[R-390] Re: [Milsurplus] Transplanting R-390 IF modules to R-390A

[Mark Huss]

Indeed, as an experiment I once dropped an R-390A into a TRR-20 (a DF 
set that used a commutated ring of vertical antennas for DF). Result was 
a much wider propeller, I.E. the resolution was not anywhere near as 
good. I was later told that was because (1) phase-shift in the 
mechanical filters, and (2) phase noise from the PTO. The R-725 
addressed both these issues.
And Ray, One thing we noted, having aligned both R-390A's and R-725's, 
was that as a rule the R-725's almost invariably had a better S/S+N 
sensitivity rating. We always suspected this was because the R-390A's 
noise floor is determined by the IF deck, not the First RF amp, as is 
usual. And the R-725, with it's R-390-style IF Deck had bandwidth 
filtering throughout the chain, not just a lump at the input. That they 
also decoupled just about every line out of the PTO and into the IF Deck 
with ferrite beads probably did not hurt either.
Finally, we used the Shop R-390A as a Shop Radio, with a used stereo 
speaker. On someones suggestion, we tried an R-725. And it sounded a lot 
better, to us, at least. Enough better that we put an R-390 IF deck on 
order and modified it to drop into the Shop R-390A. Could have been our 
imagination. But Ace, a SSG Hog, often said he could copy in-the-noise 
code better on an R-725 or old R-390 than on a R-390A. Said it sounded 
cleaner.
mikea wrote:
> On Fri, Feb 02, 2007 at 09:07:22AM -0500, Ray Fantini wrote:
>   
>> Ok, so maybe its a stupid question but why would you want to substitute
>> a IF deck with mechanical filters to one without? What's the benefit? I
>> have heard rumbling before about the A not being as good as a regular
>> 390 and thought the filters were the only difference, well maybe that
>> and the squelch.
>>     
>
> The mechanical filters tend to "ring" a bit, in my experience, where 
> the LC filters don't. If you need better phase-flatness across the
> filter passband, the LC filters are preferable to the mechanical ones,
> and it's my understanding that that's why the R-725 RDF receiver used 
> the R-390 IF deck in the R-390A chassis. 
>
> I could be all wet, of course; it wouldn't be the first time. 
>
>   


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