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Thread: Icom IC-756Pro

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  1. #1
    Join Date
    Jan 2002
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    Default Icom IC-756Pro

    Got to spend some time with a pro and would like to comment on it.

    In the past I've had many different Icom rigs, from 735 to 765, with stints running 738s and 746s and 746pros, and have had use of many others over the years. However my main rig was a 706, not known for strong signal handling ability or rx audio quality. I also had a Icom R71A rx rig alongside the 706, with the R71 being a bit better in most respects to the 706 as far as rx performance goes. I have a stereo speaker on the floor in use for the hf rigs as believe it or not I try to enjoy program content as well as music over hf - a good speaker brings out the best in a rig. So there's some background on recent ham rig employment at casa del n0sya.

    I've been doing some research on the various popularly priced used hf rigs as a replacement for the 706 (the fan is always on and I hate fan noise), and as I have a small stockpile of Icom xtal filters I decided a 756 original would do well, and allow me to put those xtal filters to good use. The 756 also has that lovely screen, and for those who have never had a tv (specan) in their radio, it's a wonderfull thing to have. Why buy a (used) radio that doesn't have a tv in it when you can for the same price as one that does? Then I noticed in reviews of the rig that all original 756 non pro screens seem to fail, and at $400 or so for the replacement screen I decided that was out. 756 originals often sell for $700, so add in the cost of the eventual new screen @ $400 and other options I feel are worthy (tcxo) and you could buy a proiii for as much.

    So I next investigated the 756pro series. I saw many pros going for $1100 or so online, with the proii going for 1300 or so, and the proiii going for around 1500. I looked at the qst reviews on each and decided that for the money the proii seemd to offer the best bang for the buck, low dollar and better than a pro. There seems to be much more similarity between the proii and the proiii than the pro and proii - as far as sensitivity, dynamic range, and other criteria. Another issue is am demodulation. I am a ambc and swbc listener aboutr as much as I'm a ham. Diode rectified am is cheap and works, but it's not the best. The software am demod in the pro series is pretty much the best I've used for a non synchro detector. If you feed it a nice sig it produces nice audio. even reminds me of the clarity and range of my old Halli tube rx, with much better highs than my Drake R8, the pro reminds me of a JRC 535 or 545 in audio quality, something I have always been looking for.

    What the pro offers standard that was optional or unavailable on the 756 original model is full dsp demodulation, a much more refined and usefull specan, a xtal oven reference oscillator, and a full color screen that seems to be much more reliable and legible than that of the original 756. Again, my heart was set on a proii and as much as I tried, each proii was sold before I could respond to the ad. A pro model became available suddenly and I decided I would look the specs over betwixt the various pros and decide if I could live with a pro rather than a proii. I found that some of the specs of the pro were better than on the proii, such as blocking and even 5KHz toip dynamic range. It also offered less odd order tx distortion at the 5th order point. All in all the pro series is within 5dB or less of each other in each spec. So a simple pro's not too shabby, and decades more advanced than what I had been using to date.

    The pro offered me looks like it just came out of the box, and these things have been around since what, 2002? Anyway the pro came home with me and replaced the 706 and R71 on the desk. The learning curve, wich is very intuitive having learned the ways of Icom with various other Icom rigs, is short save for the most cryptic and least often needed adjustments. A freak about stability, I checked u/lsb on 20MHz wwv after many hours of operating and adjusted the carrier through the wonderfully convenient case opening on the side of the rig, and the pro is now within less than a Hertz of wwv. And it stays there.

    So far as to performance, I am very impressed. It reminds me of the 765 in performance, but with that stellar tv screen. I have to say that I feel the audio reproduction is outstanding if not spectacular. An aquantance who doesn't share my love for listening to static (or understanding it) immediately noted how much better the pro sounded than the others. High praise indeed. And that screen! You can easily see distortion on signals, watch as those ufos wizz by, even lightning crashes are fascinating. I never thought a tv in a rig could be so usefull, but now I am spoiled and can never look back. All that being said, yes I'd take a proiii if it was within means, but I do not feel cheated being "stuck" with a pro.

  2. #2
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    A word on pro dsp noise reduction and filters, or lack thereof.

    The pro series employs ceramic filters in the IF just before the mixer to the final IF adc. These filters are standard MuRata plastic encased bin parts and likely have poor group delay compared to xtal or mechanical filters. Group delay is something that is very bad for things digital, like adc's and dsp's. The passband in the middle of the filter's response has little to no group delay while the edges or skirts have increasingly worse as they drop into the passband abyss. In the analog world this bears little of consequence, in the digital it is more or less of horrific import. Replacing these filters with flat (or flatter) group delay filters of similar or narrower actual bandwidth may make for a slightly better rig, this will need to be tested. One of the MuRata filters is some 15KHz wide that is employed for AM and FM modes alone, with residual filtering in each mode performed in software. The ssb/rtty/cw modes are filtered by a 4KHz wide ceramic filter. This series of filters seem to blow out around 25 to 35dB down, not a good filter really, but far better than none at all. I just happen to have some MuRata filters of similar or slightly narrower bandwidths that were made with reduced group delay in mind, and you know where I am going with that. This is the one point where Icom has allowed me to satisfy my "I can make it better" itch.....

