IC-756 Notes

Last Updated 31 May 1997

A much nicer photo of the IC-756

The IC-756 certainly looks impressive. Unfortunately, in this case, looks are deceiving.

  • Part 1: Initial Impressions by K6XX
  • Part 2: Performance Measurement by W2VJN
  • Part 3: Initial Impressions by W6CYX

    Initial Impressions of the IC-756

    de K6XX

    10 March 1997. Updated 31 May 1997

    Regarding the IC-756 MF/HF/6m rig from Icom--the one with the front panel LCD spectrum analyzer. After a short two weeks of playing, here are my opinions, which are undeniably worth every penny you spent on them.

    The '756 is positioned differently in its domestic market (Japan) than it is upon export. In the United States, it is priced as a high-medium range rig; in Japan, it is a central-medium end radio. [And it is presently hundreds of dollars cheaper in Japan!]

    Initial Notes:


    1. Nice ergonomics; this is a good "two-handed" design--the controls are separated properly so your hands can operate it without fumbling over themselves.

    2. Dual SO-239 connectors plus a separate receive-only antenna jack, and the configuration(s) may be memorized by-band.

    3. Big, clean, clear display.

    4. Transmit ALC works well, even with "fussy" amplifiers.

    5. Tuner matching range is better than its specifications.


    1. The sales literature (in Japanese, at least) boasts a 105 dB receiver dynamic range. Yet, AM broadcast band interference destroyed 160m reception until a DuneStar bandpass filter was inserted.

    2. Noise blanker threshold is non-adjustable.

    3. DSP is not in the AGC loop.

    4. CW keying is choppy (see the note, below).

    5. Printed schematic has errors (for example, where is the transmit control voltage generated?).

    Suggested Improvements

    1. Noise blanker threshold adjustment

    2. CW Keying improvement (see the note, below)

    3. A higher voltage T/R relay, or an output voltage that corresponds with the transmitter timing, for driving an external transistor switch.

    4. Microphone Compression metering option

    5. CW sidetone monitor should have a menu option that allows either (A) sidetone always active (default), or (B) sidetone controlled by the MON switch (for dual rig contesting use).

    6. The monitor output is too low, even at maximum, for useful compression/mic gain adjustment.


    The antenna tuner is active in receive mode. This adds some selectivity (but not enough to solve the AM broadcast blast-through on 160m described earlier). And this idiosyncrasy can be confusing; I spent a few minutes debugging a dead receiver on 40m after operating on 6m--the antenna tuner was ON and still tuned to 50MHz, severely attenuating 7MHz signals.

    CW keyed elements are shortened, making the keying "choppy". In "Semi-Break-in" mode, each keyed element is shortened by 5ms. A 0.33µF capacitor across the rear panel key jack fixes it. For QSK, the error is about 10ms (keying sounds terrible above 25WPM), and 0.68µF is needed. These fixes correct the timing for an external keyer or computer logging/keying system, but do not help the internal keyer's timing.

    FM modulation is controlled by the MIC GAIN control on the front panel. With it fully counterclockwise, full power is transmitted, but no modulation. It does not seem to over-deviate with the GAIN fully clockwise, however. On my unit, the PL tone is excessively loud. The supplied operator's manual does not specify which internal control adjusts PL amplitude, and the service manual is not yet available.

    I have just completed an interface box that includes a band output port and a CI-V to computer serial interface. The band output port drives things like relay-switched DuneStar band filters (Model 600, for example), remote coax switches, etc., and does not require a computer. The CI-V interface allows rig control via computer, and makes programs like CT and TR Log really shine.

