as possible.

*KEY JACK
For CW operation, connect a CW key using the supplied standard
1/4 inch, 3-conductor plug. The terminal voltage from external
electronic keyers must be less than 0.4 volts DC.
When using the IC-751 A internal iambic keyer, connect an iambic
keyer paddle using the supplied 1/4 inch, 3-conductor plug. See
SECTION 4 - 5 and 6 - 4 for more information.

* EXTERNAL SPEAKER JACK
Connect an external speaker to this jack, if required. Use a speaker
with an impedance of 4 ~ 16 ohms and remember the built-in
speaker does not function when using the EXTERNAL SPEAKER JACK.

*ACCESSORY SOCKET
This socket provides signals such as T/R switching, receiver output,
modulation output, etc. Refer to SECTION 5 - 6 for more information.

*DC POWER SOCKET [DC 13.8V]
Connect the DC power cable from the IC-PS35 internal power
supply, the IC-PS30 or IC-PS15 external power supply or other
suitable supply. Refer to SECTION 4 - 6 for more information.

IC-EX309 (OPTIONAL) INTERFACE UNIT CONNECTOR POSITION
This position is for the installation of the DP-25 connector from the
optional IC-EX309 interface unit.

* IC-PS35 (OPTIONAL) AC POWER SUPPLY SOCKET POSITION
This position is for the installation of the AC power socket when
the optional IC-PS35 internal power supply is used.


14


.
SECTION 4 INSTALLATION

4-1 PLANNING
Select a location for your transceiver which allows free access to the
front panel controls, good air circulation and rear clearance for
access to the rear panel connections.


PRECAUTIONS
Mobile or marine installations:
An optional IC-MB18 mounting bracket is available to mount your
IC-751 A. Select a location which can support the weight of the
unit, and which does not interfere with the normal operation of
the vehicle or boat. Provide protection from direct rain on boats.

Avoid using the IC-751 A in the following situations:

1) Where temperatures under --10C or over +60C are encountered. For
example, DO NOT use the IC-751 A in areas exposed to direct
sunlight or near heat-producing devices such as heaters or ranges.

2) In humid or moist places including bathrooms.


4-2 MOUNTING THE TRANSCEIVER
DO NOT run the antenna feedline near electronic instruments or
magnetic compasses.

The optional IC-MB18 mounting bracket allows installation of the transceiver
to a surface over the IC-751 A. When mounting the
transceiver aboard a boat, place the bracket in a location which
minimizes vibration and wave shock.


4-3 ANTENNA
Antennas play a very important role in radio communication. If
the antenna is inferior, your transceiver cannot give you the best
performance. A well-matched 50 ohm antenna and feedline will
provide the desired performance.

Since the IC-751 A comes standard with a general coverage receiver,
it is recommended that a long wire antenna and antenna tuner be
used when receiving outside the Amateur bands. The tuner will
ensure there is a matched condition at all times. An Amateur band
antenna generally does not give adequate performance when used on
frequencies for which it was not designed.


MOBILE ANTENNA

NOTE: To install the AH-2, an OPC-137 interface
cable must be purchased separately.
The ICOM AH-2 Antenna System may be bumper-mounted, plus it
offers automatic band switching and tuning in unison with the
IC-751A.


-15-


.
4-4 MICROPHONE
The supplied HM-36 electret condenser hand microphone or
optional SM-8 or SM-10 desk microphone may be used. Merely plug
it into the MICROPHONE CONNECTOR on the front panel. If you
wish to use a different microphone, make certain it has the proper
output level. Particular care must be exercised when wiring a
different microphone since the transceiver internal electrical switching
system is dependent upon the correct connections. See
SECTION 3 - 1 FRONT PANEL Item for MIC CONNECTOR connections.


4-5 CW KEY
For CW operation, connect a CW key to the KEY JACK using the
supplied standard 1/4 inch, 3-conductor plug. See the diagrams for
connection details.

Note that the keyed voltage when switching with semiconductors or
relays with resistors in the circuit must be less that 0.4 volts.


4-6 POWER SUPPLY
When an AC power supply is required, the IC-PS35 internal power
supply, or the IC-PS30 or IC-PS15 external power supply with
matching size and style is recommended.

CAUTION: Voltages greater than 15 volts DC may damage this
transceiver. Check the source voltage before connecting
the power cable when using with non-ICOM AC power supplies.


16


.
CAUTION: For safety purposes, remember to install FUSES in
the DC cable when connecting the IC-751 A to a
vehicle battery. Locate the fuses as close to the car
battery as practical.


4-7 GROUND
To prevent electrical shocks, TV I, BCI and other problems, be sure
to ground the equipment with the heaviest gauge wire or strap
available from a good ground to the GROUND TERMINAL on the
rear panel.


-17-


.
SECTION 5 SYSTEM INTERCONNECTIONS

5-1 AC POWER SUPPLY

5-2 EXTERNAL LINEAR AMPLIFIER
Refer to SECTION 4 - 6 for installation directions.

The IC-751 A is designed to operate into a load with 50 ohms
impedance. Therefore, any amplifier used must have a 50 ohm input
impedance for best efficiency.


USING THE IC-2KL LINEAR AMPLIFIER

NOTE: All cables required for the interconnection
of the IC-751A and the IC-2KL are
supplied with the amplifier.
The optional 500 watt, solid-state IC-2KL linear amplifier may be
easily connected to the IC-751 A with the cables supplied with the amplifier
in the same way as other ICOM transceivers. Refer to the
IC-2KL Instruction Manual for details.

It is recommended that the ICOM IC-AT500 Automatic Antenna
Tuner be used when operating with the IC-2KL and 500 watts
output (See SECTION 15). The amplifier connections are shown
below.


* USING NON-ICOM EXTERNAL AMPLIFIERS
The RELAY JACK on the rear panel of the IC-751A supplies an
output from a built-in keying relay for an external amplifier. The
ALC jack is an input for the automatic level control signal from an
external amplifier.

The specification for the keying relay is 50 volts DC maximum. Do
not exceed this limit.

The RELAY JACK is grounded while the transceiver is in the transmit
mode and it opens when the transceiver returns to the receive
mode. The output condition of this jack does not control either
the transmit or receive function of the transceiver.


-18-


.
5-3 ANTENNA TUNER
It is recommended that the ICOM IC-AT100 or IC-AT500 Automatic
Antenna Tuner be used between the IC-751 A and the antenna
system to achieve maximum performance from the transceiver.

The IC-751 A may also be used with the AH-2 Automatic Antenna
Tuner (weatherproofed) designed especially for long wire or vertical
antennas. The AH-2 covers all HF bands therefore it is suitable for
use on the Amateur bands as well as with the IC-751 A general
coverage receiver.

USING THE IC-AT100 or IC-AT500
The maximum allowable input for the IC-AT100 is 100 watts, therefore,
the IC-AT500 must be used with the IC-2KL Linear Amplifier.


USING THE AH-2
The AH-2 provides reliable matching for frequencies from 1.8MHz
to 30MHz using a 12 meter long antenna element.


NOTE: It is necessary to adjust the tuning power. Refer to the
AH-2 manual, SECTION 5 - 3 TUNING POWER.


- 19


.
5-4 RTTY TERMINAL UNIT
When operating RTTY, connect the ACC CONNECTOR pin 9 and
pin 8 (ground) via a high speed relay or through a level converter for
conversion to TTL level to your teletypewriter. The audio tones for
your terminal are available from pin 4 and pin 8 (ground). The AF
output level is about 300mVp-p for an S9 signal.

An RTTY Terminal Unit for AFSK operation can also be connected easily.

NOTE: Some RTTY Terminal Units may require the use of a
20dB audio preamplifier between pin 5 on the ACC
SOCKET and the AFSK signal output from the Terminal
Unit to provide enough drive for full RF output power
from the transceiver. The pin 5 input must be approximately
150mV rms.

Check your RTTY Terminal Unit Instruction Manual.

The AFSK signal may be connected to the IC-751 A front panel
MIC CONNECTOR pin 1 instead of the ACC CONNECTOR if the
Terminal Unit output is not high enough. An extra stage of
amplification is provided in the transceiver when using the microphone connector.

-20-


.
5-5 SSTV UNIT
Slow Scan Television operation is also possible with the IC-751 A.


5-6 ACCESSORY SOCKET CONNECTIONS

Connect the CAMERA OUTPUT on your SSTV Unit to the ACC
CONNECTOR pin 5 or MIC CONNECTOR pin 1.

The audio output signal (300mVp-p) is available from pin 4 and
the ground is pin 8.

Various signals are available from the accessory socket such as
modulation output, receiver output, T/R changeover, etc.

PIN NO.: FUNCTION
1:
Output from the squelch control stage. (+8V when
the squelch is ON.)
2:
13.8 volts DC interlocked with the power switch.
3:
Connected to the push-to-talk, T/R switch. When
grounded, the transceiver changes to the transmit
mode.
4:
Output from the receive detector stage. Fixed output
regardless of AF output or AF gain.
5:
Output from the transmit MIC amplifier stage.
(Input for the MIC gain control stage.)
6:
8 volts DC output while transmitting. (Maximum
output current is 5mA.)
7:
Input for external ALC voltage.
8:
Ground.
9:
Input for RTTY keying (MARK: HIGH level,
SPACE: LOW level). The levels for MARK and
SPACE may be reversed with an internal switch.
10:
NC (no connection).
11:
Input for TRANSVERTER control. Apply 8 volts
DC to use a transverter with the IC-751 A.
12:
Reference voltage output for band switching.
13:
Band Control Voltage output. See below.
14-24:
NC (no connection).


BAND CONTROL VOLTAGE
NOTE: The IC-751 A outputs a band control voltage when the
band of operation is changed. This signal automatically
switches accessory equipment such as the ICOM linear
amplifier or antenna tuners.


BAND (MHz): BAND CONTROL VOLTAGE
1.8: 7.0 - 8.0V
3.5: 6.0- 6.5V
7: 5.0- 5.5V
14: 4.0-4.5V
18,21: 3.0-3.5V
24,28: 2.0 -2.5V
10: 0 - 1.2V

21


.
SECTION 6 GENERAL OPERATION

6-1 SETTINGS
After all INSTALLATION instructions have been followed in
SECTION 4, including connecting a 50 ohm antenna system, set the
controls and switches as indicated below.

