IC756 PRO page
Last update: 03/08/2005 16:31:31 -0700
Background.
I received my 1st 756PRO on 1 February 2000. I'd used an
IC756 for a
year in DXing and contests, all modes. I've been quite happy
with the
IC756
and the only real problems I had was the lack of multiple filters -- I
simply needed tighter filters for SSB and PSK -- and the fact that the
DSP
was after the AGC loop making it less than useful in many cases.
The 756 PRO seems to solve these problems and contest work confirms
this. I like
the rig so much that I installed a second PRO in September 2000.
Overall Impression.
Since I was basically happy with the IC756, I did not expect to be "blown away" by the PRO -- and I wasn't. But since then I've worked a couple of contests on the PRO and then switched back to the 756 for a couple more. I really miss the PRO when I'm not using it! The two largest reasons are the incredible filters and the very quiet receiver. Things just seemed less chaotic during the contests when using the PRO.
The immediate impression of the receiver is that it is quiet. Side-by-side comparisons show the sensitivity of the two receivers the same except for 10 meters when the IC756 is more sensitive. The IC756PRO comes with a sheet showing actual tested specifications of each unit and it shows ten meter SSB sensitivity as .135 µv which is pretty good, so I assume that the 756 was just particularly sensitive. This testing was done in an extremely quiet rural area and both receivers were limited by atmospheric noise, so I don't think that this is a problem. It just results in a more stingy "S" meter. The 756PRO did a better job of receiving on ten, however. The low noise level combined with the very steep DSP filters produces an effect where signals just pop out of nowhere as you are tuning.
In operating a pretty nasty pileup for XZ0A on 20 (no, I didn't get them that time) XZ0A was S0. No movement of the meter whatsoever and only with 250 Hz bandwidth and full noise reduction could I copy the signal at all. Don't believe I could have copied it on the IC756. An S9+20 caller started up 1 kHz away. This would have rendered the signal unreadable on an IC756 with the 500 Hz/350 Hz filter combination. The AGC effects alone would have killed all chance of reading XZ0A. On the PRO, the was a mild clicking sound, but the signal remained readable.
My first XZ0A contact was at 0154 UTC on February 6 on 12 meter CW. On the 756, you could just tell that there was a signal there -- but it was completely unreadable, even with the DSP NR on. On the 756 PRO, the signal was easily readable after careful adjustment of bandwidth and heavy use of the DSP NR. I really need to get a better receiving antenna. In lieu of that, the 756 PRO is a big help.
I have tried listening to CW with a 50 Hz bandwidth. The lack of ringing makes this perfectly usable.
The really sharp DSP filtering and the AGC loop after the DSP makes the DSP noise reduction and notches much more useful. In many cases on the 756 the noise level itself caused enough AGC action to reduce the desired signal to unreadable levels. In the PRO, the NR actually improves the AGC situation by the AGC only acting on the desired signal.
Software Compatibility.
Current versions of DX4WIN32 and Writelog both work fine with the PRO. I recommend that you set either the 756 or the PRO up with a fixed rate of 9600 baud rather than use automatic. I've had no end of aggravations using automatic for the rate.
Earlier versions of DX4WIN32 do not include a rig file for either the 756 or the 756 PRO. This is trivial to resolve, simple rename the IC775 file and then edit it for upper frequency limit (54.000) and default address (50 for the 756, 5c for the PRO). Or you can use the file I use by clicking here. This file will default to 19.2kb and set the narrow filters. It also implements reading the S-meter.
Neat features not obvious from documents.
1. The band-stacking registers now store preamp, AGC, mode, and filter settings. This should be helpful in contests with frequent frequency changes as the use of one of the preamps is usually necessary on 10 and 15, frequently so on 20, and virtually never so on 40 and 80.
2. The 1/4 rate tuning mode available on RTTY on the
756 is also
available on CW in the 'PRO. This is useful with the very
narrow
bandwidths.
Noise Blanker.
Some reviewers have complained about the noise blanker on the PRO. It is essentially the same noise blanker that was used on the 756 and is equally effective on pulse-type noise. While I have not observed the phenomena yet, that would mean that it is capable of distorting weak signals in the presence of nearby strong signals. This typically gives one the impression that the large signal has "splatter". This is a common shortcoming of IF noise blankers and I don't know that there is a real solution to it.
Interesting Differences between the 756 and the 756PRO.
The following are differences between the two radios other than the obvious differences or are differences that are particularly interesting.
