A Continuing Mystery:
by Dan Petersen, WA6OIL
On 17 July,
1932, the Brisbane, Australia “Sunday Mail”
newspaper ran an article about the “Mystery Crystal
Set” in their “Our Wireless Circle” section. This
set differed from many of the standard designs of the day by
having a tuning coil with a detector winding co-wound within
the primary. This type of winding, also called a bifilar
(bye-FYE-lar) winding resulted in very tight coupling between the
primary and detector windings. The antenna and ground connections
were also unusual in that the antenna could be connected either
to the rotor or stator of the tuning capacitor. The ground
connection was not to the primary coil at all but to the common
side of the detector coil. This was a rather interesting
departure from the “normal” crystal set.
Figure one shows
the diagram of the original Mystery set. The set is pretty easy
to build and makes a nice set for experimentation. The only
tricky part is winding the coil. With a bit of patience it is not
much harder than a regular coil to wind. As can be seen the
primary and the tuning capacitor forms a parallel tuned circuit.
The antenna can be connected to either side of the tuned circuit,
causing the set to perform quite differently depending on the
antenna connection. The detector winding is electrically similar
to other designs except that the ground connection goes to the
common side of the detector circuit. Electrically, this puts a
small capacitance in series between the ground connection and the
tuned circuit while providing a solid DC ground for the detector
circuit.
Figure two shows
the physical layout of the coil. Winding this coil will require
#24 enameled wire for the primary and #30 enameled for the
detector winding. The paper coil form I got from a fabric store
that they had wound lace on. It is 3.25” in diameter and
2” long, The best way to make the coil is to take the form
and drill two 1/32” holes about ¼”apart, in line with
the windings, near one end. You will have to drill the other
holes as you wind the coil so it may be easier to make this a
two-person job. Start the #24 wire at “P1” just as if
you were winding a normal coil. Wind 12 turns onto the coil. Now
for the tricky part. You need to drill two more holes in the same
fashion as the first holes just beyond the end of the coil. Once
done, thread the #30 wire through the holes just made (position
S1). Now you must wind 25 more turns while feeding BOTH the 24
and the 30 wire so that they lay next to one another without
twisting or overlapping. At the end of this step (position S2),
drill two more holes and secure the #30 winding. Once done, wind
13 more turns of the #24 only and finish by drilling two holes to
secure the finish end of the coil (position P2). I said it was
tricky!
The rest of the
set is built in a straightforward manner. The secondary connects
to the detector circuit and the ground connection hooks to the
opposite coil connection from the detector. Note I say detector
instead of diode. You may want to use a catwhisker detector or a
germanium diode - this set works well with either.
The first
departure I made from the original design was that I used a
dual-gang 400pF tuning capacitor instead of the original
single-gang unit. I wanted to be able to lower the tuning range
if I so desired. This led to an accidental discovery that I will
elaborate on shortly. I found the operation and performance of
the "classic" Mystery set to be pretty much as the
article stated. I feel it is a very good performer and well worth
the hassle of winding the coil. The difference in performance
between the two antenna connections was striking. One side gave
me good selectivity and low but usable volume and the other side
gave me gave me a lot of volume at the expense of a good portion
of the selectivity. Here's where the accidental discovery comes
in. While connecting the antenna to the set I accidentally
connected it to the stator of the unused gang on the variable
capacitor instead of the antenna connection. What happened amazed
me. The set would now tune very sharply AND have good volume. I
double checked the connections and discovered my
"mistake" in the antenna connection. I wish to note now
that results could and will vary depending on about a dozen
different variables, most notable of which is the type of antenna
you have and how good your ground is. My antenna is 200 feet long
and about 30 feet above the ground.
Figure 3 shows
the schematic diagram of the dual-gang Mystery set. The antenna
connections go as follows: "ANT #1 connects to the same
stator terminal that the coil is connected to. The "ANT
#2" terminal goes to the frame of the capacitor and
"ANT #3" connects to the other gang's stator. The
detector circuit is the same as in the first version. The
dual-gang variable capacitor I used is available through the Xtal
Set Society.
Figures 4 and 5
are photos of the unit that I built last winter. As can be seen
in the photos. The only controls you have is the tuning knob on
the front panel and the various antenna connections.
The latest
improvement on the Mystery set was to take the variometer I
featured in a previous issue of the newsletter (Various Notes on
the Variometer) and connected it in series with the antenna and
the antenna connections on the Mystery set. Having a variable
inductor in series with the antenna introduces another tuned
circuit into the mix and makes tuning a real adventure. Adjusting
the variometer affects the tuning on the Mystery set and vice
versa. The advantage now is that you now have six different tuned
circuit configurations to choose from. Yes, I know that I am
being vague about which setting is best because the performance
will vary greatly from place to place. I have sets that work
great out here in the puckerbrush and work completely differently
in town at Mike Peebles' laboratory. I won't go into a great deal
of theory as to the operation of this type of circuit but the
variometer, when hooked between the antenna and antenna terminal
#3, comprise a series tuned circuit. This has an effect on the
electrical properties of the antenna and seems to greatly improve
the tuning capabilities.
Last spring I
took part in a crystal radio DX contest using this setup. During
the week that the contest took place I logged 33 stations in
seven States, two Canadian provinces and Mexico. The farthest
station logged was WOAI in San Antonio, Texas, a distance of 1725
miles from my location in southwestern Washington state.
How do I rate
this little Australian bugger? I would give it high marks on
performance, the recommendation that intermediate to advanced
builders give it a go and six brain-teaser points for tuning
interactions. If you have never wound a coil before, make some
regular coils before you tackle this one.
The plans for the
Mystery crystal set can be found at http://www.clarion.org.au/crystalset/mystery.html
. If you have questions on this project you can get a hold of me
at petersen@worldaccessnet.com
.
Dan Petersen,
WA6OIL
La Center, WA