1) If a supply other than the IC-PS35, IC-PS30 or IC-PS15 is used, check
that the AC power supply output voltage is 13.8 volts DC.

2) Check that the antenna connector on the feedline from the antenna is
securely fastened to the ANTENNA CONNECTOR on
the rear panel of the IC-751 A.

3) Check all connections between the IC-751 A and accessory equipment.


SWITCH AND CONTROL SETTINGS

SWITCH/CONTROL: POSITION
POWER: OFF (OUT)
AF GAIN: MAXIMUM COUnTERCLOCKWISE
RF GAIN: MAXIMUM CLOCKWISE
SQUELCH: MAXIMUM COUNTERCLOCKWISE
TONE: CENTER
MIC GAIN: CENTER
RF POWER: MAXIMUM CLOCKWISE
DIAL LOCK: OFF (OUT)
PBT: CENTER
NOTCH CONTROL: CENTER
NOTCH SWITCH : OFF
FILTER: OFF (OUT)
TS: OFF (INDICATOR OFF)
DFS: OFF (INDICATOR OFF)
BAND: OFF (INDICATOR OFF)
VOX: OFF (OUT)
COMP: OFF (OUT)
NB WIDE: OFF (OUT)
VOX GAIN: OFF
VOX DELAY: FULL
NB LEVEL: OFF
AGC: SLOW
METER: Ic
TRANSMIT/RECEIVE: RECEIVE
SPLIT: OFF (OUT)


6-2 BASIC OPERATION
This section explains basic receiving and transmitting methods.
Refer to SECTIONS 6 - 3 through 6 - 8 for detailed explanations of
each operating mode.


RECEIVING

1) Power ON.
Before beginning, confirm all switches and controls are set as stated
in SECTION 6- 1 SETTINGS.

1) Push the POWER SWITCH to turn ON the transceiver. The
FREQUENCY DISPLAY lights after about 2 seconds and the
transceiver is ready for receiving.


-22-


.
2) Select MODE.

2) Push the SSB SWITCH to select the SSB mode.

3) Select BAND.
3) Push the BAND SELECT SWITCH. The BAND SELECT INDICATOR lights. Rotate the
TUNING CONTROL to select the 14MHz band. (Note: Only the "MHz" digits change
as the TUNING CONTROL is turned.)


4) Adjust AF GAIN CONTROL.

4) Rotate the AF GAIN CONTROL clockwise for a comfortable receive level.

The green RECEIVE INDICATOR should be lit. If not, check
the position of the SQUELCH CONTROL (maximum counterclockwise).


5) Rotate the TUNING CONTROL until a signal is received. Tune
for the highest S-METER reading and the clearest audio. (Note:
The wrong mode may be selected if the signal is not understandable.)


TRANSMITTING

Before transmitting, listen in the receive mode to make sure your
transmission will not interfere with other communications.

First, set the switches and controls as stated in SECTIONS 6 - 1
SETTINGS and RECEIVING above.

1) Push the PTT SWITCH on the microphone or switch the
TRANSMIT/RECEIVE SWITCH to TRANSMIT. The red
TRANSMIT LED lights.


FREQUENCY SELECTION

2) Speak into the microphone, and note that the meter needle follows the
modulation of your voice. You are now transmitting
SSB signals.

3) If you wish to increase the output power, rotate the RF POWER CONTROL
for the desired power between 10 and 100 watts.

The IC-751 A transmitter covers all HF Amateur bands exclusively.
In addition, the receiver covers all frequencies from 0.1 to 30MHz
continuously.

1) TUNING CONTROL:
Rotate the TUNING CONTROL clockwise to increase the frequency,
and counterclockwise to decrease the frequency in 10Hz
steps while in any operating mode. Turning the TUNING CONTROL
faster automatically shifts the tuning increments to 50Hz.

2) TUNING SPEED SWITCH [TS] :
When the TUNING SPEED function is ON, the operating frequency
shifts in 1kHz increments while in any operating mode as
the TUNING CONTROL is turned.


23


.
NOTE: The IC-751 A transmits in the ranges
shown in the table, however it is the
operator's responsibility to follow the
pertinent goverment radio regulations.

3) BAND SELECT SWITCH [BAND]:
Push this switch IN and rotate the TUNING CONTROL to
change the operating band. When in the HAM BAND mode, the
VFO frequencies selected on each band are the initialization
frequencies as shown in the chart. In the GENERAL
COVERAGE mode, only the 10 and 1 MHz digits of the display
change.


BAND (MHz):
FREQUENCY COVERAGE
(MHz)
1.8: 1.800.0 - 2.000.0
3.5: 3.450.0- 4.100.0
7: 6.950.0 - 7.500.0
10: 9.950.0- 10.500.0
14: 13.950.0- 14.500.0
18: 17.950.0- 18.500.0
21: 20.950.0-21.510.0
24.5: 24.450.0- 25.100.0
28/29: 27.950.0 - 30.000.0

BAND (MHz): HAM BAND INITIALIZATION FREQUENCIES
1.8: 1.900.0MHz
3.5: 3.550.0MHz
7: 7.050.0MHz
10: 10.047.0MHz
14: 14.047.0MHz
18: 18.047.0MHz
21: 21.047.0MHz
24.5: 24.547.0MHz
28/29: 28.047.0MHz
* SSB mode selected.


MODE SELECTION
The various modes are selected by using the MODE SWITCHES in
conjunction with the FUNCTION SWITCH.


1) AM
Push the AM SWITCH to select the AM mode.
Refer to SECTION 6 - 5 for AM operation.

2) FM
Push FUNCTION and AM SWITCHES.
Push the FUNCTION SWITCH, then the FM SWITCH to select the
FM mode. Pushing the AM/FM SWITCH again returns the transceiver
to the AM mode.

Refer to SECTION 6 - 6 for FM operation.

3) CW

Push the CW SWITCH to select the CW mode, or push the FUNCTION
SWITCH then the CW SWITCH to select the CW NARROW
mode. The optional FL-52A or FL-53A narrow filter is required to
use the CW NARROW mode.

Refer to SECTION 6 - 4 for CW operation.

4) SSB [USB/LSB]

Push the SSB SWITCH to automatically select the USB mode on the
10MHz band and above, and the LSB mode on the 7MHz band and below.
Push the FUNCTION SWITCH, then the SSB SWITCH to select the opposite mode.

Refer to SECTION 6 - 3 for SSB operation.

NOTE: See SECTION 6 - 8, step 4) for USB/LSB selection when
operating in the GENERAL COVERAGE mode.

5) RTTY

Push the RTTY SWITCH to select the RTTY mode, or push the
FUNCTION SWITCH then the RTTY SWITCH to select the RTTY
NARROW mode. The optional FL-52A or FL-53A narrow filter is
required to use the RTTY NARROW mode.

Refer to SECTION 6 - 7 for RTTY operation.


24


.
(2) CW NARROW RECEIVING

The IC-751A has already FL-32A (500Hz/-6dB) filter installed.
If an optional narrow filter is installed, push the FUNCTION
SWITCH, then the CW SWITCH to select the narrow filter thereby
improving the receiver selectivity with higher shape factor.

Background noise is reduced and the signal-to-noise ratio (S/N) is
improved with the narrow filter for more enjoyable listening conditions.


OPTIONAL FILTERS
1) The chart shows the optional filters available.


FILTER:
CENTER FREQUENCY:
CHARACTERISTICS (BANDWIDTH) -6dB:
-60dB
FL-52A: 455kHz: 500Hz: 1.0kHz
FL-53A: 455kHz: 250Hz: 480Hz

2) Push the FUNCTION SWITCH, then the CW SWITCH to select the CW NARROW
mode. The CW NARROW INDICATOR lights when the function is activated.

3) Once the CW NARROW mode is selected, follow the instructions in the CW
RECEIVING section.

(3) CW TRANSMITTING
1) Insert the CW key plug into the KEY JACK on the rear panel of
the IC-751 A.
2) Set the switches and controls as shown in the chart.

NOTE: See SECTION 4 - 5 CW KEY for CW key connections

SWITCH/CONTROL: POSITION
RF POWER: MAX. CLOCKWISE
METER: Po
VOX GAIN: OFF (MAX. COUNTERCLOCKWISE)

CW SIDETONE MONITOR

BREAK-IN OPERATION

3) Select the transmit mode with the T/R SWITCH.

4) Operate the CW key. The meter movement indicates that a signal is being
transmitted.

A 700Hz sidetone oscillator is provided to monitor keying when in the CW mode.
The MONITOR LEVEL CONTROL varies the volume of the tone.
This control is located on the top panel of the transceiver.

Rotate the MONITOR LEVEL CONTROL clockwise to increase
the volume of the oscillator tone. The transceiver AF GAIN
CONTROL also varies the volume.

Refer to SECTION 7 - 4 for semi or full break-in CW operation.


6-5 AM OPERATION
(1) AM RECEIVING
Set all controls as stated in SECTION 6 - 1 SETTINGS.

  1. Push IN the POWER SWITCH.
  2. Select the desired band of operation.

 

27


.
3) Push the AM SWITCH.

4) Tune in an AM signal with the TUNING CONTROL using the meter to peak the
signal.

5) Push the FILTER SWITCH to select the narrow receive selectivity.

FILTER SWITCH: BANDWIDTH
ON (IN): 3kHz
OFF (OUT): 8kHz

6) Refer to SECTION 10 - 1 for installation instructions for the
optional FL-33 crystal filter.

NOTE: The PBT CONTROL (passband tuning) has no effect in the AM mode.


(2) AM TRANSMITTING
Transmitting an AM signal is essentially the same as for SSB transmission.
1) Select the transmit mode with either the T/R SWITCH or the PTT SWITCH on
the microphone.
2) Turn the RF POWER CONTROL maximum clockwise or set it to obtain the
desired power output.
3) Set the METER SWITCH to the Po position.
4) Adjust the MIC GAIN CONTROL for a slight indication of the meter needle
while speaking into the microphone in a normal voice.

NOTE: MIC GAIN CONTROL settings which are too high result
in overmodulation and a distorted signal. Set the MIC GAIN carefully.

