 Back
in May, 2000 appeared in the hallowed pages of the Newsletter my first
article, “It Came From the Junkbox”. This was a set in which I picked up
DX for the first time – a true thrill indeed. Before that time I always
thought that anyone who claimed to have heard DX on a crystal set had been
hit by lightning once too often while stringing antennas. Now I am a believer
as I have heard an amazing amount of stuff with the different sets I have
built. For a while I have wanted to build a DX set that incorporates several
of the tricks I have learned in my tinkerings. In this light, I designed
a circuit that uses the best of the “Mystery” set, The “Like-A-Flash, Senior”
and a new idea (for me at least), the use of a large air-wound tuning coil.
Another feature I would find handy is some way to spread out the upper
portion of the Broadcast Band. Last but not least I wanted to have a switchable
detector where I could jump between a diode and a detector stand.
The construction of the “SoICFTJ” is going to require some explanations
so bear with me. We might as well start at the antenna and work our way
back. Referring to the schematic, we see that there is an antenna and ground
connection that goes to the coil-capacitor circuit L1 – C1. C1 is a dual
gang 365pF variable available from the Xtal Set Society. L1 consists of
a 4” by 3/8” diameter ferrite rod that is available from Amidon Associates.
Something else you can do is to go down to the local Salvation Army thrift
store and buy a radio for 50 cents and pull the ferrite rod out of it.
Take off windings until you have 50 turns left and there you go. For those
of you who have the ferrite rod I first described, cover the core with
paper or masking tape and wind 50 turns of #24 onto the core. I secured
the ends of the coil with a bit of tape and then covered the whole shebang
with heatshrink tubing.
 Click
on this Image for a larger view for printing.
PARTS LIST
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C1 – 365pF dual-gang variable capacitor
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C2 – Dual-gang variable capacitor: Section “C2a” = 90pF, “C2b” = 340pF
Available from this website...
-
(Click here) Part stock number
(22-0003)
-
C3- .001uF to .002uF disc capacitor
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D1 – Germanium diode (1N34 or equiv.)
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L1 & L2 – See text
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R1 – 15,000 ohm ¼ watt carbon resistor
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S1, S3, S4 – DPST switches
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S2 – 2 pole, 6 position rotary switch (Available from this website
(Click here) , Part stock number
19-0013)
As seen in the included photos I have placed the
coil L1 on a wooden “channel” in order to be able to move L1 into L2 in
a “Loose coupler” configuration. I left the coil leads long enough to allow
free movement. The coil L1 is used in conjunction with C1 and S1 to form
a switchable antenna tuning assembly. When the switch S1 is in the “SERIES”
position, L1 and C1 form a series circuit between the antenna and ground.
Flip the switch to “S/P” and the antenna circuit is tuned with a series/parallel
tuned circuit, similar to one of Mike Tuggle’s sets. My thanks to Mike
Tuggle for his input. This type of tuning has been used in many circuits
including my “Mystery Crystal Set”. Performance will vary with the
type of antenna and ground system so there are no hard and fast rules in
the operation of this circuit. Try both series and series/parallel modes
while tuning. I find one way works well on one end of the band while the
other mode works best at the other end.
 Construction
of L2:
I wanted to have a large, high-Q coil for the main tuning coil, referenced
as L2. I also wanted to have as little dielectric effect as possible so
I came up with a scheme for an air-core inductor. I took a piece of ¼”
Baltic birch plywood and cut four disks 3-1/2” in diameter with 2-1/4”
holes cut into the centers. I then cut out eight notches, each 1/8” wide
by ½” deep into two of the disks. I then glued one of the notched
disks onto each one of the un-notched disks. While those were drying I
prepared eight ½” by 5-1/8” strips out of lexan plastic. Once sanded
to shape I taped all eight together and cut small notches every 1/16” into
one side of the strips, just deep enough to hold the wire steady. When
the end plates were ready I assembled the whole thing by epoxying the lexan
strips into the notches and clamping the whole assembly to hold it in alignment
until it dried. Later I varnished the end plates and got ready to wind
the coil. The coil was wound by drilling a small hole in each end plate
near the outside edge and looped the wire through to anchor it. Then I
started to wind. Once I finished a turn I moved the wire over one notch
and continued winding. Once finished I looped the wire through the other
small hole and voila!, a nice, big air-wound coil. I used 74 turns of #20
wire and it came out right on the money at 240uH. This being a tapped coil,
I scraped off the insulation every 10 turns and soldered a bare wire to
the coil. I then ran the tap wires to the switch S2. Please refer to the
detail photo to see what I did.
