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Author Topic: Icom R75 general coverage receiver LW/MW attenuation  (Read 433 times)

N4AEQ

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Icom R75 general coverage receiver LW/MW attenuation
« on: June 24, 2021, 11:33:57 PM »

I picked up a like new R75 from a silent key sale, Like most ham transceivers that are general coverage the
LF & AM broadcast bands are highly attenuated. I guess Icom felt the need to do the same with their R75 receiver.

Frequency SSB/CW/RTTY AM FM range 10 dB S/N 10 dB S/N 12 dB SINAD
0.1–1.8 MHz 2.0 µV 5.6 µV
1.8–28 MHz 0.16 µV 1.6 µV

Does anyone know a way to defeat/restore sensivity of LF/MV bands?
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AC2NZ

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Re: Icom R75 general coverage receiver LW/MW attenuation
« Reply #1 on: June 25, 2021, 04:32:58 AM »

I removed half of the attenuation by bridging two pads on the motherboard but the remaining part requires surface-mount surgery that's beyond me.

Complete info can be found at:  <mods.dk>

73!
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N4AEQ

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Re: Icom R75 general coverage receiver LW/MW attenuation
« Reply #2 on: June 26, 2021, 07:03:12 PM »

 Yes, I saw that mod, and like you said bridging the pad does little to help.  I don't want to do SMD/SMT surgery to fix the problem, just to old for that and would be a huge risk.  Guess the only help would be a better LW antenna as I like to DX European LW AM broadcast during winter nights.  Using a 160m dipole now and it doesn't do the job very well,
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SWMAN

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Re: Icom R75 general coverage receiver LW/MW attenuation
« Reply #3 on: June 30, 2021, 03:16:37 AM »

Most good quality receivers have a jumper that can be removed to increase sensitivity for the LW/MW bands.  I am surprised the R75 doesn't have it. Maybe you should check a little further.
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N4AEQ

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Re: Icom R75 general coverage receiver LW/MW attenuation
« Reply #4 on: June 30, 2021, 03:30:09 AM »

 I have checked, you can solder a jumper pad but then you have to remove 3 SMT resistors, change one from 10 to 100 ohm and add a 1mh choke where one of the other two resistors was, all smt.  It's listed as a communications receiver not a Ham/shortwave only receiver, should not have to rebuild the radio to restore coverage below 1800mhz. 
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SWMAN

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Re: Icom R75 general coverage receiver LW/MW attenuation
« Reply #5 on: June 30, 2021, 04:36:28 AM »

AEQ,
 I agree with you, if it were me I guess I would just leave it alone. SMT soldering is not for me either. Good luck.  Jim W5JJG
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VK6HP

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Re: Icom R75 general coverage receiver LW/MW attenuation
« Reply #6 on: July 01, 2021, 01:56:51 AM »

I'm not familiar with the gain distribution in the R75 but you're probably suffering far less than at first glance simply because of the huge increase in sky noise (cosmic and atmospheric) at MF and below.  As long as the external (antenna) noise is well dominating the receiver, you're winning. Typically, the first thing I do when working on 160 m and 630 m with amateur radios is to put in at least 10 dB attenuation to keep the receiver chain as linear as possible and to maintain the signal-to-noise ratio on e.g. WSJT digital modes. 

I assume that the modification you're referring to is this one:  http://www.sarmento.eng.br/Icom_R75/PeteGMod.pdf  If you don't want to modify the receiver, one fairly easy solution might be to use an active receive antenna, such as an amplified loop.  Unless you live in a very quiet location, just using a large non-directional antenna is likely to be a disappointment in DX s/n terms. In practice, you're likely to find that some receiver attenuation and a little gain from a pre-amp robust in IMD terms will probably work out pretty well.

73, Peter

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KM1H

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Re: Icom R75 general coverage receiver LW/MW attenuation
« Reply #7 on: July 03, 2021, 04:45:55 PM »

Ive found a 500' Beverage works quite well on the MF AM BCB and even has directivity. Ive been using a TS-930, 940, 950SD over several decades for that.

What is truly amazing to me is that Ive also copied SAQ from Sweden on 17.2 kHz quite a few times with my NE Beverage which simply seemed impossible. For that Ive used a HRO-500 with the LF-10 preselector as well as a 1930's designed USN RAK-7 which is a regenerative radio. Its type 41 audio limiter makes one hell of a nice noise blanker on CW of course.
https://www.navy-radio.com/manuals/rak7-man-4212.pdf

Carl
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N0TLD

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Re: Icom R75 general coverage receiver LW/MW attenuation
« Reply #8 on: July 04, 2021, 02:53:48 AM »

6HP's comments are pretty spot on.

I can add, I've had a much-modified R75 for a lonnng time that gets a lot of use, and one of the earliest of many mods was the Pete G attenuation mod. But before I did that I did just the jumper mod, and found some difference from stock (a few dB maybe) over LW/MW in my fairly low noise environment, making the effort worthwhile enough. Then some months later I found the skill and motivation to do the full Pete G mod and I can tell you it does make a noticeable difference from just the jumper mod.

BUT, all that said, I honestly think (at least in my relatively calm noise situation) the effect of the full Pete G mod is not all that distinguishable from the effect of just the jumper mod and a decent antenna and the R75's built in preamps. Depending on your noise floor, you may not notice that much difference either.

An aside, I completely agree that one should NOT have to rebuild a commercial receiver to have it work at its best, and if surface mount soldering and so on is not your thing I totally get it. But for me, one of the reasons I became a fan of the R75 early on is because it is so eminently modifiable, and I enjoy tinkering and improving and customizing, with all the filters and audio quality mods and so on. I even reversed the polarization of the display and its smd LED coloration so it's been a Drake R8-like display (light green characters on dark green background) for the last decade or so. 

This link should lead to an ImBB image of it:
 
https://ibb.co/cb74XWP

It's an interesting look anyway. :)

Best --
Mike
N0TLD
« Last Edit: July 04, 2021, 03:02:34 AM by N0TLD »
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