5) The speech compressor (COMP SWITCH) should be turned OFF
to prevent overmodulation or distortion.
6) Use the FILTER SWITCH to select the widest selectivity position.


6-6 FM OPERATION
(1) FM RECEIVING

  1. Push the FUNCTION SWITCH, then the FM SWITCH.
  2. Set the other switches and controls the same as for SSB reception.

The PBT CONTROL, NOTCH SWITCH and CONTROL, NOISE BLANKER SWITCH and CONTROL,
AGC SWITCH and FILTER SWITCH do not function in this mode.

3) When tuning an FM signal, tune for maximum signal strength as
indicated on the meter with the clearest audio.


* SQUELCH FUNCTION
SQUELCH: If closed, the squelch mutes all noise from the speaker when no
signal is received. This is useful while waiting for another station to call.


-28-


.
FM transmission

  1. Set the MIC GAIN CONTROL to the center position.
  2. Select the transmit mode with either the T/R SWITCH or the PTT SWITCH on

the microphone.
3) Speak into the microphone in a normal voice. The meter needle
indicates the carrier power when the METER SWITCH is in the
Po position, however the meter does not move in time with your
voice modulation in this mode.

NOTE: Most countries only allow FM operation on 28MHz or higher.


(3) FM REPEATER OPERATION
An 88.5Hz TONE ENCODER is often required to access HF FM repeaters.
The optional UT-30 PROGRAMMABLE TONE ENCODER is designed for this purpose.

NOTE: The UT-30 was preset with an 88.5Hz tone. Change the UT-30 programming if
a different frequency is required.
Install the encoder in the location shown in SECTION 10 OPTIONS INSTALLATION.
Connect the 3-pin connector from the encoder
onto J6 ("TONE" jack) on the MAIN unit.

1)When operating through repeaters, program the transmit and
receive frequencies in VFO A and VFO B.

Refer to SECTION 6 - 2 VFO A/B SELECTION for information on using the two VFOs.

2) The encoder'tone is transmitted when the FUNCTION SWITCH
is pushed while transmitting. In addition, the FUNCTION INDICATOR lights.

NOTE: The FUNCTION INDICATOR lights when the FUNCTION SWITCH is pushed while
transmitting even if the UT-30 is not installed, however no tone is transmitted.


6-7 RTTY OPERATION

(1) RTTY UNIT CONNECTIONS

(2) RTTY RECEIVING

For RTTY operation, a teletypewriter or keyboard and a
demodulator (terminal unit) which is operational with audio input
are required.

Any demodulator designed for 170Hz narrow shift with 2125/
2295Hz filters may be used with the IC-751 A.

Refer to SECTION 5 - 4 RTTY TERMINAL UNIT for system
interconnections and further information.

Audio signals for the demodulator may be taken from pin 4 of the
ACC SOCKET on the rear panel of the IC-751 A, or from the
PHONES JACK on the front panel. The level of the audio signals
from the ACC SOCKET does not vary when adjusting the AF GAIN
CONTROL, and the level is about 300mVp-p maximum.

1) Push the RTTY SWITCH to select the RTTY operating mode.

2) Set the other controls and switches the same as for SSB reception.


-29-


.
3) Tune in an RTTY signal using the tuning indicator of the terminal
unit or the cross pattern of an oscilloscope to center the receiver
on the signal. Mark frequency is 2125Hz and space frequency is
2295Hz.

Adjust the PBT CONTROL as required for the best reception.

4) If an optional narrow CW filter is installed, push the FUNCTION
SWITCH, then the RTTY SWITCH to select the narrow filter.
The receiver selectivity will be narrowed.


OPTIONAL FILTERS

FILTER: CENTER FREQUENCY: CHARACTERISTICS (BANDWIDTH) -6dB: -60dB
FL-52A: 455kHz: 500Hz: 1.0kHz
FL-53A: 455kHz: 250Hz: 480Hz

5) To tune RTTY signals with a wider shift, such as 425Hz or
850Hz, obtain a demodulator suitable for the desired shift and
use the normal RTTY mode.


(3) RTTY TRANSMITTING
1) To key the FSK circuit, insert a high speed relay coil into the
loop current circuit of the teletypewriter, and connect the relay
contacts to pins 8 and 9 of the ACC SOCKET on the rear panel.
2) The relay contacts make during the SPACE and break during the MARK.
3) Fine adjustment of the MARK and SPACE frequencies can be made by
adjusting the trimmer capacitors in the MAIN unit.
Refer to SECTION 13-4 ADJUSTMENT LOCATIONS.


USING A LEVEL CONVERTER
1) When a level converter for TTL levels is used, connect the output
of the converter to pin 8 (ground) and pin 9 on the ACC
SOCKET.
2) Apply HIGH level (5V) signals for MARK, and LOW level (0V) for SPACE.
If your equipment supplies reverse polarity signals (i.e., MARK:
0V, SPACE: 5V), slide S2 on the MAIN unit to the REVERSE
position. See SECTION 13-4 ADJUSTMENT LOCATIONS for
the S2 location.


USING AN AFSK GENERATOR
1) When using an AFSK generator with a 2125Hz MARK frequency
and 2295Hz SPACE frequency, connect the output signals from
the AFSK generator to the MIC CONNECTOR on the front
panel or ACC socket on the rear panel.
2) Push the LSB SWITCH.
3) VOX T/R switching may be used while operating AFSK for easier operation.


6-8 GENERAL COVERAGE RECEIVER
NOTE: -When in the GENERAL COVERAGE mode, it is not
possible to transmit on any frequency, including the
Amateur band frequencies.


-30-


.
1) Set CONTROLS and SWITCHES.
1) Set the CONTROLS and SWITCHES as stated in SECTION 6-1 SETTINGS. Other
controls are unrelated to operation in this mode.
2) Push IN POWER SWITCH.
2) Push IN the POWER SWITCH.

 

 


USB
/ H. P
1 1
VFO k
n
u.
n
u
n i
U 1
The meter lamp lights, and shortly thereafter, the display lights
and indicates the frequency and HAM/GENERAL mode which
are stored in the VFO A memory. The memory channel number
"01 "also lights.

 

 

3) Select GENERAL COVERAGE mode by pushing HAM/GENE SWITCH. "GENE" lights.
3) If the transceiver does not display "GENE", push the HAM/GENE SWITCH.
The GENERAL COVERAGE mode is now selected. When the HAM BAND mode is selected,
the "GENE" indicator goes out.


NOTE: In the HAM BAND mode, the letters "GENE" do not light.

4) Push the desired MODE SWITCH. When the SSB mode is chosen,
USB is automatically selected on the 10MHz band and above, and
LSB is selected on the 9MHz band and below.

Note, however, once either USB or LSB mode is automatically
selected, this mode is maintained regardless of frequency or band
changes. To change to the opposite sideband, push the FUNCTION
SWITCH, then the SSB SWITCH again.

5) Select desired band and frequency.
5) Push the BAND SELECT SWITCH, then use the TUNING CONTROL
to select the desired band of operation. Push and release
the BAND SELECT SWITCH again, and tune the TUNING
CONTROL until a signal is located.
6) Adjust AF GAIN CONTROL.
6) Adjust the AF GAIN CONTROL for a comfortable audio level.
The multifunction meter indicates the signal strength of the
received signal, therefore tune for the highest reading on the
meter with the clearest audio.

Refer to SECTIONS 6 - 3 to 6 - 7 for further information on each mode.

31


.
SECTION 7 FUNCTIONS OPERATION

7-1 VOX OPERATION
The IC-751 A has a built-in VOX (voice-operated relay) circuit which
allows automatic T/R switching by using the operator's speech modulation.

1) Set controls.
1) Set the VOX GAIN, VOX DELAY and MIC GAIN as shown in the table.


SWITCH/CONTROL: POSITION
VOX SWITCH: ON (IN)
VOX GAIN: FULLY COUNTERCLOCKWISE
VOX DELAY: 12 O'CLOCK
MIC GAIN: 12 O'CLOCK

2) Set the T/R SWITCH in the receive position. It is not necessary
to push the PTT SWITCH on the microphone.

3) Adjust VOX GAIN CONTROL.
3) Rotate the VOX GAIN CONTROL clockwise while speaking into
the microphone until the T/R switching circuit activates. Stop
adjusting the control once the circuit has activated.

4) Adjust VOX DELAY CONTROL.
4) The VOX DELAY CONTROL varies the amount of delay after your speaking stops
before the transceiver changes back to the receive mode. Rotate this control
counterclockwise to reduce the time delay. Set it to allow for short pauses in
normal speech without the IC-751 A changing to receive.

5) Adjust ANTI-VOX CONTROL.
5) Rotate the ANTI-VOX CONTROL on the top panel clockwise while receiving a
signal until the audio from the speaker no longer activates the VOX circuit.
Refer to SECTION 3 - 3 for the control location.


7-2 SPEECH COMPRESSOR OPERATION
The IC-751 A has a low distortion, RF speech compressor circuit
which provides greater talk power by improving the intelligibility
of the transmitted signal over long distances.

1) Set METER SWITCH to "COMP" position.
1) Set the switches and controls as shown in the table.


SWITCH/CONTROL: POSITION
MIC GAIN: CENTER (12 O'CLOCK)
RF POWER: MAXIMUM COUNTERCLOCKWISE
COMP: ON (IN)
METER: COMP

2) Switch to transmit and turn the RF POWER CONTROL clockwise while speaking
into the microphone until the desired RF "PEAK" output of between approximately
10 watts and 100 watts is obtained.

3) Adjust the MIC GAIN CONTROL for a reading of between 10dB and 20dB on
the COMPRESSOR (COMP) scale.

NOTE: Misadjustment of the speech compressor may cause interference to
other stations.

4) For the best transmit audio, leave the compressor OFF, or use a low mic gain
setting with the compressor ON. However, when in contact with DX stations,
or under weak signal conditions, the compressor will definitely improve the
readability of your signal when it is turned on and adjusted correctly.


-32


.
7-3 NOISE BLANKER [NB] OPERATION

When pulse noise, such as ignition noise from vehicles, makes reception
difficult, the NOISE BLANKER effectively reduces this kind of interference.

1) Rotate the NB LEVEL CONTROL clockwise from the OFF position beyond the
detent when pulse noise is present.

2) The noise is suppressed and weak signals which may otherwise have been
covered by noise can be received.

