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

[mikea]

On Fri, Feb 02, 2007 at 12:18:11PM -0500, Mark Huss wrote:
> 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.

Phase noise problems with the PTO? As in just junk on the PTO output
line, fixable by ferriting it, or other problems that required changes
to the PTO other than just ferrites? Would decoupling the PTO stuff 
make any audible changes, or is this just changes in the DF propeller
sizes from field measurements? Because if the R-390 PTO has phase 
issues, the R-390A PTO probably does, too, and I can see a purchase 
from Amidon or an Amidon supplier in my near future. 

> 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.

I've noticed that as the bandwidth gets narrower, the signals seem to 
be smeared out a little bit in time, on the R-390 *and* R-390A, but 
that effect seems to be a little exaggerated on the R-390A when I've
got the mechanical filters switched in. I think it's not all due to 
ringing in the filters, but that there's some analog to Heisenberg's
Theorem that defines a frequency-time product uncertainty that you 
have to live with -- something like 
        the product of certainty-in-frequency and certainty-in-time 
        > some constant K
where K is big enough to get bothersome for really narrow bands. 

But, again, I could be all wet. 

Certainly I do better copying code down in the grass on an R-390 than
on any of my R-390A, and it beats my riceboxes (Yaesu FT-857, FT-857D
and FT-897D) all hollow, even with DSP wound down to 60 Hz. It also 
helps keep the bedroom warm at night, which is a big bonus in winter,
but not so much in summer.

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin