73, de N2HTT

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Another radio, another VXO

Posted: 09 Jan 2017 08:29 PM PST
https://n2htt.net/2017/01/09/another-radio-another-vxo/


Well, another holiday season has steamrolled past, and as usual the need to
celebrate rose to the top of the charts once again. My lovely XYL is always
on the lookout for a ham radio themed holiday present for me, God bless
her, and this year I was able to suggest something without the usual
churning to find a suitable idea. Starting back in November, I became aware
that the Four State QRP Group (4SQRP), renown for their excellent kit
projects, was about to introduce a new QRP set paying homage to the paraset
spy radios of World War II. I havent built a kit in a while, but as soon as
I heard of this one, I was instantly lit up. What a cool idea: a little QRP
transmitter, and regen receiver, made to fit in a wooden box harking back
to the suitcase sets that went behind the lines in WW II. They had me at
wooden box.
The Bayou Jumper, a Paraset replica in a wooden box.

Of course this modern paraset replica uses a very different implementation
of the technologies incorporated in the 1940s version. Although both the
historical parasets, and the 4SQRP unit (called the Bayou Jumper) make use
of a regen receiver, the modern rig implements the receiver using FETs and
varactors, where the historical rig was all tubes.

The transmitter in the set is basically the 4SQRP NS-40 transmitter sharing
a single circuit board with the regen. The NS-40 wasÂ*another 4SQRP club
kit, a simple but very robust 40 meter transmitter, using a 2N7000
oscillator driving an IRF510 FET PA. It produces a healthy 5 watts, and
features an unusual low pass filter implemented with spiral coils built
into the circuit board, thus eliminating the need to wind two additional
toroids.
Bayou Jumper board, spiral coils visible upper right.

There is a wealth of information about both the Bayou Jumper kit, and the
historical sets available on the kit website. There is also a Yahoo
discussion group, with a lot of building resources and hints, and links to
some historical documentation of the parasets of yore. Take a look at the
files section of the Yahoo group for some goodÂ*sources of historical info
about the parasets. At the time of this writing, the first run of 100 kits
is sold out, but they are rekitting them for a second run immediately, so
keep an eye on the website if you are interested.

While perusing the historical info, I learned that the original suitcase
sets were designed to be portable and weighed 42 pounds. By the end of the
war a few years later, that weight had dropped to 9 pounds! And of course
the replica weighs much less than a single pound, including the wooden box.

So I first heard about the BayouJumper right at the end of the November,
and immediately checked the website for availability. Coming soon was the
intel at the time, but unlike other kit radios, this one had a built-in
parallel project: the wooden box. The radio is designed to fit a specific
hinged wooden box available from HobbyLobby. If you are lucky enough to
live near a HobbyLobby retail store, you can purchase just the box that
fits the radio, ordering online you have to buy the set of three, which
leaves open the possibility of other wooden packaged projects. Of course,
the woodworkers among us could home-brew something fancier (and have, check
the photo section of the Bayou Jumper Yahoo Group), but for me the prospect
of obtaining the box, and decorating it provided a pleasant diversion while
waiting for the kit to become available.

The Yahoo Group provided a wealth of advice on how to finish the box, which
I took literally. I sealed my box with several coats of Minwax
PolycrylicÂ*finish, and then used their Polyshades stains over the clear
coat. I went through a number of colors I didnt like, and wound up with a
light coat of Mission Oak, with a heavier coat of Honey Pine over it. All I
can say is that the samples at the hardware store look nothing at all like
the color I got. Mine turned out looking like a pine box with a layer of
travel grime over it. Im not complaining.
Wooden box with travel decals

Pictures of the project prototype showed it plastered with vintage travel
stickers, miniaturized to fit the scale of the box. These turn out to be
home-brew decals! A very interesting, and potentially useful technology.
You purchase you sticker images in the form of downloadable PDF or JPG
files (I got mine on etsy), and print them on special decal paper, which is
available not surprisingly from decalpapers.com. After printing your images
using high-quality photo settings for the printer, you spray the pages with
several coats of gloss plastic finish like Krylon. After they are
thoroughly dried, cut them out, soak briefly in cold water, and slide into
place on the object to be decorated. Couldnt be easier, and it actually
worked. I finished with a light coat PolycrylicÂ*to lock them in place.
Decorated box (romance side.)

With the box all prepared, and with more time to wait until the kit
shipped, I starting thinking about how to accessorize my as yet unbuilt
paraset. Shipping along with the kit are these neat little boards that
allow you to solder on a modern HC49/U crystal, and attach two pieces of
#10 solid wire to make a board that will plug into a crystal socket
intended for an FT-243 crystal.
Both sides of HC49 to FT-243 crystal adapter board.

Photos of these little boards on the Yahoo group showed a variation with
pads for a trimmer capacitor and an inductor in series, to make a
pluggable, pullable crystal. With my recent fascination with VXOs and
crystal pulling still running high (see last post), I got the idea to build
a pluggable VXO for the SKCC frequency of 7.055, using a bunch of shorty
HC49 crystals, an inductor and a polyvaricon cap. I used a piece of single
sided PCB, and just isolated the pads with a Dremel tool by hand. The
circuit is very simple: just the variable cap and the inductor in series
with the crystal. I used five crystals in parallel putting them in
parallel makes them easier to pull and increases the frequency range you
can get. Also those little shorty crystals dont have a lot of quartz in
them, paralleling them increases the current they can handle.

My finished VXO used a 10 uH choke I had lying around, a 200 pF
polyvaricon, and gives a range of about 3.5 kHz. Just enough to dial in
that guy calling CQ almost but not quite on your frequency. Ive used it
with the W1TS simple transmitter with very good results.
7055ish pluggable VXO

Finally it was just a matter of watching the website for the release of the
kit, hoping it would be in time for a Christmas delivery. By dint of
checking my phone every 30 seconds for 10 straight days, I was aware of the
kit release pretty promptly on the first day, Dec 18. I ordered pronto, and
by Thursday before Christmas, had my kit in hand to be wrapped and placed
under the tree. The minor obsession with early ordering turned out to be
worthwhile the first kit run of 100 was sold out in five days. Fear not,
4SQRP is doing addition kitting runs, and the kits will be available again
soon. Watch the site.

Santa came through, and since I wanted to use the radio from the alt-qth
later in the holiday week, I needed to get started on the build right away.
The kit is straightforward to build, and the documentation is very good.
There were a few minor inconsistencies, but checking the Yahoo Group
resources before building should alert the builder to any gotchas. I am a
slow kit builder: I check the value of each part with a meter, and mark up
both the parts list and the schematic as I progress. It took me 5 hours
over two sessions to complete the build, but I imagine a faster worker
would take much less time.

Everything worked at first power up but some major tweaking was needed. The
transmitter section was fine, working as advertised and putting out a solid
5+ watts into a dummy load. The regen was going into and out of oscillation
with the regen control, but I could not find the oscillator signal at all
using my KX3. After reading on the reflector that it could be quite a ways
above 7.3 MHz, I tried again, and finally found it up at 7.752!

Jim N5IB, one of the designers of the kit has been offering guidance on the
reflector, and indicated that moving the regen down into the ham band was
accomplished by either using the trim cap, or squeezing the turns together
on the toroid. I wasnt having any luck with the trim cap, which is only a
20 pF adjustment, so I took a look at the toroid on a photo of a completed
board on the Yahoo group. Big difference, so I squeezed the coils to make
them look like the photo, and that did the trick.
Toroid before squeezing
Toroid after squeezing

I wound up with a tuning range of 7.030 7.175 which is kinda high, but
covers all the areas I am interested in. While I was testing with
everything opened up, it was going from 7.015 up, but it seems to have
jumped up once everything was in the box. Jim provided a hint on moving the
range down a bit adding a few pF across the base of the trimmer cap, and I
may revisit this as some point. But for now it is working fine and I
decided not to mess with it further. I painted the coil with some home-brew
coil dope (artisanal) since I wasnt able to get my hands on any clear nail
polish.
Artisanal Coil Dope

My regen goes in and out of oscillation at about 75% of the pot range, and
I wasnt able to budge it with the corresponding trimmer cap. Anyway, I can
get it to go into and out of oscillation at both ends of the tuning range,
thats all it needs to do, so again I decided it was good enough.

