[R-390] RE: HSR-2 vs 390a

[John Kolb]

On Tue, 27 Jan 2004, Cecil Acuff wrote:

> Those sound like some good ideas to check out...I had thought about the IF
> deck mod to replace the Mechanicals with some LC filtering or ceramics.  I
> know many new radio's now days can be ordered with the Collins mechanicals
> or ceramic filters so there must be some level of interchangeability.
> Good thoughts...
> Cecil...

Older mechanical filters such as those used in the R-390A are
100 Kohms in/output resistance. Most ceramic filters, and the
new torsion bar mechanical filters are 2 Kohms in/out. So swapping
ceramic filters into the 390 would require some additional
impedance transformation circuitry.

The Yeasu frDX-400 used transformers to get from the tube
impedance levels down to the 2K for Kokusai or Murata filters,
and another transformer to match back to the following IF amp grid.

I've got some mechanical and ceramic filter curves on my site
<http://members.cts.com/king/j/jlkolb>
Some of the ceramics look pretty good.

John

> ----- Original Message -----
> From: "Drew Papanek" <drewmaster813@hotmail.com>
> To: <r-390@mailman.qth.net>
> Sent: Tuesday, January 27, 2004 5:13 PM
> Subject: [R-390] RE: HSR-2 vs 390a
>
>
> >
> > On an Electric Radio article comparing the simple HSR-2 vs the R-390a, Dan
> > Merz wrote:
> >
> >
> > What caught my attention was the statement -
> > >"Comparison to my very well working 390a was dramatic,  I could CLEARLY
> > >hear signals that were well under the hash level of the 390a, but very
> > >comfortable copy on the homebrew receiver."
> >
> > (snipped)
> >
> > But what I'm wondering
> > >about,  are there other factors that would make the 390a inferior in
> > >certain cases to a simple receiver as expounded in this article?
> >
> > The aforementioned statement regarding HASH level of the R-390A could have
> > some basis even for a well-functioning R-390A.
> >
> > The R-390A's mechanical filter response has extremely sharp skirts;
> > wonderful in crowded ham band conditions but not so good in impulse noise
> > situations.
> >
> > Hash as we sometimes define it is a form of impulse noise. Those filters
> > will ping and ring, "smearing" out the pulses and making interference
> worse.
> >   One listening to a high quality AM signal with the R-390A can hear a
> > slight raspiness to the audio; the filters ring on transient modulation
> > peaks.
> >
> > The R-390 non-A has LC filtering (the IF cans) which has a more gradual
> > skirt characteristic largely devoid of ringing and hence does far better
> in
> > the presence of impulse noise.  That also accounts for the superior audio
> > quality of the non-A.
> >
> > I have an el cheapo Hallicrafters S-120 which is a low entry level general
> > coverage receiver.  Tube lineup is the All-American Five minus the 35W4
> > (selenium instead).  Normally in side by side comparisons a signal that is
> > not even detectable on the Halli will be armchair copy on my R-390A. But
> > enter strong 60 Hz power line hash and on moderate strength signals the
> > great '390A is humbled by the lowly Halli.
> >
> > Rolling your own R-725 by stuffing the non-A IF module into an R-390A has
> > its merits.  However, that module can be hard to come by.
> >
> > Here are some ideas (possibly without merit) I've been pondering for
> R-390A
> > IF module modification.
> >
> > (Forum Tranquility Preservation Statement: Performing the following
> > modifications would constitute heresy!  Shame upon he who would attempt
> > them!)
> >
> > 1.Replace one or more of the "mechanism a philharmonic" (mechanical
> filters)
> > with a filter comprising cascaded LC sections. These could be modern
> > miniature IF cans with added impedance matching in and out.  Modern
> ceramic
> > filters plus impedance matching schemes could also be used.
> >
> >
> > 2.Bypass 16 KHz filter entirely, add attenuation to match losses in the
> > remaining filters.  Change values of or  remove the Q spoiler resistors in
> > the R-390A's IF cans.  Realign with less (or even no) frequency
> staggering.
> > Fiddle with cathode resistors to reduce the gain which was raised by can
> > mods. The IF cans so adjusted would then define bandwidth in the switch
> > position labeled "16 Kc".  That would be the high fidelity, impulse noise
> > resistant mode.
> >
> > Using the narrower bandwidth positions would then provide mechanical
> > filtration at the original bandwidths (provided the filter in use had a
> > narrower bandwidth than newly defined by IF cans). Original function (and
> > susceptibility to impulse noise) in the narrower positions would be
> > preserved.
> >
> > These modifications might be attractive to one whose IF module has a
> defunct
> > 16 KHz filter, if not for the fact that the 4Khz filter fails most
> > frequently.
> >
> > 3. Keep the Philharmonic Quartet (16, 8, 4 ,2 Khz) intact but add relay
> > switching to put the Quartet on hold and swap in LC or ceramic filtration
> as
> > desired.
> >
> > 4. Add that panadaptor output jack you've been thinking about and use it
> to
> > feed an external IF/detector/audio unit.
> >
> > A better "modification" might be to acquire the R-390 non-A.
> >
> > Drew
> >
> > "Vicariously repairing and defiling R-390x via advice to others"
> >
> > _________________________________________________________________
> > Rethink your business approach for the new year with the helpful tips
> here.
> > http://special.msn.com/bcentral/prep04.armx
> >
> > _______________________________________________
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>
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