    All in all I am very satisfied with the filter performance of the pro, they are quite steep; once a signal is off the passband edge you no longer hear it. I set the pro on ssb as 3.3KHz for a wide, 2.8 for medium, and 2.1 in honor of all those Collins rigs with 2.1 mechanical filters. I like wider response, and find on a low noise band that the 3.3 effective dsp filter sounds lovely. Dsp filters are known for high quality sound results, and they often sound a good deal wider than an analog of similar bandwidth as the dsp filters are almost straight down and have no group delay at all, whereas the analog filters have to have just so much rolloff at the edges as it distorts the signal and adds more and more group delay, wich is also a form of distortion. I often crank up the dsp nr to allow me to use the 3.3 rather than go to a narrower bandwidth without dsp or nb.

    On am I employ the standard 9Khz, 6KHz, and 3KHz filters just as the rig came from Icom. 9KHz width is awesome for ambc listening as ambc stas are either 9KHz or 10KHz apart, and for any swbc sta that is in the open, 9KHz offers just lovely audio. The 6KHz is perfect for swbc and any other slightly narrow need for am demod, swbc stas are packed 5KHz apart, so the 9KHz will let in a good deal of 5KHz heterodyne at times - hit the filter switch to drop from the 9KHz filter to the 6Khz and there goes the 5KHz heterodynes. The 3KHz is perfect for when the am sig is really being encumbered by other stas or noise. And something I noticed is the 3KHz filter really is 3KHz wide, but sounds so much better than a analog 3KHz filter on the same am signal it kinda amazes me.

    For cw and narrow rtty use I employ a wide of 500Hz, medium of 250Hz, and narrow of 50Hz. All with the bpf icon enabled wich does wonders for eliminating key clicks and other odd "dsp artifact" noises alleged to the pro series. Wonderfull filters to satisfy my non directional beacon lust.

    Dsp Noise Reduction with the pro is very nice, some of the best I have used so far. My first foray into dsp was with the dsp40 from rat shack. You may remember that one, it was just a passband filter and het filter, no noise reduction other than via reduced bandwidth. A neat toy for lack of better. My next experience was with the TS 870 by Kenwood, a very nice rig as far as I am concerned, wonderfull once set up with proper agc decay and filter bandwidth. I have to say that I was not impressed with the 870s dsp noise reduction, it did about as much apparent signal reduction as it did noise, but at low levels and with low noises to account for, the 870 dsp nr was fine. All I am looking for in dsp nr is to get rid of the slight and always present white noise on hf, and when nr is set to low values the pro dsp nr does just that, and does it very well without degrading the desired signal. That is very important; one of the three laws of robotics err dsp algos: do not degrade the desired signal. And with that, running the pro's dsp nr to full clockwise does not seem to result in dsp wraparound, where the dsp nr algo is attacking the desired signal as much as it is the undesired. In most cases where with other rigs I know I would have just given up trying to make the desired signal worth listening to, the pro hasn't let me down. I havenj't had it long but lately we've had horrid conditions on hf so it does get a daily workout of all features in the attempt at hearing and understanding the desired signal. I have also used a Heil audio frequency dsp box and it did very well on low level white noise - better than the 870 nr, I consider the Heil unit one of the better stand alone dsps, but it didn't take to much knob twisting to make it wraparound.

    The one glaring fault I find in the pro so far is the analog noise blanker. In a word, useless. No, worse than useless because so far in most every case, sure it gets rid of poweline noise of white noise but it also distorts the desired signal almost every time. My 706 was really bad in that respect but the pro has the 706 beat by a mile. I am thinking the internal nb adjustment of thr pro is far too agressive, reducing it may make for an actually useable nb. And that bears testing, of course.

  3. #3
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    A neat test for showing off the power of dsp rigs:
    http://www.ab4oj.com/icom/w8lx.html
    And a vid of the test:
    http://www.microhams.com/binfile/talks/WWV-low.wmv

    After reading about this "test" I tried it, noticing that the 756pro has much better audio than the rig in the linked video.

  4. #4

    Default

    The Icom 756Pro offered me looks like it just came out of the box, and these things have been around since what, 2002?
    First shown in Tokyo, late 1999 ... Introduced to N. America market in January 2000.
    Received mine from AES in March 2000, and promptly used or ARRL & CQ SSB contests.

    Recently sold to friend, upgrading from earlier Icom IC-745 model.
    Nullius in verba

  5. #5
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    If you have a pro you might want to do some fairly simple mods to get rid of the nb circuit artifacts backwashing into the 8v feed to the rest of the 756. The mods consist of replacing/adding a cap or two to filter out the nb noise from the rail as discussed here:
    http://www.n1eq.com/tech/page_06.html
    http://www.ab4oj.com/icom/756pro_nb/main.html

    I never noticed these noises untill I did some in depth testing with some very sensitive headphones and now I can't help but notice them. As soon as I get some smt caps the fix will be in. Viva la 756 pro!

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