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

[Drew Papanek]

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"

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