[Icom] IC-751A vs. IC-765

[Adam Farson]

Hi Chuck,

As one who has owned both radios, I believe that I can provide you with some
insight based on my experience with both of them.

I purchased a new IC-765 in June 1990. The first thing I did was the W2ISB
PBT Mod. In their haste to comply with a patent-infringement writ, Icom's
engineers stripped out PBT and left the IC-765 with two horrors:

1. A crude IF Shift implementation which bypasses the 9 MHz IF filter when
IF Shift is enabled.

2. Mis-terminated SSB IF filters, causing up to 6 dB of passband ripple
(corrected in S/N > 3000). This causes severe attenuation of "highs" in the
recovered audio when the reconstituted PBT is cranked up.

These issues can be addressed by looking for an IC-765 with S/N > 3000 and
performing the W2ISB PBT Mod. This is in a downloadable archive:
<http://www.qsl.net/ab4oj/download/download.html>

I also recommend replacing the FL-30 ad FL-96 SSB filters with an FL-80 and
FL-44A (or Inrad 109). This will tighten up the PBT, and significantly
improve adjacent-channel selectivity. In fact, the FL-80/FL-44A pair is used
in the IC-761, IC-745, IC-751A and the NATO variant of the IC-781. It is
arguably the best SSB filter combination Icom has ever offered.

That said, what is the difference in performance between the IC-751A and the
IC-765 with the modifications described above?

The IC-765 uses a DDS synthesiser, as opposed to the classical PLL of the
IC-751A. DDS offers much lower phase noise than a conventional PLL design;
the benefits of this are a quieter receiver, and superior strong-signal
behaviour. Strong signals outside the IF passband are less likely to raise
the noise floor in the passband. The IC765 also incorporates an excellent,
microprocessor-controlled autotuner.

The ARRL Lab Test on the IC-765 (QST, Dec. 1990) described the 765 as "one
of the five best receivers they had ever tested". Its blocking dynamic range
is around 150 dB. The significance of this is that if you can locate a
late-series IC-765 and are prepared to do the modifications described above,
the upgrade is well worth the trouble and expense.

Otherwise, your IC-751A will do a better job in many ways. (I assume that
your 751A is the PBT version). The synthesiser is a bit noisy by today's
standards, but the receiver front end is very good, and the FL-80/FL-44A SSB
filter pair offers adjacent-channel rejection and aggressive PBT action to
which the stock IC-765 cannot compare.

In the event, I sold my IC-765 when I acquired an IC-751A in mid-1992. From
my perspective, the IC-751A behaved better under crowded band conditions.



-----Original Message-----
From: icom-admin@mailman.qth.net [mailto:icom-admin@mailman.qth.net]
On Behalf Of cpro32
Sent: Wednesday, October 30, 2002 18:23
To: icom@mailman.qth.net
Subject: [Icom] IC-751A vs. IC-765


I am thinking of upgrading from a 751A to a 765. The 751A is certainly a
nice rig, but my only rig for comparison was a Swan 700. I'd like to hear
some opinions and wisdom from the members of this list as to whether it's
worth the time and money to upgrade. My interests are DX and a little bit of
casual contesting. I am particularly interested in a comparison of receiver
performance. Will I really be able to tell the difference between the two
rigs? The 751A does not have an internal PS or tuner, but that's not a big
problem for me. It would be great if someone who has owned both rigs can
comment. Thanks.

Chuck KF9PW