    The component cost of the full interface is relatively low: well under $20 at Digikey prices. I had all the parts in my junk box. You can probably build it for between $0 and $10 (but then, there is that $25 minimum order problem)

    If you are interested, the design is on this web site at K6XX's Band & CI-V Interface for Icom Rigs

    The IC-756 was the second rig [temporarily] at K6XX

    ADDENDUM: 31 May '97

    All of the above remains true after two months of operation, including some contest work. The flaws in the 756 were fatal, and I sold the radio. The radio is simply grossly overpriced for its performance. Its unforgivable sins are:

  • Receiver dynamic range does not meet specification (my neighbors BLOW AWAY the front end)
  • The noise blanker is not adjustable, and is nearly always completely ineffective
  • The DSP is only adequate, and although advertised as "IF DSP", since it is outside the AGC loop, it might just as well be in the audio chain.
  • The speech processor is merely a simple compressor, and adds too little "punch" to the signal.

    73 & GL de Bob, K6XX


    Posted by Permission from George, W2VJN.

    From: w2vjn@rosenet.net (George Cutsogeorge)

    To: cq-contest@contesting.com (CQ CONTEST)

    Date: Thu, 13 Mar 97 10:00:24 GMT

    Subject: IC-756 Tests

    Received from the Contest Reflector 13 March 1997

    IC-756 S/N 01020 3/8/97
    K6AW and W6OTC were nice enough to lend me an IC-756 to run some lab tests on.  
    K6AW has covered the operator impressions and features very well in a prior 
    note, so my intent here is to provide some performance numbers.  Where 
    possible, performance numbers of an IC-765 have been included.  You may note 
    that some performance numbers usually given are absent.  This is due to the 
    lack of certain types of test equipment such as a spectrum analyzer in my home 
    Conditions: stock SSB filters, CW-optional 500 Hz@9 MHz and stock SSB@455 IF.
                (Numbers in parentheses are for a reference IC-765 S/N 4616)    
    SENSITIVITY-    SSB Noise Floor = -130 dBm, Preamp OFF  (-128)
                                    = -135 dBm, Preamp 1 IN (-134)
                                    = -137 dBm, Preamp 2 IN
                    CW Noise Floor = -134 dBm, Preamp OFF   (-133)
                                   = -139 dBm, Preamp 1 IN  (-140)
                                   = -141 dBm, Preamp 2 IN
    SELECTIVITY-    Small signal, AGC off, SSB = 1900 Hz @ -3 dB    (1020 Hz)
                    "      "             CW  = 480 Hz             (330 Hz)
                    Large signal, AGC on,  SSB = 2570 Hz            (2260 Hz)
                    "        "              CW = 1070 Hz            (680 Hz)
    BLOCKING-   Noise limited for both radios within a few kHz of the desired 
                signal and not enough generator output to measure at 500 kHz 
                offset.  Both radios are quite good.
        Additional conditions: The third order, blocking and reciprocal mixing 
        tests were performed with a home brewed two tone generator using low noise 
        FET crystal oscillators.
        Tone spacing = 100 kHz;
            Intermod products at the noise floor for two -32.5 dBm tones.   (-29.5)
            Third order intercept = +18.2 dBm   (+22.7)
            Dynamic range = 101.5 dB            (103.5)
    AGC - Extremely flat from minimum to maximum signal .
           Rise       = 1 - 3 ms, Fast to Slow.         (3 ms.)
           Undershoot = 20%, all speeds.                (none)
           Overshoot  = 20%, all speeds.  Small signal to large.(7% Fast, 17% Slow)
           Recovery   = 100 ms Fast, 400 ms Med, 1.4 sec Slow (30ms Fast, 0.6s Slow)
    RECIPROCAL MIXING -phase noise
        Measured with the radio offset from a crystal oscillator by the amount 
        indicated and adjusting the input level to raise the noise floor 3 dB.
                Offset         IC-756              IC-765
                3 kHz       -56 dBm, S9+16      -44 dBm, S9+38
                5 kHz       -48                 -37
                10 kHz      -39                 -30
                15 kHz      -34                 -26
                20 kHz      -31                 -23
                50 kHz      -22,  S9+52         -16,  S9+65
        S meter readings above are for the indicated tone level on the respective 
    POWER OUTPUT-power supply set to 13.8 VDC
                Band        Tuner off       Tuner on
                1.8 MHz      98 watts       90 watts
                3.5         100             93
                7.0         101             95
                10          102             95
                14          102             93
                18          103             95  
                21          104             92
                24          103             91
                28          103             87  
                50           85             77
    KEYING - tested at 40 meters, no frequency effects noted.
        Rise time = 0.7 ms for the first dot and 1.0 ms thereafter
        Fall time = 3 ms down to approximately 3% , then drops rapidly to zero.
        Delay to RF out, full break in = 6 ms to a 3% pedestal, 16 ms to start of 
        Delay to RF out, semi break in = first dot as above, then 6 ms to a 3% 
        pedestal, 10 ms to start of rise.  
        Note: the pedestal level stays constant as the power output is reduced.
        Key up to RF off = 0, starts fall time immediately.
        There is a pronounced dip or undershoot in power output approximately 75 ms 
        after key down to about 60% of full power at 100 watts and substantially 
        lower for lower power outputs.  The good news is, it is virtually 
        unnoticeable over the air.
        The keying is shortened as the delay to the rise and from  the fall are not 
        the same.  It is quite noticeably light.  At 40 wpm the duty cycle of dots 
        is 40%.  The so called "weight control" for the built in keyer only adjusts 
        dash length, not both dots and dashes as it should. The side tone is a true 
        representation of the key down periods.  There are some key clicks in the 
        side tone (not terrible) which are  more noticeable during full break in 
    SPEECH PROCESSING--average power increase due to processing.
        Peak power output is 100 watts.  White random noise is put into the mike 
        input and the level is adjusted for an ALC reading according to the 
        instruction manual.
        Average power output, no processing   = 17 watts    (17 watts)
        Average power output, with processing = 23 watts    (42 watts)
        Average power increase                = 1.3 dB      (3.9 dB)

