[ARC5] ARC-5 Transmitter

[David Stinson]

zeno wrote:

> 1. How was it keyed originally,...
In the original design, all B+ voltages were supplied by a dynamotor
which provided 550 VDC with good regulation.  Screen and Oscillator
voltages were developed with a resistive divider network.
The transmitter was keyed by a relay making and breaking the
550 VDC output of the dynamotor.  Believe it or not,
with a properly operated (as in "operated as designed) transmittter,
there is little chirp with this system.

> ...and aside from the "museum" 
> consciousness, is adding the key jack as suggested in this article 
> considered an actual circuit "modification"...
Yes, indeed.  Any unoriginal hole drilled in your unit
changes it from an historic artifact into a hanger queeen.
That said- one of your BC-696s has silver tape over what
used to be a key-jack.  I covered that hole when I put the
transmitter back into "original design" configuration.
Since the hole is already there, there is not harm in
reinstalling a key jack and doing whatever other mods
you might contemplate.  I do not recommend the usual
mods you see in the ARRL or "conversion" handbooks.
They key badly, tune incorrectly and offer other frustrations.
Running the filaments on 12 VAC introduces hum in the
master oscillator output and impacts overall frequency
stablity.  You can buy 24 volt power supplies that will
deliver a regulated 3 or 4 amps from surplus dealers
for a few dollars, so there's no good reason to
destroy the filament string and hack out the relays;
you gain nothing and loose system stability.

>  How are you purists
> keying this? Where are you plugging in your key?

Pin #5 on the SCR-274N and ATA transmitters
and pin #3 on the AN/ARC-5 transmitters keys the
transmitter select relay and the antenna relay.
With a lead lifted on the antenna relay and
the contacts on the selector properly sequenced
as I wrote in one of the posts, it works
splendidly as a keying relay.  I've run my
bug at 25 WPM with no trouble.

> 2. I do not recall in this article, 

I respectfully suggest that you put aside the article and
forget everything in it.  With the exception of those
written by Gordon White, they almost universally were
written from a flawed understanding and give bad advice.

> that they suggest adding a coax 
> connector for the antenna... 
Ill-advised and needless.  The "ARC-5" transmitters
cannot match a coax-fed, 50-ohm antenna in their native state.
The simplist fix (not perfect, but it will work just fine)
is, as I wrote before, to put a 1000 volt or better,
NP0 type capacitor in series with the output lead, then
connect that to your coax feedline.  Something around
50 pFd will work best at 7 MC, something between 75 and 125 pFd
on 3.5 MC will do.  There's a lot of "wiggle room" on these values,
unless you want to get it perfect.  Marty's suggestion of a
large variable in series will work, too.  And Mike's UnUn will
also do the job, and is my personal preference.

> Were some hams in the 50s simply connecting 
> their antenna wire to the original terminal,..
Yes; they hooked 50-ohm antennas to the output,
mistuned it to get as much power out as they could
with the serious mismatch and got pink tickets
for harmonics, just exactly as you would if you
mis-tuned any other transmitter's output and
used it.  Tune the transmitter properly
and it's cleaner than many modern transceivers.

 > and if so, what would be
> the most effective and simple antenna that one could use now, to avoid 
> what seems like complexity as indicated in the discussion we are 
> currently having. 
Kindly speaking, I don't think a series cap is all that complex.
You could use a short, end fed wire or "T" as they did on the
body of the aircraft, and the transmitter will tune it without
any changes at all.  You won't get much in the way of signal out,
though.

> 3. I assume that one would need to add a good grounding terminal to the 
> unit. Cannot say I understand how that was done in the airplane, but for 
> ham use, I assume those who are using these units are grounding the 
> chassis to a good earth ground. Does this not mean adding some kind of a 
> terminal lug to this chassis?

In the original design, the transmitter was one part of
a larger system.  It fit into a mounting rack which road on
a shock mount.  The ground lead ran from the mounting rack
to the aircraft body right at the mount.
No need to drill a hole to ground your transmitter;
unscrew any of the screws and use it to attach a ground
lug at that location.


> What we need is for some 
> mentor to publish a new manual or pamphlet on the best current way(s) 
> for getting the ARC-5 transmitter into ham use operation. 
Did you receive a copy of the three posts I made on
getting these units running?

> Anyone ever hooked up an ARC-5 transmitter using an old Johnson Matchbox 
> with balance line?
I ran one into an MFJ matchbox feeding a 200-foot-long closed,
horizontal loop antenna at just a few feet above ground.
It had an excellent signal.  I checked into the 7290 Traffic Net
with it several times, just for "kicks."

> For example, if one already had one of those power supplies indicated in 
> that article mentioned above from the ARRL website, what specific 
> additions to that power supply would remedy the problems?

I can't get the article; it's for "members only."
But I've seen them many times over the years and they're
all about the same- misleading and poorly regulated.
If you can provide 400 to 600 VDC
at 150-200 mA with good regulation and 24 VDC at 2.5 amps,
and build the very simple voltage divider I outlined,
you will have all the power supply you need to run
any "ARC-5" transmitter.

I use the original dynamotor, which is inherently well-
regulated.  I defer to those members of the list that
are smarter than me (there are lots of them ;-) to
advise both of us on the best way to keep a 500-600 volt
power supply happily delivering 150 milliamps without
jumping 60 volts back and forth... fellas?

If you have a unit that has already been drilled and the
relays have already been removed, that actually complicates things.
One of the worst things about ham modifications is that they leave
you not knowing what you've got.  There's no way for me
to tell exactly what's been done to your modified BC-459
unless I can see it first hand, so
it's difficult to advise you.  With an unmodified rig,
I can tell you exactly what's needed.
The article you mention may help you wire the filaments
and key the radio, but IMHO it will be less satisfactory
than using the keying relay.  It can still work, of course.
And I strongly recommend
using DC on the filaments, even if they have been rewired
for 12 volts.
Don't listen to any advice about "de-TVI-ing" or output
tuning in any of these articles; they are universally in error.
Use the techniques for antenna tuning we've discussed.

GL OM DE Dave AB5S