While hunting for the tuning limits, I was turning the rig on and off by
plugging and unplugging the power cord, and I broke one of the tabs on the
power jack. It had been mentioned on the Yahoo group that they are fragile
and not to bend them too much on installation. Luckily, it was the ground
side that broke, and I was able to solder the wire directly to the ground
side of the jack. That was the worst mishap I encountered with the build,
everything else went smoothly.

Well one week after Christmas is famously Straight Key Night (oh yes, also
Hogmanay) and what better time to try out my new Bayou Jumper on the air. I
have collected a fair number of crystals over the years, and so have lots
of frequency options available on 40 meters. Because of SKN, the CW portion
of the band was happening, leading to a textbook example of:
N2HTTs Axiom of Operating Rock-bound

For any given crystal frequency there will be either:

A. Someone calling CQ exactly 500 Hz above or below you transmitter
frequency with his passband cranked down tight, or:


B. Someone exactly on your crystal frequency, in hour two of a four-hour
rag chew.


The upper end of 40 meters was too busy to shoe-horn in, but luckily I have
a few rocks up in the old Novice CW portion of 40 meters, above 7.100 MHz.
The Bayou Jumper is shipped with a 7.122 crystal (apparently the QRP-ARCI
frequency) so thats a good spot to look for them. I finally chanced across
Tommy, AA4TB, calling CQ spot on 7.1175, one of my rocks. He was also QRP,
running an HW-9, and we had a short but pleasant contact although he was
faint and there was lots of QRM on my side. He must have been mystified
when I told him my rig was a Bayou Jumper, and I was so excited that I
forgot to send my SKCC number, but all in all, an excellent first QSO. A
while later I worked Bill, KB0BWY on the same frequency, for my second and
final SKN contact this year. The QSO with Bill is the current standing
distance record for my Bayou Jumper at 1400 km.
On the air for SKN!

Aside from breaking in the Bayou Jumper, SKN was kind ofÂ*a bust for me this
year as suddenly, on Friday afternoon (Dec 30th) I completely lost the
ability to be heard on the air. I usually check for my signal on Reverse
Beacon Network, but at some point Friday I just disappeared. I tried
different rigs, actually put up a new antenna (40 meter sloper dipole),
went QRO, switched coax, but all to no avail. I wasnt getting out. It wasnt
until Sunday afternoon that I took the MFJ tuner out of the circuit, and
just as suddenly I was back! Something must have fried in the tuner; I have
it on the bench but havent had time to check it out yet. Unfortunately it
was the last item I switched out, so I was down for most of the event.

After playing with the Bayou Jumper for a few days, Im beginning to
understand how the hams of yore could operate successfully using simple
regen receivers. Ive built a couple of regens, which work great, but I just
couldnt figure out the interplay between transmitter and receiver. Regens
are so sensitive, they get blown away by the local signal when you
transmit. Forget about hearing yourself in the receiver as a sidetone, or
spotting you transmit frequency.

The Bayou Jumper revealed the secret to me: when you are transmitting, turn
the receive audio OFF. Works like a charm, just mute the darn thing. No
need to spot your transmitter, just get the receiver in the neighborhood of
where you are calling, and sweep up and down a few kHz looking for a reply.
It couldnt be easier. I was definitely over thinking the whole process. I
am going to try my old regens out with one of my home-brew transmitters, to
see if I can get this to work.

Oh yes, one other thing: the Bayou Jumper has no sidetone. Actually not a
big deal, your sending will be fine even without one. Where were going, we
dont need sidetones

73 (and Happy New Year)

de N2HTT