[ARC5] Using SCR-274N

[Bill Cromwell]

On Sat, 2013-03-30 at 20:11 -0700, Mike Everette wrote:
> Perhaps the point has already been made, perhaps done to death... but,
>  to my way of thinking, the ultimate SCR-274N/ARC-5 receiver setup is
>  the "ARC-5 Triple Superhet" idea published in QST in the late 50s. 
>  This yields a triple conversion receiver, by combining one of the HF
>  receivers which has been stripped down and its BFO made into a second
>  conversion oscillator to take the IF of the high freq receiver down to
>  455 kc to feed into a BC-453 or R-23/ARC-5.

Hi Mike,

I thought about doing something like that, too. To do it will require
some expense in cash (probably not a lot) and quite a bit of effort (but
it's a hobby). Better performance is had with *ANY* receiver if the main
selectivity is closer to the antenna as opposed to being at the end of
the last IF or even at the end of the audio output. That makes less
difference for casual operation. It also is less of a problem if the
front end is less susceptible to overload. Using "ARC-5" command
receivers *as-is* it's easy enough to avoid overload due to the way the
gain control controls. You just have to be a radio operator - not an
unpleasant proposition. I think the effort should be put into something
a little more productive and a little less destructive.

> 
> This yields an excellent receiver.  I built one once (then, like an
>  idiot, sold it at a hamfest), and can testify to its many merits.  It
>  runs rings around Ikensu junk of today, and would give many
>  sophisticated vintage receivers a run for the money.

I have read testimony from a couple of hams who let those go and
regretted it.

> 
> Such an approach avoids miscegenating an SCR-274N or ARC-5 receiver
>  with solid state thingies (I would say JUNK, but some might not like
>  that... hmm, I don't think I care....)

If you don't mind the extra drain on the power budget a lot of outboard
accessories can be used to help sort signals built around tubes. You
could even use a passive audio filter in the low level audio stages
(ahead of that single ARC-5 audio tube) for some help. No active devices
at all. The RAK/RAL do that quite effectively (but they do have some
loss). And there are quite a few ways to use something like a
"Select-O-Ject" and other vacuum tube processing to reduce otherwise
unmanageable QRM. I'm not sure if a SCAF filter would be practical in
hollow state. It's somewhat more sophisticated than the select-o-ject
(but duplicates it) and not as complicated as a full blown DSP unit.
Adding a xtal filter at the original IF frequency seems a lot more
desirable than making virtually irreversible mods to a receiver that
already does a pretty good job "as-is". With those bathtub caps gone
(replaced with new caps) there is room for an xtal filter module under
the chassis.

> 
> But as for "Using SCR-274N," even AS THEY COME, they are VERY usable.  
> 
> I'd like to point out that back in the day, a lot of us used what we
>  could get our hands on CHEAP.  Many Novices cut their teeth on Command
>  Set receivers which they got for anywhere from free to $5 or $10, with
>  no added IF filtering or other stuff.  And, the darn things will hear
>  any signal on the air, be it emanating from Outer Mongolia, B-F-E, or
>  wherever.

I remember 'back in the day'. $5 or $10 was about what I earned working
in people's yards or shoveling snow for two or even three weeks. And
um... how many kilowatt stations were transmitting in the novice band to
overload those receivers? My 40 meter receiver arrived already recapped
and ready to rock n roll. It has the widest IF channel of all of my
receivers even my regens! I applied an antenna and power before I even
picked up the spilled foam peanuts from the floor. No problem copying
any one of a dozen signals that were present in the passband. It just
sounded different than my Ten Tec or Kenwood with the wide filters
selected. The scaf filter plugged in where the cans plug into the radio
turned it right into Cadillac! Maybe it would be back to Ford Pinto
status in the sweeps...maybe not. They really are "very usable".

>   
> 
> Granted, you may hear a lot of other signals in the passband; but
>  that's where SKILL comes into play.  The human brain is by far a
>  better selective filter than anything else, if properly trained.  It
>  may come as a surprise to many in today's plug-n-play, computer
>  (de)generated world that this is still a basic truth. 

Each of us have a completely unique hearing response (as with all of our
other responses). Most of us can do that to one degree or other. Many of
us could do it quite well. For me it's like being in a busy, popular
restaurant surrounded by many conversations. We listen to one of
interest (probably our lady friend or XYL) and ignore the rest. However,
if a loud fight breaks out at the other end of the place or the music is
too *LOUD* it gets more difficult and we throw in the towel. Most of the
time band conditions on 80 and 40 meters are just not that crowded. I
think back in the day more transmitters had there own "character' and
that helped too. In that crowded restaurant your lady's voice has it's
own sound and that helps. Today most of the rigs on the air sound
exactly the same and when computers send the CW there isn't even an
accent from the fist!

>  
> 
> Even a barn-door, four-track-railroad-wide Command Set receiver still
>  beats stuff like the Hallicrafters S-38 from back in the day.
> 
> But that ARC-5 Triple Superhet will slice signals thinner than Pringles
>  potato chips.... ga-ron-teed.
> 

The measured specs tell us the R-23/BC-453 can be choked down to about 1
kc wide at the peak and 2 or 3 kc further down the slope where more
doesn't matter as much. That's a whole lot better than 15kc!! or the
entire novice band at once (grin).


> If I have time to get it going I may just build up one of these
>  receivers and take it on Field Day this year.  Gaaa-aaaaAAAAAAh-LEEE!
> 
> For whatever these late-Saturday-night thoughts may be worth....
> 

They're worth a lot. It pays to review the basics from time to time and
those radios will force us to do that.


> 73
> 
> Mike
> W4DSE


73,

Bill  KU8H