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Update! I entered this radio configured as a hobby class set
(single coil set with a wave trap) for the
2006 Crystal Set Receiving Contest.
Fasten your seat belts, friends and neighbors. This will be a wild
ride. I am proud to introduce you to my new Multi-Radio. I didn't know if I
should list this in my tube or crystal section, so you are seeing it in both.
I have built modular sets before. They had a separate antenna tuning unit
that could be moved from one radio to another. I am taking this a step further by
using interchangeable circuit modules. In seconds I can
change my radio from a crystal set to a one tube radio. Not only can I select the
detector type, but also if the radio is to have one or two tuned circuits.
I may build a transistor module too. I got the idea for this set from my
breadboard project. The mechanics of that
project worked so well that I did the same with this project. Each chassis base,
made from 1/8 (3mm) thick garolite is fastened to the oak base with two thumbscrews
and two T-Nuts under the base. By removing the screws and disconnecting the
circuit from the variable capacitor, a new board can be placed in that spot in
seconds. I have become a real quick change artist.
Why did I make this radio? Perhaps my shrink, Doctor Peebles could answer this better
than I, but I will attempt an answer. It has to do with cost. The two most
expensive parts in my dx crystal sets are the big litz coils and the premium
quality variable capacitors. By making a board with one LC circuit, I am able
to reuse these parts over just by making a new receiver module. Several of
you have commented that Schmarder guy is pretty tight with his money. This proves it.
If you would like this kind of economy, several great radios with only one
litz expense, please continue reading.
The detector board is 17x8 inches or 43x20 cm. The variable capacitor is
one of those 350 pf "holy grail" type capacitors. It combines all what is
best in a capacitor, ceramic insulators, silver plated plates and a great wiper
arm. These are hard to come by, so keep an eye out for them. The coil is
made from 660/46 litz wire. The form has a
2 inch hub (50mm) with a 7 inch outside diameter (18cm). There are 48 turns
on this coil, producing an inductance of around 240 micro henries. The form
is made from polystyrene. If this litz is outside your budget, 165/46 litz will
work ok too. Put the rest of the money in a better variable capacitor.
By the way, a vernier drive is essential for the easy operation of this
radio. Tuning 120 channels over 180 degrees rotation is tedious.
I would like to describe the tube module first. This is a single
1625 tube receiver with only 12 volts on the plate. How does it work with so low voltage?
I feel it is the high current heater that boils the electrons off the cathode
in such an intense manner that any plate voltage will attract them. For those
of you that don't want to take chances of electric shock with high voltages,
this is the project for you. I drew two diagrams. One is how I wired my dx
set, and the other is a "lite" version that will still work great. The circuits
are below. My comments only relate to the full schematic.
The 4 diodes is an audio peak clipper. In my
past dx radios
I had trouble with noise pops causing near deafness. The diodes limit the
peaks. There is also a volume control. Sometimes
I turn the regeneration control way up to narrow the receiver response. That
also makes the radio much too loud. It is good to be able to control the volume
independently.
This set uses the famous Bogen T725 transformer as a match between
the tube and earphones. A three position switch selects the tap. You can
use your favorite taps, or just forget the switch and connect to one directly.
The tickler is made from a small spider coil.
There are 6 turns of 40/44 litz wire wound on the form. This coil is about
an inch (24mm) from the main coil. The regeneration control is set 1/2 to
2/3 up on the pot, which is the optimum range. If you have too many turns
on this coil, the audio will be lower at the point of oscillation. The
regen pot operates very smoothly too. Proper phasing between the main
tuned circuit and tickler is required. If your set doesn't go into oscillation
when the regen control is turned up, please turn the tickler coil around.
The tube module is powered by a 12 volt @ 500 ma wall wart power
supply. Since the heater draws 450 ma, and the rest of the radio only a few
milliamps, this is a perfect supply. If you need one, I have
a bunch of them. Since under near full load
there is some hum ripple present, I added a filter network to smooth
the dc. That filter network consists of a 560 ohm resistor and
a 1000 uf capacitor. The resistor isolates the part of dc to be filtered
only to the plate and screen supply.
This story is kind of long, isn't it? Now it is time to describe
the crystal radio module for my multi-radio. Again, I used
ideas from my past radios to make this set. My crystal detector uses
a true Hobbydyne
circuit. It uses a differential capacitor, like Jim used. Differential
capacitors are hard to find, so go ahead and use a single trimmer
from the coil to the diode. A small variable
capacitor on one of the legs of the differential capacitor is for tracking
adjustment. If adjusted properly, as you turn the differential capacitor,
you won't have to retune the main tuning control. This doesn't work perfect
for me, but it is an improvement.
My crystal module uses a single FO-215 germanium diode. As always,
a 1N34A can be used. The FO-215 diode provides higher performance in that
with the good impedance match it provides between the large litz coil and
output transformer, the selectivity is improved.
This module has a built in Bogen T725 transformer. But since this
radio has large litz, a great variable capacitor and Hobbydyne circuit, the
output impedance is larger than the input of the Bogen. So I have provided
a quarter inch jack with a cut off switch so that an
external transformer can be connected. Some examples of what I can
connect are shown on my transformers page.
To finish up the module, I used a 1 pole, 3 position rotary switch to
select the transformer tap that is best for the headphones that I am using.
The Antenna Tuning Unit also has a nice dual section variable capacitor.
This one has a 400 and 200 pf sections. A dual 365 will work, but I like
this one better because of the lighter antenna coupling that the 200 pf section
offers. The crystal set and tube modules can be attached to this board for
a single tuned circuit. The detector board can still be used as a wave trap.
When this board is used only as an antenna tuner, the capacitance
change caused by the module being removed is enough to mess up the calibration.
I put a small trimmer capacitor on a separate garolite strip that can be
connected to compensate for the difference in the capacitance. Also, since
this capacitor combo only allowed tuning to 540 khz (antenna dependant),
I added a 33 pf fixed capacitor to act as a padder to get me down to
530 khz.
The capacitor shaft has to be isolated from the vernier. This
is accomplished with a ceramic shaft coupler. I added a wire from the
ground connection to the vernier drive.
This is because the hand capacitance detuned the circuit.
Now there is very little shift when I remove my fingers from the
tuning knob.
One of the finishing touches on this set is the dial. Having a
dial calibrated in kilohertz makes it fun and easy to know where you are.
After I found the radio to be as good as it could be, I began the
calibration of the dial. I used the tube
module so I could get a zero beat note with the stations. I start at 1700 and
work my way down. Many times I can count ten beats for the next 100 khz
down. Of course, this is in the evening. A signal generator or digital
readout MW radio helps.
I also calibrated the dial in wavelength (meters). I wanted to get used to
this wavelength measurement technique and what better way than to have
a dial with both calibrations. The dial material is white styrene which
pretty well hides the tag edges at a short distance away. These dial markings
are now available from me. I can also provide the styrene you can cut in
a circle. To use the labels, you need a knife and pair of small side cutters.
The labels are cut. At the edge closest to the dial, I cut close to
the marking at an angle, so the label edges don't stick over the dial edge. Your hand
has to be somewhat steady. As you go down the dial, if the distances
crowd the labels, just leave off some of the labels.
Here is my labels offer page.
Like my schematics below? You can make them too and it is easy.
Check out Gary, WD4NKA TubePad
method. I have expanded the symbols for my use and you are welcome
to them at my downloads page.
How does this radio work? It works great! The tube section works
as well as it's higher voltage cousins. The crystal radio has that sharp
tuning appeal. I love the smell of litz being tinned in the morning!
It smells like great DX in the evening!
Good DX! Dave - N2DS
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