Modifications for the ICOM - RogerK

AM wide...............(none)................ 6 KHz (CFW455IT)AM normal............ 2.8 KHz (FL-70)....... 2.8 KHz (CFJ455K5)SSB/CW/RTTY wide..... 2.8 KHz (FL-70)....... 6 KHz (CFW455IT)SSB/CW/RTTY normal... 2.3 KHz (FL-30)....... 2.8 KHz (CFJ455K5)At first glance, one might think that it would have been more logical to use both 2.8 KHz filters in the SSB "wide" position(similar to the AM "normal" position), rather than a 2.8 KHz filter followed by a 6 KHz filter. (Since the narrower of the filters inthe IF chain really determines the overall bandwidth, the 6 KHz filter is actually doing nothing.) However, if things had been setup this way, replacement of the 2.8 KHz 3rd IF filter with the optional FL-44A 2.4 KHz high grade crystal filter would haverestricted the SSB "wide" bandwidth to 2.4 KHz. The SSB "wide" and "normal" bandwidths would then have been virtuallyidentical.Observe that the installation of the FL-44A does in fact reduce the AM "normal" bandwidth from 2.8 KHz to 2.4 KHz. Here's howthe filter chart would look for an R71A with both the optional FL-44A and the optional FL-32 CW narrow filter installed:2nd IF3rd IFAM wide.................. (none)................ 6 KHz (CFW455IT)AM normal................ 2.8 KHz (FL-70)....... 2.4 KHz (FL-44A)SSB/CW/RTTY wide......... 2.8 KHz (FL-70)....... 6 KHz (CFW455IT)SSB/CW/RTTY normal....... 2.3 KHz (FL-30)....... 2.4 KHz (FL-44A)Narrow-all modes......... 500 Hz (FL-32)....... 2.4 KHz (FL-44A)Having the razor sharp 2.4 KHz bandwidth of the FL-44A accessible in the AM mode can sometimes be great for critical DXing,but AM audio at that bandwidth does lack sufficient fidelity for comfortable listening.With some minor rewiring of the filter selection hardware, the 2.8 KHz AM bandwidth can be restored in "FL-44A"-equipped R71Areceivers. This is done by making the AM "normal" setting select the same 2.8 KHz (FL-70) / 6 KHz (CFW455IT) filter pair whichis used in the SSB "wide" setting. For experimental purposes, I had temporarily configured my R71 to select this filter pair in theAM "wide" setting in order to make direct receive fidelity and skirt selectivity comparisons against the 2.4 KHz AM "normal"setting.My findings revealed that with the receiver tuned directly to a broadcast station, both bandwidths produced very similar soundingaudio without much fidelity. But by tuning the R71A's main tuning dial 1.5 KHz away from the AM carrier frequency, the 2.8 KHzfilter passed the carrier and enough of one of its sidebands to provide halfway decent fidelity. (It was noticeably less than the 6KHz bandwidth, but still quite usable.) The same trick was also tried with the narrower, 2.4 KHz passband by tuning the maindial 1 KHz off frequency; but even then the fidelity was still too restricted for comfortable listening.Since the 2.8 KHz bandwidth seemed to isolate noisy AM signals just as well as the 2.4 KHz setting in most of my tests, I hadinitially considered replacing the 2.4 KHz setting with 2.8 KHz in order to take advantage of the additional fidelity. However, I didencounter some relatively rare instances when trying to tune in a weak AM station adjacent to a strong local broadcaster wherethe 2.8 KHz/6 KHz filter pair allowed some odd residual noise from the strong adjacent signal to become embedded in thedesired signal. The noise was not there when using the 2.8 KHz/2.4 KHz filter pair. This revelation led me instead to devise away of making both bandwidths available in the AM mode (in addition to the 6 KHz bandwidth).TECHNICAL DISCUSSION:For the following technical discussion of the IF filter selection circuit of the R71, you may wish to refer to your schematic diagramor service manual. The filter select IC is IC3 on the main PC board. It has three input pins (pins 9, 10, and 11); the logic state ofwhich are determined by the operating mode (AM, FM, SSB/CW/RTTY) and by the position of the "wide" and "narrow" filterswitches on the front panel. The configuration of these settings determines which IC output pin goes high. (Output pins are pins1, 2, 4, 5, 12, 14 and 15.) Diodes connected from the output pins to the various filters determine which filters are activated byeach specific mode-bandwidth combination. There are two output pins (pins 1 and 4) which are only activated in the FM mode.These outputs are unused (not connected to anything) since the optional FM board has its own IF filter which is automaticallyselected in the FM mode.The following table from the service manual shows the input-output relationship of IC-3:INPUT PINMODE 9 10 11 OUTPUT PINSSB/CW/RTTY normal L L H 14SSB/CW/RTTY wide H L H 5**SSB/CW/RTTY narrow* L H H 12AM normal L H L 15AM wide H H L 2AM narrow* L H H 12FM normal H L L 1FM wide H L L 1FM narrow* H H H 4* assuming optional filter switch (S1 on main PC board) is ON.** this output was incorrectly listed as pin 3 in theservice manual.