#Icom ic 756 pro ii cat pinout how to#
How to hear it: Find a band that has a lot of strong signals. It was surmised that this signal was coupling into the 8V supply rail, and getting coupled into the output of U311, which is a two-stage low level audio preamp and voltage controlled gain stage. Our working hypothesis was that the rumble signal was actually the AM detected version of the entire 455kHz IF passband, which is used in the Noise Blanker to derive a gate signal, used to blank noise pulses. The performance characteristic led quickly to the only point in the audio chain where the rumble could be introduced, and to a pretty good first guess as to its source. The Probable Cause: K1KP did the schematic analysis. With a single data signal in the passband, it will appear to be a mirror image of that signal, but lower in pitch. Tune off that loud signal of interest and you still hear the growly-rumbly, thumps or bumps, as long as the loud signal remains within the 7.5KHz wide upper IF. This led to reports of “Shadow Signals” or “Ghosts” in the radio. On CW, it sounds like thumps and bumps…like muted or low-pass filtered key clicks. The signal is actually INTERFERING WITH ITSELF. On SSB, it sounds like a growly-rumbly sound, under the signal of interest. Once you notice this leakage, you’ll also discover that it is present even underneath the signals you are trying to copy, both on CW and SSB. The ICOM sp21 speaker does not have sufficient LF response to permit hearing the problem. Headphones plugged into the front panel reveal the problem. You cannot hear it in the rear panel audio, on acc1 it is injected after that point. You cannot hear it in a speaker, although it contributes interference. You WILL hear it, when the right amount of interference is present. To be clear: The Noise Blanker Drive signal in every 756PRO is leaking into the main audio channel. Standardized tests require a certain amount of simplicity, in order to be both meaningful and repeatable. The problem is dependent on input signal levels and density, and quite variable. Casual operating comments in the review text for the most part failed to identify this particular anomaly, even though it very likely degraded the results of some intermodulation distortion tests. This brief paper describes both a source and correction for what we now identify as “The Noise Blanker Drive-Signal Leakage Problem.”Įarly reviews by RSGB and DLARA, and ARRL remained within their structured testing schemes, for consistency. Now that we know their source, it makes perfect sense that their reported characteristics varied, and that some folks didn’t hear them at all.
These signal artifacts were variously described as ‘phantom signals’ or ‘ghosts’ or ‘rumble’, and were so variously reported, as to be completely rejected by ICOM.
These signals took on a lot of characteristics, and weren’t observed by all amateurs. Shortly after Icom’s IC-756 Pro HF/6 meter transceiver was introduced into the US, one of the authors observed that signals within the upper IF bandwidth could be heard, even though they were well outside the lower IF filters. You may not rewrite or reproduce it, in any way, nor may you remove the copyright paragraph. You may give copies to other amateurs, in printed or soft form.
You may provide links to sites where this piece is found. This short version is provided for web posting through specific web sites, previously authorized. This condensation may not be distributed or republished, without express permission of the authors, or of ARRL. Correcting the IC-756Pro Noise Blanker Drive-Signal Leakage ProblemĬorrecting the IC756Pro Noise Blanker Drive-Signal Leakage Problem by Jim Jarvis N2EA and Tony Brock-Fisher K1KPĬOPYRIGHT: This piece is a condensed version of Technical Correspondence, QST, June 2002, p.68.