1. The IC 756 performs SSB compression in the audio system. This is contrary to what the documentation for the radio says. As a consequence, the audio input on the accessory socket (ACC2) is not subject to compression so that the COMP switch doesn't mean anything during AFSK data operations. The IC 756PRO apparently uses RF compression which is implemented after both the microphone input and the ACC2 input. The good news is that this should be superior compression. The bad news is that you should remember to turn the compression off when operating AFSK.
2. In both of my IC 756 radios, the ACC2 input required about 440 mv. to drive the radio to 50 percent power on 20 meters using the SCS PTC-II TNC. The IC 756PRO only requires about 100 mv. to drive the radio to the same level. This is a high impedance input and I found that the PTC-II expects low impedance and I experienced some power supply artifacts in the transmitted signal. The simple addition of a 1k resistor accross the "MOD" input solved this easily.
3. I had been using a Heil HMP preamp with my Pro Set headset. This caused distortion on the IC 756PRO so I removed the preamp. Everything seems to be fine without it. Since the 756 appears to also function fine without the preamp, it will be retired.
Random Observations.
1. The multiple filters are nice. However, I'm not
overly fond with
the implementation of the stored settings. Basically, in
defining a filter,
you are presented with a bandwidth adjustment and the two pass band
tuning adjustments, all of which are then stored. The problem
with this is that
it presumes a center frequency. In SSB this center is 1500
Hz. In RTTY
it is calculated from the mark frequency and the shift. In CW
it is calculated
from the pitch control. This concept works fine for RTTY and
CW, and is
not bad for SSB. But it is distinctly lacking for digital
modes that are implemented through SSB, such as PSK31. If
your PSK31 decoder is centered
on 1500 Hz, fine. Mine defaults to 1300 Hz and the
combination of PBT shifts that center on there only implement down to
500
Hz bandwidth. I can
change my decoder to center on 1500 Hz, so this is not a real problem,
just a nuisance.
A better implementation, IMHO, would be to allow setting bandwidth and
center frequency and then allowing PBT around those. This is,
in effect,
what you get with CW with the pitch control and on RTTY with the
mark/shift information.
2. There is now a USB-data and LSB-data mode. At
first glance this
seemed great. The problem is that the only apparent different
difference between
USB (or LSB) and USB-D is that the microphone input is turned
off. The "mode" uses the same filter definitions, preamp
settings, and AGC
settings and worse, leaves the compressor on. So if you want
really narrow data and
a default filter for it, you lose one of your three SSB filter
settings. A
better implementation (which would have been trivial) would be to treat
SSB data as a completely different mode with its own filters and
settings --
which would also allow the mode to be saved in the band-stacking
memories.
3. The key click problem on the sidetone when using the
speaker seems to be
gone. The sidetone keying is noticeably softer.
4. Many people have questioned the use of DSP only for
filtering,
contending that roofing filters would be necessary to prevent overload
of
pre-DSP stage. This question is partially answered by looking
at the circuit diagrams. The is a 15
kHz filter in the 64.455 mHz IF and two 15 kHz filters in the 455 kHz
IF. I don't see anything narrower for
CW/RTTY as has been suggested as necessary, but in CW mode if you
reduce the DSP
bandwidth to 500 Hz or narrow, a little yellow "BPF" indicator shows
on the display which is not documented in the manual. Whether
this is a DSP
optimization for narrow (probable) or a narrower crystal filter
(doubtful) is not obvious.
I have concluded after a lot of use of the PRO that it would be great
to add a
single automatic narrow roofing filter in the narrower CW
modes. This
would remove the occasional, and hardly noticeable "rumble"
encountered when very strong DX pileups 5 kHz up are present and the DX
signal
is extremely weak.
5. The 'PRO implements some additional features to facilitate
FM repeater
operation. You operate on a repeater by selecting the output
frequency in
the MAIN VFO and then holding down SPLIT for two seconds to set the SUB
VFO to
the repeater input frequency. The same action also turns on
the tone
generator for CTCSS access. The theory is that you'll then
store the
primary frequency in a memory and then get to the repeater later by
accessing a
memory channel. Note that the memory also stores the
specified tone
frequency. There are two problems with this theory.
First, the
memory does not store the transmit frequency, when you decide to
transmit on the
repeater, you have to go through the SPLIT operation first.
The second
problem originates from the first. Since the two-second SPLIT
activation
not only sets the offset frequency, but also turns on the tone, you are
always
transmitting with tone. Not a big deal, but it could result
in accessing a
repeater that one didn't plan on accessing. I don't operate
repeaters on
10 or 6, so I don't much care, but if one really wanted to use the
radio for
that, this would be a nuisance. If ICOM really wants to
support the
repeaters, they need to store in the memory that there is an associated
input
frequency.
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