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Hi there! This is another of my
recently revised radios. Not that I built them wrong to begin with, but
I come across new and better materials to make my sets. I don't mind
going back and revisiting a set now and again.
Building on the success of my last radio,
I proudly offer this crystal set for consideration for your next
construction project. This is another one of my dual circuit
crystal radios. The idea for this set comes from Mike Tuggle's
Lyonodyne 17
and experience that I have gotten from my past sets.
I found that with a very simple switching system, I can make this
radio's circuit act like two different ones. I have wanted to build
a Lyonodyne type set for a while but couldn't resist adding the extra
switch.
This radio features the use of three variable capacitors.
While at first, this is quite a chore to tune, after a while you would
get used to where the dial is to be set for certain stations. You also
have a choice of two radio circuits in one radio. One is the single
coil that works very well and is featured in my
#12 and other sets. A hidden feature of this
radio circuit is that the unused coil / capacitor
becomes a wave trap! This wave trap
can be used to reduce the signal of a loud interfering blowtorch
station, allowing you to hear other, weaker stations that are
close in frequency.
The two coil receiver is similar to the Lyonodyne
circuit. The series tuning capacitor has been transposed so it
is in the antenna line, rather than the ground line. I also decided
to use two capacitors, rather than one capacitor. This increases
the tuning complexity of this radio. The circuit was explained
in some forums, but I just couldn't wrap my arms around using
one ganged capacitor, so I used two. Since
this is my radio, I built it my own way. It is kind of a "german"
in me.
The antenna coil is the original one I used in this
set. I could have used 660/46 litz on both coils, but this works
dandy the way it is. The best inductance in the antenna coil will
depend on your antenna. It is cut and try but you have leeway.
My antenna is a single wire about 75 feet long.
The detector coil is made from 660 strand 46 gauge
litz wire. You can find it for sale here at
1n34a.com.
This is the first coil I wound with the big litz. It came up short from
the 240 µH
that I wanted for my other project but will work pretty well in my
single / double coil arrangement. The worst case is that the
low end of the band can't be tuned without an extra capacitor.
The coils are mounted using dowel
rods and 1/2 inch square wood blocks. I have used this scheme
in my #23 and
#16 sets with good results.
I made it so the antenna coil could slide to adjust the amount
of coupling used. Most of the time I use them fully apart for
the best selectivity. If you experience real weak reception,
you can move the coils closer together.
My revision included changing the detector circuit to match
the extra high performance of the detector coil. This included using
the 100k to 1.5k ohm audio transformer. You can pick out the transformer
that meets your headphones requirements, or build a
matching box. The coil and
the transformer turn this set into a high performance radio. If you
can, try to find good quality variable capacitors. The ones commonly
available really aren't that good in high performance radios. The capacitors
I selected have a ceramic insulation on the stator.
The circuit details is where it starts to get interesting. Look
at the circuit below while I rattle on about this set. When the
switch is set to double, you see an antenna input, series capacitor
connecting to a coil / capacitor parallel circuit. If you set
the switch to single, you see an antenna input, series capacitor
also connecting to a coil / capacitor parallel circuit. In the
case of the double switch position, there are two tuned circuits,
compared to a single tuned circuit in the single position. I just
found that highly interesting and also helped me in how I should
tune this set.
Here is how I tune this radio. There are three
variable capacitors, the series (in series with the antenna),
the antenna tuning (across the antenna coil) and the detector
tuning (across the tapped coil). Tuning to maximum means that
the capacitor plates are fully meshed and minimum means the
plates are open. Ok? Here we go.
Single position tuning instructions: Tuning starts
at the high end of the band and works downwards. Set the
detector and series capacitors fully open.
Fully mesh the antenna tune capacitor
( if you are near the low end of the band, this capacitor should
be fully open as it is a wave trap). Rotate the series capacitor
as you listen for stations. If you hear a strong station,
and want to increase the selectivity (at the cost of some sensitivity),
mesh the plates of the detector capacitor a little and then peak the
series capacitor to maximize the signal. After you make your first
pass, set the detector capacitor at a third of rotation and start
tuning again. Do this until you have reached the bottom of the band.
The antenna capacitor can be tuned to tune out a loud station so
you can hear others. Try it.
Double position tuning instructions: Tuning is a lot
like in the single position. Set the antenna capacitor at minimum.
Rotate the detector capacitor a little at a time while sweeping
the series capacitor. You will be tuning starting at the high end
of the band. When you hear a signal, then start peaking each
capacitor starting with the antenna, then detector, then back to
series. Keep repeating that until the signal is at full strength.
Having numbered knobs and keeping a log of the settings helps finding
the station again.
As a little "extra", I have included a little circuit
to use if your sets don't tune the full range. It is a padder
arrangement. Instead of hooking two gangs of a capacitor together
for full range tuning, adding a padder capacitor will allow you
to tune only to the bottom of the band. That way, you can spread
out your tuning range to fit the full rotation of the capacitor.
It does require the use of a 2 gang capacitor.
I haven't talked too much about construction details
on this page. I have a new
construction hints page on my site.
Also, take a look back through my previous sets for little nuggets
of hints. If you have a question or two, or would like to chat
about crystal sets, I am only an e-mail away.
Best wishes and good DX. Dave - N2DS
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