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Hello again crystal set fans.
This is my
rendition of the J.W. Miller crystal radio that was made in the
fifties. I was fortunate to find one of these sets on ebay and
even luckier that I was able to obtain it. You folks that are really
into this diode radio thing will recognize this set. The Miller set
was originally designed as a Hi-Fi AM radio tuner. There was no
earphone connection, only a wire with an RCA connector to plug
into a fifties amplifier.
My radio attempts to copy the circuit closely but using
some of my own ideas. The main difference between the Miller
and my circuit is the use of two of those honeycomb coils that
I had in my junk box. You might recognize these coils from my
#14 and #15 radios.
As usual, my radios aren't very well planned, but seem to work
out in the end. This one is no exception. First, I try to use the materials
that are around my house. The oak base was for a tube set, but I can make
more of the bases. The garolite is 3/16 inch thick and was left over from
my #3 project. I bought the vernier dial a little
while ago not knowing where it would end up. The tuning is a little sloppy,
but it serves well for this set. The capacitor I got in a trade deal. The
shaft was too short to use with the vernier so I added an extension.
As I was fitting the pieces together, I noticed that the vernier dial
would be very close to the base board. I decided that the capacitor
had to be raised. I had a small block that was trimmed and ready to
be finished that fit perfect. As luck would have it, I had already
drilled the holes for the metal brackets in the front of the garolite, but
they were far enough apart so the piece of would fit in between
the two brackets. So you see, not all my luck is bad, just most of it.
The wiring is quite straight forward. I used the fahnestock
clips and solder lugs. I used some terminal strips for tie points and
a little bare wire never hurts the looks. The coils have an extra
winding. It is green in color. I could have removed it but I might
have damaged the coils. Besides it really makes the coils look cool!
The original design used ferrite core coils. The core is about 5/16 inches
in diameter. I didn't count the windings as I wasn't going in this direction.
The coils are mounted at about 90 degrees from each other.
This is to prevent coupling between the two coils. The coils are mounted
to the base with a little assembly that I thought of. There are two
pieces of wood, a half inch square and 1-3/4 inches long. I drilled
a 9/64 inch hole through it for the wood screw that attaches the little
block to the base. I then drilled two 3/16 inch holes partially through
the wood, perpendicular to the mounting hole. I then took two inch and
a half, 3/16 inch diameter dowel rods and placed them in the holes.
After the measurements were worked out, I stained all the pieces to
match the base. I really like how they turned out, if I do say
so myself.
These two 240 micro henry coils have to be fairly close
in value for this set to work right. Mine are well within 1% of each other.
I did this using my inductance meter.
Another method would be if you had a grid dip oscillator hanging around,
plug in each coil and tune a close by radio until you hear the oscillator.
If they tune real close, you are all set. Otherwise when you are
winding the coils keep good track of how many turns you wound.
Next, I had to build the bifilar coil for this project.
I cut a 3 inch piece of quarter inch dowel and
drilled one small hole through the dowel at one end. Then, another hole
2-1/2 inches away from the first. I wound
about 50 turns with two strands of 30 gauge insulated wire, side by side.
The center connection of the coil is made by
taking the two ends on one side of the coil and connecting them
together. If your radio doesn't work and you have checked out everything
else, try reversing the coil connections.
After the set is built and you are receiving stations,
tune to the highest frequency station you can hear and adjust the
trimmer capacitors for maximum. This step is important for maximizing
the signal heard.
The results? Well it sure looks great, but it is low
on performance compared to my other sets. I found the same to be
true with the Miller set. In all fairness, the Miller set was designed
for use with an amplifier. It was built for selectivity, not sensitivity.
I am working on a multi-position switching
system (up to 6 receivers). This will let me quickly switch from
set to set. I will then be able to give you all a better idea of
performance. I will say that this is probably an "urban" receiver.
If you have plenty of strong stations and you need to separate them,
I would say that this is your set.
This radio uses a band-pass circuit made up of the
dual gang variable capacitor, the 240 uH coils and the bifilar
coil. This gives much better selectivity than the single tuned
circuits that are commonly used.
More information on the Miller set can be found at Scott's site.
Best wishes from -- Dave N2DS
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