3) When the NOISE BLANKER TIMING SWITCH is set in the
WIDE (locked IN) position, the noise blanker effectively removes
long duration pulse noise such as the "woodpecker".

NOTE: The noise blanker may not work as well when strong
signals are on nearby frequencies, or when the noise is
continuous rather than pulse noise.

If the desired receive signal becomes distorted by the
action of the noise blanker, either place the NB WIDE
SWITCH in the OUT position, or reduce the NB LEVEL
CONTROL (turn counterclockwise) until the signal is clear.


7-4 CW SEMI/FULL BREAK-IN OPERATION
The IC-751 A has both semi break-in and full break-in CW capability when using
the VOX function. This means the transmit/receive switching is performed
automatically by the transceiver whenever the keying starts or stops.
The receive to transmit switching time is almost instantaneous whereas the
transmit to receive switching time may be varied by an adjustment.

SEMI BREAK-IN OPERATION USING A STRAIGHT KEY
1) Set the controls and switches as shown in the table.

SWITCH/CONTROL: POSITION
MODE: CW or CW NARROW
VOX: ON (IN)
KEY SPEED: OFF
VOX DELAY: 10 O'CLOCK
TRANSMIT/RECEIVE: RECEIVE

2) Set the transmit release delay time to suit your keying speed by
adjusting the VOX DELAY CONTROL while keying. Rotate the control clockwise
to increase the delay.

3) Begin sending with the CW key and the transmitter will switch between
transmit and receive automatically.


SEMI BREAK-IN OPERATION WITH AUTOMATIC KEYING USING AN IAMBIC PADDLE

Automatic keying is possible simply by connecting an iambic paddle.
The operator merely controls the number of dots or dashes transmitted
without regard to spaces between the elements, or the timing
of the elements, within a character. When used correctly, the resulting
CW is generally easier and more enjoyable to copy at the receiving end.

1) Set the switches and controls as shown in the table.

NOTE: The CW WEIGHT CONTROL is installed on the electronic keyer unit.
Refer to SECTIONS 12-3 (3) and 13-2 for the control location.


-33


.
SWITCH/CONTROL: POSITION
MODE: CW or CW NARROW
VOX: ON (IN)
KEY SPEED: 12 O'CLOCK
VOX DELAY: 10 O'CLOCK
TRANSMIT/RECEIVE: RECEIVE

2) Adjust the KEY SPEED CONTROL for a suitable keying speed.

3) Set the transmit release delay time to suit your keying speed by
adjusting the VOX DELAY CONTROL while keying. Rotate
the control clockwise to increase the delay.

4) The transmit/receive switching is automatically performed by the transceiver.


FULL BREAK-IN OPERATION WITH AUTOMATIC KEYING USING AN IAMBIC PADDLE

1) Set the switches and controls as shown in the table.

SWITCH/CONTROL: POSITION
MODE: CW or CW NARROW
VOX: ON (IN)
KEY SPEED: 12 O'CLOCK
VOX DELAY: FULL (MAX. COUNTERCLOCKWISE)
TRANSMIT/RECEIVE: RECEIVE

2) Adjust the KEY SPEED CONTROL for a suitable keying speed.

3) The transmit/receive switching is automatically performed by the
transceiver.


7-5 SPLIT (DUPLEX) OPERATION
(Example)
Receiving: 7.057MHz/CW
Transmitting: 7.255MHz/LSB

Receiving
(VFO A)


The purpose of the split function is to allow an operator the option
of transmitting on a frequency different from the receive frequency.
Push the SPLIT SWITCH to turn the function ON or OFF.

1) Store the frequency 7.057MHz in VFO A and 7.255MHz IN VFO B.

2) Push the VFO A/B SWITCH to select VFO A, then push the SPLIT SWITCH to
select the split function. The letters "SPT" on the display illuminate.

3) The receive frequency is now set for 7.057MHz and the transmit frequency
is 7.255MHz. Push the VFO A/B SWITCH once again
in order to receive on 7.255MHz and transmit on 7.057MHz.

Each VFO also stores the operating mode in addition to the operating
frequency. This allows crossmode contacts to be made easily.


7-6 FM REPEATER OPERATION
A tone encoder is often required to access HF FM repeaters. The
UT-30 PROGRAMMABLE TONE ENCODER is available as an
accessory for use with the IC-751 A.


Refer to SECTION 10 - 5 for installation instructions.

34


.
1) After installing the UT-30 in the transceiver, push the FM MODE SWITCH
and program both the transmit and receive frequencies
into VFO A and VFO B, respectively.

2) Push the SPLIT SWITCH to select the split (duplex) operation.

3) Select the transmit mode with either the T/R SWITCH or the PTT SWITCH.

4) Push the FUNCTION SWITCH while transmitting. The red LED lights and
indicates the programmed tone is being transmitted.

5) Push the FUNCTION SWITCH again to cancel the tone.

NOTE: If no UT-30 is installed, the FUNCTION INDICATOR
lights but no tone signal is transmitted.


7-7 PASSBAND TUNING (PBT) OPERATION
Passband Tuning is a system designed to electronically narrow the
bandwidth (selectivity) of frequencies that will pass through the
receive crystal filter.

1) The PBT CONTROL has a detent at the center (12 o'clock) position. This
position provides the widest bandwidth, and the
control should normally be left here.

2) The PBT CONTROL may also be used to adjust the tone of the receive audio
for maximum intelligibility.

3) While receiving in the LSB mode, narrow the bandwidth by turning the PBT
CONTROL counterclockwise to eliminate
interference from lower frequencies (interfering signals are high
pitched). Turn the PBT CONTROL clockwise to eliminate
interference from higher frequencies (interfering signals are low pitched).

4) While receiving in the USB mode, the bandwidth is reduced in the
opposite manner. Narrow the bandwidth by turning the PBT
CONTROL clockwise to eliminate interference from higher
frequencies (interfering signals are high pitched). Turn the PBT
CONTROL counterclockwise to eliminate interference from
lower frequencies (interfering signals are low pitched).

NOTE: The passband tuning does not function when the transceiver
is in the AM or FM mode.

 

-35-


.
7-8 NOTCH FILTER OPERATION
This circuit provides high attenuation at one particular frequency in
the IF passband. It may be used to reduce or eliminate interfering
heterodyne signals that fall within the passband of the receiver.

1) Push the NOTCH FILTER SWITCH to the ON position (IN).

2) Adjust the NOTCH FILTER CONTROL to minimize interference.


7-9 FILTER SWITCH OPERATION
This switch selects the different combinations of the internal receive filters.

 

 


PBT-®-NOTCH

FILTER
SWITCH
SSB
CW/RTTY
*1
CW/RTTY
NARROW
AM
FM
OUT
2.3kHz
*2 500Hz
500/250Hz
*4 8kHz
15kHz
IN
2.6kHz
*3 2.3kHz
500/250Hz
*5 3kHz
15kHz

 

FILTER SWITCH

"I: CW NARROW or RTTY NARROW mode requires either an
optional FL-52A or FL-53A narrow filter.

*2, *3: Internal preset switch S4 on the MAIN unit reverses these
narrow and wide filters:
FILTER SWITCH OUT: 2.3kHz
FILTER SWITCH IN : 500Hz
*4, *5: Internal preset switch S3 on the MAIN unit reverses these
wide and narrow filters:
FILTER SWITCH OUT: 3kHz
FILTER SWITCH IN : 8kHz


7-10 RITWTX OPERATION
RIT OPERATION

By using the RIT circuit, it is possible to shift the receive frequency
up to 9.9kHz to either side of the transmit frequency without moving
the transmit frequency. This is.useful for fine tuning stations
which call off frequency or for compensating for frequency drift.

1) Push RIT SWITCH to turn ON the RIT.

Receive : 14.267.8MHz
Transmit: 14.267.8MHz
"RIT" displays.
1) Push the RIT SWITCH once to turn the RIT circuit ON. The
letters "RIT" and the amount of shift are displayed.


-36-


.
2) Rotate INCREMENTAL TUNING CONTROL counterclockwise.

Receive : 14.2639MHz
Transmit: 14.2678MHz

USB
VFO I
1.8 RIT
-3.3
n i
U 1
2) The receive frequency can be shifted in 10Hz steps by adjusting
the INCREMENTAL TUNING CONTROL. Rotating the control
in the (+) direction raises the receive frequency, and in the (--)
direction lowers it.

3) Push RIT SWITCH to turn OFF the RIT.
n i
u I

n? b 1.8
3) To turn OFF the RIT function, push the RIT SWITCH again.
The letters "RIT" and the amount of shift are no longer displayed.

 

 

4)
Push the RIT SWITCH again to turn ON the RIT.

4) When the RIT circuit is OFF, the transmit and receive frequencies
are the front panel displayed frequency. They are the same
regardless of the amount of shift previously selected with the
INCREMENTAL TUNING CONTROL. However, the RIT shift
value is stored in a memory for later use when the RIT function is
again activated.

5) Push the RIT/ATX. CLEAR SWITCH.

5) To clear the RIT shift, push the RIT/ATX CLEAR SWITCH. The
shift value resets to 0.0 and the receive and transmit frequencies
become the same whether the RIT function is ON or OFF.

6) Push FUNCTION SWITCH, then push RIT/zfTX
CLEAR SWITCH.
6) To add the RIT shift value to the displayed frequency, push the
FUNCTION SWITCH, then push the RiT/ATX CLEAR SWITCH.
The RIT shift value automatically resets to 0.0.


ATX OPERATION
The ATX circuit allows the transmit frequency to be shifted up to
9.9kHz to either side of the receive frequency without moving the
receive frequency. This function is similar in operation to the RIT function.

1) Push the ATX SWITCH to turn ON the ATX circuit. The letters "ATX" and the
amount of shift are displayed.

2) Rotate the INCREMENTAL TUNING CONTROL counterclockwise.
2) The transmit frequency can be shifted in 10Hz steps by adjusting
the INCREMENTAL TUNING CONTROL.


37


.
3) RIT function activated.

Receive : 14.2639MHz
Transmit: 14.2678MHz
3) As with the RIT function, the amount of shift selected with the
INCREMENTAL TUNING CONTROL is maintained in a memory
whether the ATX function is ON or OFF. A shift value
selected while the RIT was activated becomes the ATX shift when
the ATX function is turned ON.

