[ARC5] Using SCR-274N - For the FUN of it

[Mike Everette]

--- On Sun, 3/31/13, Bill Cromwell <wrcromwell at gmail.com> wrote:

>
Regarding the ARC-5 Triple Superhet,

> 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.

Destructive?  What be this heah, Destructive...?

Most of us have encountered hamfest-special Command Set receivers that could (charitably) best qualify as parts-sets, but are more or less intact -- just missing some key parts like the audio transformer, or maybe either having had the rear apron sawed off or the chassis butchered to mount a power supply to the chassis.  

Why gut something that doesn't deserve it? 

All you need to find is most any garden-variety junquer BC-454, BC-455, R-26 or R-27; and we all know there are a LOT more BC-453/R-23 out there than anything else.  Get an R-23 if possible because it has the AVC already self contained.  Tie the RF amp and one IF stage left from stripping down the HF receiver to the AVC line from the low freq radio, and that solves most of the overload problem.

If you want some selectivity ahead of the antenna, you'll be surprised what a simple antenna tuner can add... something to transform 50 ohms to the hi-Z input of the HF command receiver.  Passive device.  No muss, no fuss, no solid state mess.  

Want a little more pre-selectivity?  Add a tube stage to the tuner, as a regenerative RF amp.  Yeah... regeneration.... that funky stuff invented by Major Armstrong back in 19-freakin'-13.  It still works.  Take a look at older ARRL Handbooks from the 30s-40s-50s-60s and you'll see some circuits for regenerative preselectors.

(Dinosaur radio... still works great. And... No Phase Noise!)

It is not all that difficult to add a product detector and S-meter to this setup, especially if you are lucky enough to have a rack... just build the extra circuitry into the rack's interconnect box.  Again, go through those old Handbooks for ideas.  

There were 2 tubes sprouting from the box in my rack, for the product detector and an S-meter amp.  Well, at first I used a 1629 tube instead of an actual meter; it was pretty cool to watch the tube winky-blink at you.  

And, the LF and HF receivers had individual gain controls.  I probably didn't need to do that, and seem to recall intending to eliminate one and tie the gain lines together, but you know what the road to hell is paved with, so I never did.

You don't have to have a rack to do all this but it's handy.  (Yes, the rack I used had been gutted.  I did not destroy one.  Nope, never would do that; I had better sense.)

My setup included HF receivers for 3-6 and 6-9.  Changing bands was a simple matter of throwing a switch on the rack to transfer the B plus and AVC.  I let all the filaments run continuously so either band would be hot and ready.  If I revisit this system, I'd love to find a suitable 1.5-3 receiver so use for 160 meters.  By the way, the 6-9 receiver I had came with an unintended bonus.  Someone had hacked it to cover about 6.9-10.1 mc so it'd receive WWV.  It might have even been possible to fudge it a little more to 30 meters but I never did that.

Yes, I well remember that 40 meters at night was practically a lost cause.  Radio Moscow or rrrrrrrRRRADIO HABANA -- KOOBA every 10 kc from 7.1-7.2 (or even higher) with signals that seemed capable of frying a front end; and the BBC in between; and who knows what else (Ukraine?  Albania?).  Who needed a BFO?  Lots of CW signals could be heard riding in on the SWBC signals.

But, guess what... It COULD be done!  And, it was.  And above all, it was huge fun!  Even without a KomPOOTer. (They take up space better devoted to RADIOS, and they also generate noise; so who needs 'em?)

And by the way, when I'm listening to CW, if I hear a signal with even a little chirp on it, that gets my attention because I figure the guy or gal is running something interesting, not just another Ikensu Kamikaze Super Zot Ray 70000.  And a hand-sent fist, any time -- be it from a straight key or a bug -- from any rig, is equally appealing.

73

Mike
W4DSE