    COMMENTS-- Good Features.

        1. Sensitivity is excellent and compares to the best radios.
        2. Selectivity is good if cascaded filters are used.
        3. AGC performance is excellent.
        4. Dynamic range is very good.

    COMMENTS-- Could use improvement.

        1. Phase noise performance is  not as good as it could be.
        2. CW keying needs improvement.  Side tone clicks should be eliminated.
        3. The audio compressor is not adequate for competition.
        4. The AGC is too flat.  A few dB of tilt helps when picking signals out of 
           a pile up.

    This radio comes very close to being a competitive contest radio, but in my opinion it doesn't quite make it. An RF speech processor, improved keying and better close in phase noise performance would make the difference. There are a lot of us out here that would pay a bit more to get that kind of performance.

    George Cutsogeorge, W2VJN@rosenet.net

    Umpqua, Oregon


    IC-756 Evaluation by W6CYX

    IC-756 tested had 500Hz 9MHz CW filter, but no 500Hz 455kHz CW filter.

    I. Unacceptable Items

    1. CW keying is very choppy in both semi & QSK, especially above 30 WPM.
    2. 16V DC maximum rating on amplifier T/R relay is completely unacceptable.

    II. Overall Listening Impression

      Audio quality on CW & SSB seems muffled. It lacks clarity and presence. Could it be that Icom installed bypass capacitors to ground in the audio stages to cover up the white noise instead of really solving the problem? Receiver seems to be noisy although it can hear the weak signals. For example, to get the same S-meter readings as the TS870S on 20m, you need to use Preamp #1, But when you take out the preamp, it sounds more like the TS870S.

    III. Minor Complaints

    1. Why can't paddle be plugged into the rear panel key jack instead of only the front panel?
    2. Noise blanker is completely ineffective on AM. Poor overall noise blanker performance. No level control.
    3. AGC loop should not be outside of the DSP filter loop. Weak signals near strong signals are completely lost due to AGC pumping. (Having the 455kHz CW filter would help).
    4. Even in its fastest position, the AGC is too slow to suit my taste.
    5. If find the LCD & panoramic frequency display to be more of a gimmick than useful. I miss having a "real" display.
    6. The manual is totally vacuous as to theory of operation or circuit descriptions.
    7. Limited choice of filter selection on menu--e.g., impossible to receive SSB with both wide filters.

    IV. Good Things

    1. I like the GMT clock on the display.