On the IC-756Pro III vs. the IC-781: Some thoughts from my shack

by Adam Farson, VA7OJ/AB4OJ

Q: Would I be better off with a new IC-756Pro III or a used IC-781?

A: There is simply no way to compare the analogue IF filtering of the IC-781and other radios of that era with the DSP IF filtering (which includesa -70dB notch) in the IC-7800, IC-756Pro series and IC-746Pro. George W5YR's IFFilter Performance Page illustrates this very dramatically.

The IC-781 was a good strong-signal performer, but again, both George and Ihave written notes on the strong-signal behaviour of theIC-756Pro II. I have since found that the Pro III represents a worthwhileimprovement over the Pro II.

I bought an IC-781 in mid-1993, and replaced it 5 years later with anIC-756. I found that the little "original" 756, with its integrated DSP NR,outgunned the IC-781 + outboard JPS NIR-12 on weak and noisy signals. Furthermore, theFL-223/FL-222 narrow SSB filter pair in my IC-756 offered adjacent-channelrejection unattainable with the IC-781. The step down from 150W to 100W was nota concern, as I almost always have the amplifier in the signal path. (Mustadmit, though, I did miss the IC-781's tuneable pre-AGC IF notch; the IC-756offered a post-AGC auto-notch.)  Since then, an IC-756Pro (2000), Pro II (2002) and Pro III (2005) have replaced the IC-756.

The IC-781 was probably one of the very best analogue radios made, with a fewcaveats. I had to replace the stock FL-96 SSB filter with an FL-44A, toimprove adjacent-channel selectivity and tighten up Twin PBT operation. I alsoreplaced the FL-102 AM filter with an FL-223 (1.8 kHz SSB), for "SSBNarrow". The IC-781 uses the same DDS chipset as the famous IC-x75 series ofall-mode radios; its reciprocal mixing noise (measured at 10 kHz offset) is lower thanthat of the FT-1000D by 1 dB (RF Preamp off) and 11 dB (Preamp on). 

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 anexcellent mod to address the ALC spike). The vertical sensitivity of theSpectrum Scope is much inferior to that of the IC-756Pro II. The CRT has a longlifespan, 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 aresome work-arounds for the heat problem - a $140 fanretrofit, and reducingthe mains voltage to 100V. Floyd K8AC also discusses IC-781 heat issues on hiswebsite.

My IC-781 never had any synthesiser problems, but the chassisconstruction dictates the use of special extender harnesses for servicingthe PLL modules should the need arise. Also, with the radio now discontinued, spares and servicesupport will become increasingly difficult as the years go by. 

We still come back to the basic question. You will be paying about $3000 fora used IC-781, plus another $250 for the filter upgrade. I am sure that youwill be happy with an IC-781, unless something breaks and you cannot get itfixed at reasonable cost. For some $300 less, you can buy a brand-new IC-756ProIII with a warranty. With those greatcontinuously-variable DSP IF filters, you will never need to buy anothercostlyanalogue filter.

Copyright 2003-2005 A. Farson VA7OJ/AB4OJ. All rights reserved.

Last revised: September 25, 2019

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