Push RIT SWITCH to turn OFF the RIT.
Push ATX SWITCH to turn ON the ATX function.

4) RIT already activated.

4) If the RIT and ATX functions are both activated, the receive and
transmit frequencies will be equal but shifted from the displayed
frequency by the shift value. Varying the INCREMENTAL
TUNING CONTROL changes both the transmit and receive
frequencies simultaneously.

 

 

Push ATX SWITCH to turn ON the ATX function.

Receive : 14.2639MHz
Transmit: 14.2639MHz
Push RIT SWITCH to turn OFF the RIT function.

5) Push ATX SWITCH to turn OFF the ATX function.


5) Push the ATX SWITCH again to turn OFF the ATX function. The letters "ATX"
and the amount of shift are no longer displayed.

6) Push ATX SWITCH to again turnON the ATX function.

6) When the ATX function is OFF, the transmit and receive frequencies
are as indicated on the frequency display regardless of
what amount of shift is selected. Activation of the ATX function
recalls the shift value from memory.

7) To add the ATX shift value to the displayed frequency, push the
FUNCTION SWITCH, then push the RIT/ATX CLEAR SWITCH.
The value of the ATX shift automatically resets to 0.0.


7-11 MONITOR OPERATION
The transmit IF signal may be monitored while using SSB if desired.
This circuit is included to allow checking of the quality of the transmitted
signal, especially useful when using the RF speech compressor.

1) Turn the MONITOR SWITCH on the top cover ON.

2) Adjust the MONITOR LEVEL CONTROL on the top panel for a comfortable
audio level.

3) Wear headphones while using the monitor function to prevent howling
sounds caused by feedback.
See SECTION 3 - 3 TOP PANEL for the control and switch locations.


38


.
7-12 MICROPHONE UP/DOWN OPERATION

FREQUENCY CONTROL

1) Each push of the UP or DN (down) SWITCH on the supplied microphone
changes the operating frequency one increment up or
down respectively.

2) Holding either switch down changes the operating frequency continuously
in the same manner as turning the TUNING CONTROL.
The tuning rate must be set with the TUNING SPEED SWITCH.

This up/down function is effective for changing your operating
frequency when using the VFO mode, or when using the MEMORY
CHANNEL mode with the DFS SWITCH pushed IN.

MEMORY CHANNEL SELECTION

1) When using the MEMORY CHANNEL mode, the memory channel may be changed
with the microphone up/down switches.
Also, in the VFO mode with the DFS SWITCH pushed IN, the
displayed memory channel number may be changed in the same way.

2) The memory channel or the displayed memory channel number changes every
two seconds if the microphone switches are held down.


7 -13 AGC OPERATION

The IC-751 A has a fast attack/slow release AGC system which
maintains the peak voltage of the rectified IF signal from the IF
amplifier circuit for a brief period of time. This circuit keeps irritating
background noise from being heard during the short pauses in
speech. The damping effect of the AGC therefore allows accurate
S-meter readings to be taken of the peak IF signal strength.

1) For normal SSB reception, select the SLOW position.

2) For CW or AM signals, or for SSB signals with short interval fading,
select the FAST position. In the FAST position, the time
constant of the circuit is shortened.

3) In the OFF position, the AGC circuit is deactivated and the
S-meter gives no signal strength readings. In this case, the RF
GAIN CONTROL is useful to reduce the receiver gain when
strong signals are received. Adjustment of the RF GAIN
CONTROL affects the meter reading as described in item(8)of
SECTION 3-1 FRONT PANEL.


7 -14 PREAMP/ATT SWITCH OPERATION
PREAMP SWITCH OPERATION

Place the PREAMP/ATT SWITCH on the top panel in the PRE AMP
position when receiving weak signals. An RF preamplifier is inserted
in the receive path which increases receiver sensitivity.

ATTENUATOR (ATT) SWITCH OPERATION

Place the PREAMP/ATT SWITCH in the ATT position when receiving
very strong signals. In the ATT position, the RF preamplifier is
removed from the receive path and a 20dB attenuator is inserted.
This helps to reduce interference.

Leave the PREAMP/ATT SWITCH in the OFF position for normal operation.
See SECTION 3 - 3 for the switch location.


-39-


.
SECTION 8 MEMORY AND SCANNING OPERATION

The IC-751 A has thirty-two memory channels numbered 01 through
32. One frequency and the operating mode may be assigned to each
memory channel whether the transceiver is in the VFO mode or the
MEMORY CHANNEL mode.

Remember the following when storing frequencies in memories:

MEMO 01, 02: These channels are used as band limits for the
PROGRAMMED SCAN function.


8-1 MEMORY CHANNEL SELECTION
1) Turn ON the POWER.
1) When the power switch is turned ON, the transceiver automatically
selects VFO A and displays "01".

2) Push the VFO/M SWITCH. "MEMO" displays.
2) Push the VFO/MEMORY SWITCH to place the transceiver in the
MEMORY CHANNEL mode.

The frequency, operating mode and HAM/GENE mode which
have been stored in memory channel 01 are displayed. The
letters "MEMO" above the memory channel number indicate that
the MEMORY CHANNEL mode is currently selected.

3) Rotate the TUNING CONTROL.
Turning the TUNING CONTROL clockwise.

3) Rotate the TUNING CONTROL to select any of the memory
channels. When memory channels not yet programmed are
selected, only the MHz and kHz decimal points appear on the
display. In this case, the transceiver continues to operate on the
memory channel or VFO which was selected immediately prior to
selecting the non-programmed channel.


8-2 PROGRAMMING MEMORY CHANNELS
Any frequency, operating mode and HAM/GENE mode may be
memorized in any memory channel.

1) Select operating parameters.
Frequency, mode and HAM/GENE mode will be memorized.

For example, select "15.725MHz", "AM" and "GENE" with
VFOB.


-40-


.
2) Push IN the DFS SWITCH and select a memory channel.


2) Push IN the DFS SWITCH, then select a memory channel using
the TUNING CONTROL. For example, select memory channel 10.

3) Push VFO/M SWITCH to check present contents of selected memory.
3) To check the memory channel contents, push the VFO/MEMORY
SWITCH. The contents of the selected memory channel are
displayed. If you do not wish to erase the contents of this particular memory
channel, select either a different memory
channel which contains a frequency that can be erased, or select a
channel that is vacant. Vacant channels display only the MHz and kHz decimal
points. Return to the VFO mode by pushing
the VFO/MEMORY SWITCH.

4) Push the WRITE SWITCH to store the new data.
The display appears as below after pushing the VFO/M SWITCH again.

4) Push the WRITE SWITCH. The previous memory contents (if
any) are erased and the new frequency, mode and HAM/GENE
mode are stored in the memory channel.


In the example, the frequency 15.725MHz with the AM mode is
now programmed in memory channel 10, and ready for use with
the GENERAL COVERAGE receiver.


8-3 MEMORY CLEARING

  1. Select memory channel.
  2. Push FUNCTION SWITCH, then push either WRITE SWITCH or M ? VFO SWITCH.
  1. Select the memory channel to be cleared (erased).
  2. Push the FUNCTION SWITCH, then push either the MEMORY WRITE SWITCH

(WRITE) or the FREQUENCY TRANSFER
SWITCH (M ? VFO).
The memory channel is now vacant.


8-4 MEMORY SCAN
This function is used to automatically scan all programmed memory channels.

1) Program memory channels.
1) Program the desired frequencies into memory channels.

NOTE: Random frequencies were stored in the memory channels
at the factory for testing purposes.

2) Adjust SQUELCH CONTROL.
2) Adjust the SQUELCH CONTROL to quiet the noise output from the speaker.

3) Push SCAN SWITCH.
3) Push the SCAN SWITCH, and the IC-751 A begins scanning the programmed
channels from the highest channel towards the lowest channel. The scan skips
unprogrammed channels if there are any.

4) Scan stops when receive signal opens squelch.
4) Provided the squelch is closed when the scan begins, the scan
stops when a receive signal opens the squelch. After approximately
10 seconds, the scan resumes.

5) Use SCAN SWITCH to manually stop/start scan.
5) Push the SCAN SWITCH to stop the scanning function manually.
Turning the TUNING CONTROL also stops the scan. Push the
SCAN SWITCH again to resume scanning.


41


.
8-5 MODE SCAN
The purpose of this scan is to selectively monitor those memory
channels which contain frequencies programmed with the same
mode (FM, USB, LSB or CW).

1) Push VFO/M SWITCH.
DPush the VFO/MEMORY SWITCH to select the MEMORY CHANNEL mode.

2) Adjust SQUELCH CONTROL.
2) Adjust the SQUELCH CONTROL to quiet the noise output from the speaker.

3) Push MODE-S SWITCH.
3) Push IN the MODE SCAN (MODE-S) SWITCH.

4) Push SCAN SWITCH.
4) Push the SCAN SWITCH.


The transceiver begins from the channel selected in step 1), and
steps through each memory channel with the same mode,
stopping only when a receive signal is detected.


8-6 PROGRAMMED SCAN
The purpose of this scanning function is to monitor a particular
section of the band.

1) Store the scan limits in memory channels 01 and 02.
1) Store the frequencies of the high and low limits of the desired scanning
range in memory channels 01 and 02. See SECTION 8 - 2 PROGRAMMING MEMORY
CHANNELS, if necessary, for instructions on how to write frequencies into
memory channels.
The scan begins from the high limit of the range regardless of
which channel has the higher frequency stored.

NOTE: Store both scanning limits in MEMO 01 and MEMO 02 using either the HAM or
the GENE mode. HAM mode limits must be in the same band.

2) Select VFO A or B with VFO A/B SWITCH.
2) Use the VFO A/B SWITCH to select the VFO you wish to use.

3) Adjust SQUELCH CONTROL.
3) Adjust the SQUELCH CONTROL to quiet the noise output from the speaker.

NOTE: If the RECEIVE INDICATOR remains ON, for example
if the SQUELCH CONTROL is rotated fully counterclockwise,
the scan will not stop at any signals.

4) Push SCAN SWITCH.
4) Push the SCAN SWITCH to start the scan. The transceiver scans
from the high limit towards the low limit. The scanning rate
depends on the TUNING SPEED SWITCH position.

