A: 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-7800, IC-756Pro series and IC-746Pro. George W5YR's IF Filter Performance Page illustrates this very dramatically.
The IC-781 was a good strong-signal performer, but again, both George and I have written notes on the strong-signal behaviour of the IC-756Pro II. I have since found that the Pro III represents a worthwhile improvement over the Pro II.
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 IC-781 + outboard JPS NIR-12 on weak and noisy signals. Furthermore, the FL-223/FL-222 narrow SSB filter pair in my IC-756 offered adjacent-channel rejection unattainable with the IC-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 IC-781's tuneable pre-AGC IF notch; the IC-756 offered 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 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. I 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 mixing noise (measured at 10 kHz offset) is lower than that 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 an excellent mod to address the ALC spike). The vertical sensitivity of the Spectrum Scope is much inferior to that of the IC-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. Floyd K8AC also discusses IC-781 heat issues on his website.
My IC-781 never had any synthesiser problems, but the chassis construction dictates the use of special extender harnesses for servicing the PLL modules should the need arise. 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 an IC-781, unless something breaks and you cannot get it fixed at reasonable cost. For some $300 less, you can buy a brand-new IC-756Pro III with a warranty. With those great continuously-variable DSP IF filters, you will never need to buy another costly analogue filter.
Copyright © 2003-2005 A. Farson VA7OJ/AB4OJ. All rights reserved.
Last revised: June 16, 2018