[Icom] Icom 781 vs. Icom 756PRO II
Adam Farson
[email protected]
Fri, 11 Jul 2003 17:22:03 -0700
Hi Charlie,
There is simply no way to compare the analogue IF filtering of the IC-781
and other radios of that era with the DSP IF filtering (which includes
a -70dB notch) in the IC-756Pro2 756Pro and 746Pro. George W5YR's comparison
of the IC-765 and IC-756Pro2 filters illustrates this very dramatically:
http://www.qsl.net/ab4oj/icom/passband/passband.html
The 781 was a good strong-signal performer, but again, both George and I
have written notes on the strong-signal behaviour of the 756Pro2 here:
http://www.qsl.net/ab4oj/icom/ic756pro_notes.html
Here is an excerpt from an e-mail I sent about six months ago to another
fellow-ham who had the same question concerning the IC-781, IC-756Pro II
etc.:
I bought an IC-781 in mid-1993, and replaced it 5 years later with an
IC-756. I found that the little "original" 756, with its integrated DSP NR,
outgunned the 781 + outboard JPS NIR-12 on weak and noisy signals. Also, the
FL-223/FL-222 narrow SSB filter pair in my 756 offered adjacent-channel
rejection unattainable with the 781. The step down from 150W to 100W was not
a concern, as I almost always have the amplifier in the signal path. (Must
admit, though, I did miss the 781's tuneable pre-AGC IF notch; the 756
offered a post-AGC auto-notch.)
The 781 was probably one of the very best analogue radios made, with a few
caveats. I had to replace the stock FL-96 SSB filter with an FL-44A, to
improve adjacent-channel selectivity and tighten up Twin PBT operation. Also
replaced the FL-102 AM filter with an FL-223 (1.8 kHz SSB), for "SSB
Narrow". The IC-781 uses the same DDS chipset as the famous IC-x75 series of
all-mode radios; its reciprocal noise-mixing figure is 5 to 7 dB better than
that of the FT-1000D.
The IC-781 has a fairly serious heat issue in its power supply, and a 200 ~
250W initial RF output spike due to slow ALC attack. (Mel VE2DC devised an
excellent mod to address the ALC spike). The vertical sensitivity of the
Spectrum Scope is much inferior to that of the 756Pro II. The CRT has a long
lifespan, but replacements are unavailable from Icom; they will sell you a
$1200 LCD retrofit kit which is not nearly as sharp as the CRT. There are
some work-arounds for the heat problem - a $140 fan retrofit, and reducing
the mains voltage to 100V.
I never had any synthesiser problems with my IC-781, but the chassis
construction dictates the use of special extender harnesses for servicing
the PLL modules. Also, with the radio now discontinued, spares and service
support will become increasingly difficult as the years go by.
We still come back to the basic question. You will be paying about $3000 for
a used IC-781, plus another $250 for the filter upgrade. I am sure that you
will be happy with a 781, unless something breaks and you cannot get it
fixed at reasonable cost. For some $750 less, you can buy a brand-new Pro
II with a warranty (and a PS125 thrown in). With those great
continuously-variable DSP IF filters, you will never need to buy another
costly
analogue filter.
Cheers for now, 73,
Adam VA7OJ/AB4OJ
-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of W0YG Charlie Summers
Sent: Friday, July 11, 2003 09:06
To: Icom Reflector
Subject: [Icom] Icom 781 vs Icom 756PRO II
Has anyone really done an on line real time A/B test under a variety of
conditions for these two radios? Is so, how does the 756 PRO II stack up?
Please no anecdotes on I got a S9 plus 20 report in India, I am looking for
some real info with A/B type scenario.
73,
Charlie, W0YG..>>