5) Scan stops when a signal opens the squelch. Scan
resumes after 10 seconds.
5) The squelch opens when a signal is received. This stops the scan
automatically if the SQUELCH CONTROL has been set as
explained in step 3). After approximately 10 seconds, the scan
resumes downwards from the frequency on which it stopped.

6) Pushing the SCAN SWITCH while the scan is operating, or during
the 10 second halt period, clears the scanning function. Transmitting
or rotating the TUNING CONTROL also cancels the
scanning function.


8-7 SCAN RESUMPTION
After stopping due to a received signal, all scans resume after a fixed
time of approximately 10 seconds. However, switch S10 may be
used to override this feature.

The factory setting is the SCAN RESUME position. When placed in
the SCAN CLEAR position, the scan function automatically clears
when a receive signal opens the squelch circuit. To continue scanning,
push the SCAN SWITCH again.


-42-


.
SECTION 9 OTHER FUNCTIONS

9-1 VSWR READINGS
The IC-751 A has a built-in VSWR meter to check antenna matching
as an aid in avoiding problems caused by high VSWR.

  1. Set the METER SWITCH to "Po".
  2. Select the RTTY operating mode.
  3. Place the TRANSMIT/RECEIVE SWITCH at TRANSMIT.
  4. Adjust the RF POWER CONTROL located on the front panel to place the

meter needle at "SET" on the SWR scale.

5) Set the METER SWITCH to "SWR". Read the VSWR from the SWR scale.

 

NOTE: Always use a 50 ohm antenna system in order to obtain
full output power and to avoid possible damage to the
transmitter. ._-.-


9-2 WWV/WWVH RECEPTION
The signal from radio station WWV or WWVH may be used for
alignment of a frequency counter, the marker oscillator or the frequency
display.

1) Select the 10MHz band using either the HAM BAND or GENERAL COVERAGE
mode.

2) Push the AM SWITCH.

3) Use the TUNING CONTROL to select 10.000.0MHz.

Reception of WWV/WWVH is also possible on 2.5, 5 and 15MHz.
Use the frequency which has the strongest signal at your location.
Refer to SECTION 12-3 ADJUSTMENTS for further information.


43


.
SECTION 10 OPTIONS INSTALLATION


10-1 FILTERS

FILTER CHARACTERISTICS

WARNING: Disconnect the power cable from the IC-751A before
performing any work.

MODE
FILTER
CHARACTERISTICS


Center
Frequency
-6dB point
-60dB point
CW/RTTY
FL-52A
455kHz
500Hz
1kHz
CW/RTTY
FL-53A
455kHz
250Hz
480Hz
CW/RTTY
FL-63A
9.0106MHz
250Hz
800Hz
CW/RTTY
*FL-32A
9.0106MHz
500Hz
1.34kHz
SSB
FL-70
9.0115MHz
2.8kHz
5kHz
SSB
*FL-80
9.0115MHz
2.6kHz
3.8kHz
SSB
*FL-44A
455kHz
2.3kHz
4.2kHz
AM
FL-33
9.0100MHz
6kHz
20kHz

NOTE: The filters marked with a"*"are supplied with the IC-751 A.

FILTER COMBINATIONS

MODE
FILTER
SWITCH
9MHz
FILTER
455kHz
FILTER
STANDARD
BANDWIDTH
P.B.T.
USB/LSB
OUT
FL-80
FL-44A
2.3kHz
YES

IN
FL-80
CFJ455K5
2.6kHz
YES
CW/RTTY
OUT
FL-32A
FL-44A
500Hz
YES

IN
FL-80
FL-44A
2.3kHz
YES
CW/RTTY
NARROW
OUT
FL-32A
FL-52A/
FL-53A
500Hz/
250Hz
YES

IN
FL-80
FL-52A/
FL-53A
500Hz/
250Hz
IF SHIFT
AM
OUT
THROUGH
CFW455HT
8kHz
NO

IN
THROUGH
CFJ455K5
3kHz
NO
FM
OUT
THROUGH
CFW455E
15kHz
NO

IN
THROUGH
CFW455E
15kHz
NO

FILTER SYSTEM


[A] [B]
[C] [DUE] [F][G]


IFILTEfl SWITCH]

2nd IF Amp.o--

RTTY OPTION
-M-! \ '" T\ ?H,L_\..J.J


e'^
FILTER REVERSE SWITCHES
The FILTER SWITCH on the front panel of the IC-751 A selects
between two receive filter systems for the SSB, CW, RTTY or AM
mode when it is switched between the IN and OUT positions. The
IN/OUT relationship of the FILTER SWITCH may be reversed by
using the internal FILTER REVERSE SWITCHES.


-44-


.
E

13 J13
NORMAL
NORMALS R NC
J J
REVERSE
REVERSE
S3 S4

(1) FL-52A, FL-53A INSTALLATION
S3: Reverses the IN and OUT positions of the FILTER SWITCH when using the AM
mode.

S4: Reverses the IN and OUT positions of the FILTER^SWITCH
when using the CW or RTTY mode.

These are 455kHz filters for use with the CW NARROW or RTTY
NARROW mode.

 

 

 

(2) FL-63A INSTALLATION

(3) FL-70 INSTALLATION

No special tools are required to successfully complete the installation
of these filters. Install them at position [F] in the photo.

1) First, remove the top cover.

2) Insert the optional FL-52A or FL-53A into position [F] as shown in the
photo.

3) Check the operation of the filter using the FILTER COMBINATIONS TABLE
for a guide. If necessary, refer to SECTION 6 - 4
CW OPERATION for instructions on how to select the CW
NARROW and RTTY NARROW modes.

This is a 9MHz narrow filter for the CW or RTTY mode. The filter
replaces the standard FL-32A which is supplied with the IC-751A.
Install this filter at [A] position.

1) Remove the top cover, then the 11 screws from the MAIN unit.

2) Lift the right edge of the MAIN unit upwards taking care not to damage
the sockets and plugs that are installed on the unit.

3) The filter must be installed at position [A] in the photo, therefore
remove the FL-32A original filter from the MAIN unit using
de-soldering braid.

4) Install the FL-63A at position [A]. Orient the label on the filter in
the same manner as the other filters already installed.

5) Bend the leads and mounting tabs flush against the opposite side of the
printed circuit board, and solder. Trim the ends of the
leads with diagonal cutters.

6) Replace the MAIN unit and screws, and the top cover.

There are no adjustments required after installation is completed.

This is a 9MHz wide filter for SSB which may be installed in one of
two ways:
a) To replace the standard FL-80 with the FL-70.

b) To replace the standard FL-32A with the FL-70.

a: Use the FL-63A installation method given previously. Place the
FL-70 in the position where the FL-80 is now installed.

b: 1) Use the FL-63A installation method given previously. Place
the FL-70 in the position where the FL-32A is now installed.

2) The FL-70 operates only on CW or RTTY if it is installed in
the FL-32A location.

Therefore, perform the following modification so the FL-70
may or may not be used as desired.


45

.
3) W98 and W99 jumper wires on the MAIN unit must be changed as shown in
the diagram on page 45.

4) The FL-70 and the CFJ455K5 can now be selected by pushing the FILTER
SWITCH on the front panel IN. The receive
bandwidth will be 2.8kHz.

(4) FL-33 INSTALLATION

The FL-80 and FL-44A can be selected by placing the
FILTER SWITCH at the OUT position. The bandwidth will
then be 2.3kHz.

This is a 9MHz filter for the AM mode. The filter replaces the
FL-32A and requires the relocation of jumper wires.

1) Use the FL-63A installation method given previously. Install where the
FL-32A is now installed.

2) Change the jumper wires as shown in the diagram.

3) This installation causes the FL-33 to be selected whether the FILTER
SWITCH is at the IN or OUT position when using the
AM mode. Only 455kHz filters wilt be changed by the FILTER
SWITCH.


10-2 IC-PS35 INTERNAL POWER SUPPLY

WARNING: Disconnect the power cable from the IC-751 A
before performing any work.


INSTALLATION

1) Turn the transceiver upside down. Remove PLATE (A) attached to the rear
panel by unscrewing the four screws. These screws
will be used later.

2) Attach the IC-PS35 to the bottom cover with the supplied screws and
insulating washers. Also insert the insulating gasket between
the IC-PS35 and the bottom cover.

3) Pass the DC power cable attached to P1 through the hole of the AC power
socket plate as shown in the illustration, then insert
the bushing into the hole.

Attach the AC power socket plate to the PLATE (A) position
using the screws which previously held PLATE (A). The AC
power socket should be towards the bottom of the transceiver.

4) Pass the P2' connector from the power socket unit to the inner chassis
through the hole in the rear chassis. Connect this P2'
connector with the P2 connector from the IC-PS35.

5) Position the cables in the rear chassis. This prevents magnetic coupling
between the cables and the VCO coil cores.

6) Replace the top and bottom covers of the transceiver. Plug P1 from the
IC-PS35 into the DC Power Socket on the transceiver.

7) Connect the supplied AC power cable into the newly installed AC power
socket on the rear panel of the IC-751 A. Connect
the AC power plug into an AC power outlet.

8) Push the IC-751A POWER SWITCH to apply power to the transceiver.


-46-


.
10-3 IC-EX310 VOICE SYNTHESIZER UNIT

INSTALLATION
After being installed, the voice synthesizer announces the displayed
frequency when the SPEECH SWITCH on the front panel is pushed.

1) Turn the transceiver upside down.

2) Install the IC-EX310 unit as shown in the photo using the four supplied
screws. Insert the 2-pin plug shown in the photo into
J2of the IC-EX310unit.

3) Plug the 8-pin plug from the IC-EX310 into J12 on the LOGIC unit.

4) Adjust the volume and speech speed if necessary. The procedure is
described below.

5) Replace the top and bottom covers on the IC-751 A.

ADJUSTMENT
1) Adjust the speech volume and speech speed, if necessary, before the top
and bottom covers are replaced.

2) Connect a power source to the transceiver and turn ON the POWER SWITCH.
Push the SPEECH SWITCH on the transceiver
to have the displayed frequency announced in English.

3) The volume of the announcement is adjustable with R16 on the voice
synthesizer. Adjust R16 for a comfortable audio level.