 And
Now . . .The Rest of the Story:
I used one of Mike Peebles dual-gang variable capacitors for the main
tuning control. These beauties have 340pF on one gang and 90pF on the other.
They also sport a plastic tuning shaft and a stout metal frame. I wired
the 340pF gang to a switch, S3, to enable me to switch this gang in or
out of the circuit. The 90pF section remains in the circuit all the time.
With the switch closed (“LO” position) the set tunes over the entire Broadcast
Band. Switching the 340pF section out of the circuit (“HI” position) spreads
the upper third of the BCB over the entire 180-degree swing of the tuning
cap. In a “regular” circuit you will find that the upper half of the band
is scrunched into the last 20% of the tuning range. The “bandswitching”
reduces this problem considerably.
 The
remainder of the circuit is relatively straightforward. The detector select
switch, S4, is connected to the common terminal of S2, the detector tap
select switch. This switch allows me to select between a catwhisker detector
and a germanium diode. You may note in the photo that the diode assembly
consists of two alligator clips mounted to a terminal strip. I can easily
change diodes for evaluation purposes with this gizmo. The eagle-eyed will
also notice that there is no crystal in the detector mount and that it
is not even connected to the rest of the circuit. This picture was taken
when I had the detector mount temporarily disconnected. I like to have
both detector systems in place on a DX set for there are times that a catwhisker
detector works better than a germanium diode! Theory dictates that this
is not possible but theory also says bumblebees can’t fly. Anyway, most
of the DX chasers include a catwhisker detector in their arsenal of tricks.
The ability to change diodes quickly also adds to the versatility. Capacitor
C3 acts as an RF shunt while R1 guarantees a DC path to ground regardless
of the type of phones you use.
 Operation
of the SoICFTJ:
Once I got the whole whiz-bang together it was time to try it out in
the real world. I connected it to my wind-damaged “RMS Titanic” antenna
and the ground rod. One end of the antenna is about 3 feet off the ground
and the other is still up about 30 feet. It sags badly and is spliced in
two places but I still get signals in. I’m gonna have to get up on my hind
legs and fix that bugger before winter sets in. I set the switch S1 to
“SERIES” and set the antenna-tuning cap C1 about half meshed. S3 was set
to “LO” and the detector switch was set to “DIODE”. Pop a set of cans on
my ears and let’s see if this thing hears. Tuning across the band I find
that the set has exceptional selectivity and good volume. I can easily
separate local stations at 750 and 860 KHz. You may have noticed in the
overall picture of the set the oversized knob on the tuning control. This
is for a finer “feel” when tuning. I am teetering on the edge of mounting
a vernier drive on the tuning control. A large knob is a good way to go
if you can’t get a hold of a vernier.
So the set works fine in the daylight. Let’s see how it dances in the
dark. After nightfall I tried again. As it was still early evening in the
summer I didn’t expect much. Much to my surprise I picked up KGO-San Francisco,
KBOI-Boise, KFBK-Sacramento and numerous “peanut-whistles” in rural Oregon
and Washington. I tried out the “HI” position on the band switch and found
that the upper part of the band was nicely spread out. The “HI” band starts
at about 1100 KHz and extends to 1750 KHz. I can hear the telltale murmuring
of numerous stations in the background so I stay where I am and listen
to see if any one of them rises out of the babble. One does a bit and it
turns out to be Fresno, California. Not bad for a summer’s eve. As with
all sets in the genre, this one has tuning interactions between the positions
of the variable capacitors and the position of L1. You just have to work
with a new set until you get used to its idiosyncrasies. Logging the positions
of the various controls when you get a station tuned in helps a lot.
This set was a challenge to build but I consider it a job worth doing.
The tuning coil was the star of the show in my estimation closely followed
by the suggestion by Mike Tuggle to use a ferrite rod coil for L1 rather
than an air-wound coil. I always like to have a tapped tuning coil as it
gives the operator an added edge when the balance between selectivity and
sensitivity has to be modified. It’s a winner in my estimation.
Dan Petersen – WA6OIL
La Center, WA
e-mail: petersen@wa-net.com
True wit: Do not be afraid to try something new. Remember, amateurs
built the Ark and professionals built the Titanic. |
