HFU HF Underground
Technical Topics => Equipment => Topic started by: Matt285 on October 27, 2019, 1409 UTC
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I was curious. I own an Icom 718 which I use for TX and RX. The Icom R75 is based off the same cabinet design and is almost identical. I know there obviously different, but since the price is so similar despite lacking transmit capabilities I was wondering. How different are the two in the receiving aspect. There must be a reason why the R75 sells for almost the same price? Thanks
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Have seen this same question discussed in other forums.
The consensus is that in spite of similar appearances, the R75
is the superior receiver. The 718 is a dual conversion superhet
while the R75 employs triple conversion.
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Thanks for the comparison. I always kind of knew the R75 would likely have superiority in the receive aspect. Thanks for the technical opinion aspect of things.
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Most all Icom analog mid range hf rigs have about the same ears as far as sensitivity and dynamic range goes, I doubt you could tell them apart in the same mode and filter bandwidth.
Most of them also follow the same IF schemes of 70mHz 1st IF, 9mHz 2nd IF, 455kHz 3rd IF feeding the demodulator or yet another mixer for a fourth IF, and often use the same demod chips. The dual conversion rigs will IF from 70mHz to 455kHz. You will see the exact same circuitry blocks in the schematic, parts values, and general layout in all of them from about the IC745 onwards, main dif being now they're all smd rather than thru hole components, have a dds rather than a pll, and some are dsp/sdr. I suppose it's the same for Yazoo and kw rigs too.
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For a more detailed analysis, you may want to check out the reviews in QST.
See January 2000 for the R75 and July 2000 for the 718.
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Thanks. Will do.
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While their outward appearance is similar, the R75 has a superior receiver. It has been a while since I compared the schematics, but I recall that they were significantly different.
The 718 is OK accept for one glaring fault: the receiver AGC is terrible. I have a 718 I use on MARS nets and it is literally exhausting to listen to SSB for extended periods of time. The hang time is far too short so the receiver just “pumps”. I have found it necessary to ride the RF gain control to make it tolerable to listen to.
I haven’t seen any mods to fix this. The nearest “fix” is one that turns on the AM mode AGC time constants while you are using SSB - this is supposedly better, but not perfect.
Other than the AGC, I’ve got no complaints about the 718. It is a good value for the money (though my advice for anyone considering purchasing one today is to save up a little bit more and buy a IC-7300 instead - you will be far happier with it.)
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Poor agc action allows for imd to be created in band. This is what tires the ears and makes weak signal work tougher. The IC765 has a brilliant mod to decrease the in band imd by replacing a cap in the agc line that clears things up dramatically.
https://www.ab4oj.com/dl/765_108.pdf
I suppose the same could be done for the rest of the Icom line since Icom still hasn't learned to do agc sans imd.