4) The W1 jumper wire controls the speech speed. Cut W1 to increase the
speech speed.

5) Replace the top and bottom covers of the transceiver when adjustments
are completed.


10-4 CR-64 HIGH-STABILITY CRYSTAL UNIT

INSTALLATION

1) Turn the transceiver upside down.

2) Unscrew the six screws retaining the PLL board. Unplug the connectors
indicated in the photo, then turn the board over to
view the printed circuit side.

3) Using de-soldering braid, remove the solder from the original crystal
unit terminals and grounding lead. Remove the original
crystal unit and grounding lead from the board.

4) The mounting location for the high-stability crystal unit is shown in
the photo. The holes for the unit terminals are predrilled. If the holes are
filled with solder, remove the solder using desoldering
braid.


47


.
5) Orient the unit so that the crystal and heater terminals are
inserted into the correct holes as indicated in the photo. The
terminals are labelled on the bottom of the unit.

Position the crystal unit flush with the board, bend the leads
against the foil on the board and solder.

6) Trim the terminals even with the solder points.

7) Replace the PLL board and re-install the connectors. Replace the top and
bottom covers.


10-5 UT-30 PROGRAMMABLE ENCODER UNIT

INSTALLATION

1) Remove the top cover.

2) Install the unit where IC3 is located using the supplied double-sided
tape.

Orient the unit as shown in the diagram.

3) Plug the 3-pin connector into J6 on the MAIN unit.

4) Program the unit for the frequency required using the programming chart.
The UT-30 is factory programmed for 88.5Hz.

5) Replace the transceiver cover.

Refer to SECTION 7 - 6 FM REPEATER OPERATION for instructions
regarding operation.

NOTE: Install and solder a jumper wire in each position indicated
by "I" in the table below.

SUBAUDIBLE TONE ENCODER PROGRAMMING CHART
FREQUENCY P1 P2 P3 P4 P5 P6 FREQUENCY P1 P2 P3 P4 P5 P6 67.0 131.8
171.9136.5174.4141.3177.01146.2179.71151.482.51156.785.41162.288.5167.991.5
173.8194.8179.9197.4186.21100.01192.81103.51203.5107.21210.7110.91218.1114.
81225.7118.81233.61123.0 1241.81127.31250.311

 

 

Above unit is programmed
for an 88.5Hz tone at the
factory.

-48


.
SECTION 11 CIRCUIT DESCRIPTION

11 -1 RECEIVE CIRCUITS
(1) RF CIRCUITS
RF UNIT

The receive signal from the antenna connector is fed to J10 of the
RF unit in the receiver circuit when D4 is turned OFF and RL1 on
the CONNECTOR unit is turned ON.

(2) IF CIRCUITS RF UNIT
The incoming signal to the RF unit passes through an "I" attenuator,
consisting of R92 and R93 on the RF unit, for 20dB of attenuation
when the PREAMP/ATT SWITCH is set to the ATT position.

The BPF switching voltage is obtained with IC1 and IC2 by decoding
the B1 - B11 band signals from the LOGIC unit.

The ON/OFF switching voltage is provided by IC2. R13V is provided
from the OR gate consisting of D5 and D6. The attack time of
this control voltage is determined by R9 and R10. The immediate
release is provided by D1 and D2 when switching to the transmit
mode. . . . . _.

Q9 and Q10 comprise a double-balanced mixer using low-noise
2SK125 J-FETs. The mixer, driven with 13.8V, provides an
excellent noise figure and converts the incoming signal to the
70.4515MHz first IF signal.

The first LO output signal from the PLL unit is fed through a
high-pass filter, is amplified by Q2, is filtered by a low-pass filter,
and then is applied to the first mixer as its local oscillator signal
(70.5515 - 100.4515MHz). R18, L13 and C14 are for feedback to
improve the frequency characteristics of Q2.

The first IF signal is filtered by the FI1 monolithic crystal filter
(7.5kHz/-3dB) and is then amplified by the Q8 dual-gate FET.
The second gate of Q8 is controlled by the AGC voltage.

The signal is fed through the D19 T/R switching diode and a
high-pass filter to the second IF mixer of the IC3 double-balanced
mixer where the signal is converted to the 9.0115MHz second IF
signal. The signal is then filtered by a low-pass filter to remove the
local oscillator components and is fed to the MAIN unit through
P3.

MAIN UNIT
The second LO signal (61.44MHz) from the PLL unit is fed to IC3
as the local oscillator signal for the second mixer.

The IF signal from the RF unit passes through a noise blanker circuit,
and is supplied to a 9MHz filter through the Q33 amplifier. The
signals always are passed through the noise blanker which is controlled
by a noise blanker gate consisting of D68 - D71 called a
double-balanced diode switch. If the noise blanker detects pulse
noise, the blanker puts out a control voltage via D72 which momentarily
cuts off the signal.

The 9MHz filter circuit consists of FI2, FI8 and a bypass circuit.
The signal from the filter circuit is supplied to the IC10 mixer
through the Q84 buffer amplifier, and is converted to a 455kHz
signal. The signal is then fed through Q39 to a 455kHz filter circuit
consisting of FI3, FI4, FI5, FI6 and an optional filter.

49


.
The output from the filter is converted to a 9MHz signal again by
the IC11 mixer through the Q42 buffer amplifier. FM signals from
the FI6 filter are fed to a special FM circuit. The 9MHz signal is
supplied to an amplifier circuit consisting of Q43 and Q44, then is
supplied to the AGC or the DETECTOR circuit. Finally, the signal
is fed to the AF amplifier circuit.

FM signals from FI6 are supplied to amplifier and limiter circuits
consisting of IC12, Q45 and IC13. The limiter circuit eliminates
noise or AM components, then the signal is supplied to a frequency
discriminator and is detected. The detected signal passes through a
C-R network for de-emphasis and is supplied to the AF amplifier
circuit.

The AF amplifier circuit consists of a pre-amplifier and an AF power
amplifier which amplifies the signal to a sufficient level to drive a speaker.

A switching circuit consisting of Q74 and Q75 Is installed at both
the input and output of the pre-amplifier circuit in order to cut off
the AF signal by means of a squelch signal. The output from the
pre-amplifier is also supplied to the ACC socket on the rear panel.

The AF signal from the detecter circuit is amplified by IC19 (b).
Then it is fed to the IC19 (a) pre-amplifier through the TONE
CONTROL circuit consisting of R394, C258 and the TONE CONTROL
on the front panel. The signal is supplied to the IC18 AF
power amplifier through the AF GAIN CONTROL on the front
panel. The AF power amplifier has a 2.6W output capability under
an 8 ohm load with 10% distortion.


11-2 TRANSMIT CIRCUITS
(1) AF CIRCUITS
MAIN UNIT

The AF signal from the mic connector is fed through the MIC GAIN
CONTROL on the front panel to Q34 and Q35 where the signal is
amplified. The tone control circuit is installed between Q34 and
Q35, and varies the frequency response of the mic amplifier. The
signal is also supplied to the ACC socket through R148 and C83.

IC9 is a double-balanced mixer which generates the SSB and AM
signals. IC9 puts out DSB signals formed by the AF signal mixing
with the BFO signal. In the AM mode, an offset voltage is fed to the
modulating input terminal of IC9 through R159 and D75 so the
carrier level may be controlled by simply adjusting the offset voltage.


RF COMPRESSOR
The DSB or AM signal from IC9 is fed to the 9MHz filter and the
filter eliminates one sideband resulting in an SSB signal. The signals
are fed through Q84 to the IC10 mixer. IC10 puts out a 455kHz
signal which is fed to the Q38 matching amplifier through the
Speech Compressor circuit when the circuit is turned ON. Then the
signal is converted to 9MHz again, and is fed to the transmitter IF
amplifier by Q1.

RF UNIT
The 9.0115MHz IF signal from the MAIN unit passes through the
LPF and is converted to 70.4515MHz by the IC3 double-balanced
mixer. The HPF eliminates the 9.0115MHz signal component from
the mixer output signal, and the L30 and C67 resonator eliminate
the 61.44MHz 2nd LO signal component. The signal is amplified by
the Q11 dual-gate FET which receives an ALC voltage at its 1st gate.


50


.
The amplified signal is fed through a double-tuned filter composed
of L32, C77 and L33 to eliminate spurious components, and then
fed to the Q12 and Q13 transmit mixer. The frequency of the local
oscillator ranges from 70.5515 - 100.4515MHz which converts the
IF signal to the desired frequency of 0.1 - 30MHz.

In the transmit mode, D45 is turned ON and the signal is amplified
about 20dB by the Q14 wideband amplifier and is output to the PA
unit through J8. C171 and R86 are provided to taylor the frequency
characteristics.


(2) PA CIRCUITS
The RF signal input from the RF unit through P1 is amplified by
the Q1 class A amplifier. The output from Q1 is converted to a
balanced output by L1 and is amplified by the Q2 and Q3 class AB
push-pull amplifier. The negative feedback circuits inserted between
the collector and the 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 set value from deviating due to variations of D1
characteristics.

The output of Q2 and Q3 is fed to the L4 impedance matching
section and is amplified by the Q4 and Q5 class AB push-pull amplifier
to provide 100 watts output power.


11 -3 PLL CIRCUITS
The PLL unit outputs two oscillator signals for the RF unit, i.e., the
variable first local oscillator output (1st LO) of 70.55 - 100.45MHz
necessary for the first mixer, and the fixed local oscillator output
(2nd LO) of 61.44MHz necessary for the second mixer. In addition,
the marker signal is also generated in this unit and is sent to the RF unit.


PLL IC CHIP
IC1 (M54929P) is a multi-function IC containing a phase comparator,
a programmable divider, a reference frequency oscillator circuit, a
divider and a swallow counter controller. By using this IC with
IC2 (M54466L, a swallow counter), it can perform pulse swallow dividing.


-51


.
VCO CIRCUIT

LOOP SYSTEM
The performance of the VCO is very important for PLL operation.
In order to obtain a high carrier-to-noise (C/N) ratio and a stable
oscillator output, four separate VCOs are used. Each is assigned a
quarter of the total necessary bandwidth.

The power supply for the VCOs is doubly regulated when compared
with the reference frequency oscillator. Furthermore, coreless coils
are used for the oscillator coils in order to obtain a high Q as well
as immunity from external induction.

