[Icom] IC-781, IC-756Pro II etc. Was: KK7TV 781 Mods

Thu, 13 Mar 2003 22:50:37 -0800

Hi Chuck,

Here is an excerpt from an e-mail I sent about six weeks 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 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 $500 to $750 less, you can buy a brand-new Pro
II with a warranty (and a PS125 thrown in). With those great
continuously-variable 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 [email protected]
Sent: Thursday, March 13, 2003 18:51
To: [email protected]
Subject: Re: [Icom] KK7TV 781 Mods

Are you saying that in some ways the IC-781 is superior to the DSP 756 Pro
II
?  The original writeup described the 781 as extremely quiet etc.  How would
some say the 781 is better?  Curious in Ohio.

Thanks,

Chuck
n8gmb