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Hello again radio friends. Since this is my #50
radio, I thought I would make it a big one.
It is much like my
#35 radio that I built last year.
I liked that one so much that I decided to build another. I can't
think of a better crystal set design for DX. It is kind of a
"$X" set too (Dollars Unknown).
Since I made my
#35 and #48 sets, I found
that this is about as good as it is going to get!
This radio turned out very well. Plans? Plans? I have no
stinkin plans! I wasn't even sure of all the parts I was going to use
until the last moment.
I had these very expensive capacitors I wanted to use in a project. So now
I present to you my #50 high performance DX crystal receiver.
My DXceiver is built in a modular style. That is, the antenna
tuner is on one board and the detector is on another. This is so I
can separate the coils for the best coupling. The base is 6 x 15
inches (15 x 38 cm). This gives me plenty of room to construct
the radio.
The antenna tuner uses a four gang, 250 pf per gang variable
capacitor. This is a high quality, low loss type with ceramic insulators.
Using such a capacitor insures that the Q, or tuning sharpness gained
by the large litz wire coil is not destroyed. Mounting the capacitor
on ceramic or styrene standoffs is a must. Also an insulated shaft
coupling is also essential. If your shaft is metal, then when you place
your hand near the knob, the receiver will detune. This is called the
"hand capacitance effect".
If you don't have a four gang capacitor, just use a dual like
I did on my #35 set. I added the link switch to
be able to switch one gang either across the tank circuit or across the
antenna capacitor. The switch can be left open for better tuning at
the high end of the band. However, I found there was not a lot of
advantage in this arrangement, so if you want to leave it out, you
won't miss much.
The coil is made with
660 strand, 46 gauge litz wire. The inductance
is about 150 micro henries. I made
a styrene spider coil form 6-1/2 inches (16.5 cm) wide. The bottom has holes to
attach to the base with a small piece of wood. Styrene is a very
good rf insulator, and your coil will be very efficient. Careful
dressing of the wires helps the efficiency too. The terminal block/link switch
is made with styrene too.
The detector board also has a 660/46 litz coil, but this coil is
larger. The width is 7.75 inches (20 cm). The inductance is a little over
250 micro henries. A very high quality 440 pf tuning capacitor is used. The
tuning range is very wide (about 480 khz - 1.8 mhz) due to the very low
distributed coil capacitance. A 365 capacitor will most likely give you
the full band coverage.
With such a sharply tuning system, using
vernier dial drives
is essential. If you don't, the tuning will be difficult and you will
miss a lot of stations. The white styrene dials are attached to the vernier drives
with two small screws. I made the dial 4 inches in diameter (10 cm). This
is large enough to put good scale markings on the dial. I found that
putting frequency markings on the dial is best for quickly finding
stations. You have to be a little careful putting these markings on
the antenna tuning unit. Changing the antenna, will shift the dial somewhat.
However, when I recently doubled the length of my antenna, I found it
made very little difference. The detector unit will not shift because of
a different antenna.
Moving right along. . . , the selectivity is controlled by a differential
capacitor. This is the "Hobbydyne" circuit recently developed by
Jim Frederick in Florida. The Hobbydyne allows for a very light coupling
to the tank circuit by the detector diode. This increases the circuit
Q, thus making a very sharp tuned radio. And folks, this does tune
sharp. If you don't have a differential capacitor, you can use a
regular small value variable capacitor between the tank and the
diode switch.
I have been putting in diode selecting switches in all my
high performance sets. This gives me a good way to test and evaluate different diodes.
My favorite diode overall is the
FO-215 diode made 40 years ago or
so by ITT. At the top end of the band, I like to use a schottky
diode as better selectivity can be achieved with this diode. I also
have a 1N34A as my benchmark diode.
After the diode switch (which should be a low loss type), there
is a resistor with a capacitor in parallel. This is called a
"Benny". This circuit
provides the same DC load to the diode as the impedance that the
transformer provides (100,000 ohms). This resistor value isn't real
critical. The capacitor offers a low impedance to audio frequencies,
and that energy gets transferred to the transformer.
The audio matching transformer I used is a surplus 100k ohm to
1500 ohms. This gives me a good match to my sound powered phones.
This part will be difficult to get, so I would recommend that you
use a Bogen T725 auto transformer instead.
For the price, it's the best thing going. The transformer is used
to match the fairly high detector impedance to headphones. This is
another one of those essential parts.
A feature I added was a method to disconnect the internal
transformer so I can use external types. If you aren't going to be doing
that kind of experiment, you can wire the radio output directly to
the transformer input (O to X terminals).
If you are familiar with crystal set reception but have never
used a large litz type set, you are in for a treat. This baby does tune
very sharp. I hear a station on every frequency! At night, the frequencies
between 1230 and 1490 are filled with a lot of low powered stations.
Once you tune to a frequency, you have to sit a while until one station comes
up out of the mud. It is a lot of fun to DX with a crystal set.
Ok, that's about it for this radio. I had a lot of fun building
this one. I have fun building them all! My #51 set is already to be
started. This one will not be a solo effort. I will have a little help.
It should be ready in about a month. If you have questions about the
sets I have built, please use the contact link at the bottom of this page.
73 and good crystal DX. Dave - N2DS
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