The output of the VCO is separated into two parts after passing
through the Q22 buffer amplifier. One part is amplified by Q23,
and after impedance matching by Q25, is output to the RF unit
as the 1st LO. The output level is about 0dBm/50ohms. The
other part is fed back to the PLL loop through the Q26 buffer amplifier.

The VCO signal is then mixed with the in-loop LO by IC3 to be
down-mixed. The output from the mixer is passed through the
bandpass filter with a bandwidth of about 40 to 70MHz to eliminate
the spurious components. The output is then amplified by the Q16
and Q17 cascade amplifier, and input to IC2 to form the PLL.


11-4 LOGIC CIRCUITS
The functions of this unit include the control of frequency, the
processing of BPF, LPF and mode signals, the output of data for the
PLL unit and display unit, etc. This unit includes an 8-bit NMOS
CPU, a 4-bit 1k word CMOS RAM, a multi-purpose custom IC and
I/O expander IC's.


(1) CPU INPUT CONTROL CIRCUIT
A multi-function CMOS IC is used. This custom IC is contained in a
40-pin DIL package.

(a) An external L and C are connected to pins 18 and 19 to give an
approximate clock signal of 100kHz.

(b) ATS from pin 32 is HIGH if the TUNING CONTROL is rotated faster than a
set speed. The set speed is fixed by the values of C6 and R7 connected to
pin 21 (TO). The high level is used as a strobe signal which switches the
dial-pitch (tuning rate) of the matrix input.

(c) M1 and M2 at pins 38 and 37 are used to switch the multiplication
factors of the input pulses from the TUNING CONTROL. 200 pulses per one
rotation are obtained by 50 pulses x 4. For RIT/ATX control, the multiplication
factor is two times to give 50 pulses x 2 = 100 pulses per one rotation.


(2) DISPLAY CIRCUIT
The luminescent display tube (DS1) is driven by the IC1 and IC2
drivers. These ICs contain such functions as input data latches,
clock oscillators, timing counters, segment decoders, etc.
The clock frequency is set by C2 and C6.

Displays for the RIT/ATX shift frequency and memory channel are
driven by IC2, and other displays are driven by IC1.

The signals for the display of "RTTY" through "GENE" are sent
from the LOGIC unit to each segment. The segments are switched
by the digit signals, TO to TO from IC1 and T3 from IC2, and lit
dynamically. The letters "RIT", "ATX", "-" and "DUP" are connected
to the same digit in the tube, thus the necessary word is
selected by T4, T1 and TO digit signals and dynamically lit.

52


.
SECTION 12 MAINTENANCE AND ADJUSTMENT

12-1 CLEANING
After sitting in your ham shack for a period of time, the IC-751A will
eventually require cleaning. Remove the knobs from the front panel and use a
mild, soapy solution sparingly. Do not use strong chemicals or cleaning
solvents. Wipe dry before replacing the knobs on the panel.


12-2 MAINTENANCE

NOTE: Before performing any work on the transceiver, make sure that the power
cable is detached from the transceiver.


(1) PREPARATION
Remove the top and bottom covers by unscrewing the six screws from each cover
and the three screws from each side of the transceiver.


(2) FUSE
Locate the cause for a blown fuse before replacing it and attempting
to operate the IC-751 A again.


Fuse Ratings:
Inside IC-751 A
3A
Supplied DC cable :
20A
Optional IC-PS35 POWER SUPPLY :
10Afor 120V AC
5A for 220 - 240V AC

(3) BACKUP BATTERY
The backup battery location is shown in SECTION 13- 2.
The IC-751 A uses an advanced, highly reliable RAM integrated circuit
which is completely separate from the transceiver's main microprocessor.
The purpose of the battery is to provide power to the RAM so it retains all
memory information during power failures, or when the unit is unplugged or
turned off.

The usual life of the lithium battery exceeds ten years. It is advisable
to monitor the lithium battery carefully and replace it if there are repeated
cases of display malfunction.


NOTE: Battery replacement must be done by an ICOM
Authorized Dealer or ICOM Service Center.


(4) ORDERING PARTS
For the fastest service, supply all of the following information when
ordering parts from your dealer:
Equipment model and serial number
Schematic part identifier (e.g. IC5, Q23)
Printed circuit board number
Part number and name
Quantity required


12-3 ADJUSTMENTS

(1) BRAKE ADJUSTMENT
It is unnecessary to remove the bottom cover for the brake adjustment.

The TUNING CONTROL tension may be adjusted to the operator's preference.
The screw adjustment is located on the bottom side of the transceiver cabinet
below the TUNING CONTROL. The method for adjustment is as follows:

1) Rotate the TUNING CONTROL continuously and smoothly in one direction.

2) Adjust the brake adjustment screw either clockwise for tighter tension,
or counterclockwise for looser tension as desired.


53


.
<2) SIMPLE FREQUENCY ADJUSTMENT
A very accurate frequency counter is required to align the frequency
of the IC-751 A. However, a simple check may be performed by
receiving radio station WWV, WWVH or other standard frequency signals.

1) Select the AM mode and GENERAL COVERAGE mode.

NOTE: This adjustment calibrates the reference oscillator, therefore it is
unnecessary to calibrate on any other frequency.

2) Use the TUNING CONTROL to set the frequency display to the exact
frequency of the standard frequency station (10.000.0MHz
for WWV/WWVH). Check for a strong receiver signal.

3) Turn ON the MARKER SWITCH on the top panel.

4) Adjust the MARKER CALIBRATOR on the top panel for zero beat. Zero beat
exists when two signals are on exactly the same frequency resulting in
no audible tone.

5) Turn OFF the MARKER SWITCH.


(3) ELECTRONIC KEYER WEIGHT CONTROL
Features designed into the custom microprocessor include contact debouncing,
RF immunity, self-completing character generation, dot memory and
weight control.

The keying speed can be varied continuously from 5 to 45 words per minute.

The R8 WEIGHT CONTROL is set fully counterclockwise at the factory which sets
the DOT:SPACE:DASH ratio at 1:1:3. DOTS and DASHES increase in length
if R8 is turned clockwise.

(4) CW SIDETONE FREQUENCY ADJUSTMENT

1) This control is factory set for a 700Hz tone.

2) Connect a CW key to the KEY JACK on the rear panel and select the CW mode.

3) Close the CW key and adjust R374 for a suitable tone. Refer to SECTION
13-2 BOTTOM VIEW for the adjustment location.


(5) SCAN SPEED ADJUSTMENT
1) Activate the PROGRAMMED SCAN. Refer to SECTION 8 for information on the
scanning functions.

2) Adjust R14 for a suitable scanning speed. Refer to SECTION 13-2
BOTTOM VIEW for the adjustment location.


(6) BFO ADJUSTMENT
1) Connect a frequency counter to CP1 on the MAIN unit. Refer to SECTION 13-1
TOP VIEW for the CP1 location.


MODE: RECEIVE FREQUENCY (MHz): TRANSMIT FREQUENCY (MHz): ADJUSTMENT

USB: 9.01300: 9.01300: C202
LSB: 9.01000: 9.01000: L33
CW: 9.00990: NO SIGNAL: L34
RTTY: 9.008475: 9.008475: L35
AM: NO SIGNAL: 9.01000: L33
FM: NO SIGNAL: NO SIGNAL: ---


(7) TRANSMIT AUDIO ADJUSTMENT
The bass and treble response of your transmitted signal may be altered with the
R155 adjustment on the MAIN unit. Refer to SECTION 13-1 TOP VIEW for the
R155 location.


54


.


55


.
13-2 BOTTOM VIEW

figure, not shown.


13-3 RF UNIT

Figure, not shown.


56


.
13-4 ADJUSTMENT LOCATIONS

Loop Filter

 

J6 (Connector for UT-30)
R290
(Subaudible Tone --
Deviation Adj.)
C20 (RTTY MARK Freq. Adj.)

L29 (FM Transmit Freq. Adj.l"P155
(Transmit Audio Adj.)
JR159 (AM Carrier Level Adj.)

-R1S8, R161 ISSB Carrier
Suppression Adj.)
R121 (S-Meter Adj.)

 


C18 (RTTY SPACE Freq./170HzAdj.)
C15 (RTTY SPACE Freq./850HzAdj.)
S8 (Power Selection Switch)

BOW
-C202 (USBI
-L3S (RTTY)
-L33 I LSB I
- L34 (CW)

BFO Adj.

 


100W

S1 (RTTY SHIFT Selection Switch) 170Hz ?--850Hz

-S2 (RTTY Polarity Switch)
NORMAL --? REVERSE (MARK : H) (MARK : L)

 

 

Filter Preset Switch
NORMAL
1 REVERSE
-Space for Optional Filters
(FL-52A, FL-53A)
- L36 (PBT Freq. Adj.)
Space for UT-30

 

 


J12 (Connector for IC-EX310,
R14 (Scan Speed Adj.)
--Space for IC-EX309 Interface Unit


J15, J10 (Connectors
for IC-EX309)

Space for IC-EX310 Voice Synthesizer Unit


Scan Resumption Switch
L2 (Reference Osc. Adj.)

 

 

TUNING CONTROL
Brake Adj.


57


.
SECTION 14 BLOCK DIAGRAM

Figure, not shown.


58


.
SECTION 15 OPTIONS

IC-PS30 AC POWER SUPPLY 13.8V, 25A
IC-PS15 AC POWER SUPPLY 13.8V, 20A
IC-PS35 INTERNAL POWER SUPPLY 13.8V, 20A
IC-2KL 500W LINEAR AMPLIFIER
IC-AT500 500W AUTOMATIC ANTENNA TUNER
RC-10 FREQUENCY CONTROLLER
SM-10 COMPRESSOR/GRAPHIC EQUALIZER DESK TOP MICROPHONE
SM-8 DESK MICROPHONE
IC-SP3 EXTERNAL SPEAKER
IC-EX310 VoiCE SYNTHESIZER UNIT
UT-30 PROGRAMMABLE TONE ENCODER UNIT
CR-64 HIGH-STABILITY CRYSTAL UNIT


59


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T

ICOM INCORPORATED
1-6-19, KAMIKURATSUKURI, HIRANO-KU,
OSAKA 547, JAPAN

A-0762
Printed in Japan



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