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This is continued from the previous page.
The Contra Connection
Before we get started with the formal discussion of the construction of my #68 and #69
radios, I felt that special attention should be dedicated to the winding of the contra
coil. Of course, you can just go out and
buy a coil, but this web site is
dedicated to the homebrewer. To make this coil, you need to know the minimum and maximum
values of your tuning capacitor. You need a decent
LC meter to check the
inductance of your coils so that they can be balanced. (That little meter is still only
$100 after all these years). You will also need a good way to calculate the coil you
need to wind. Our friend Jeff Welty is our bridge between crystal sets and
PHP programming. His
contra coil calculator
is excellent! I used it in designing my coils. Thanks again Jeff.
Looking at the picture above of my #68 radio coil, you will see that I have marked
the 4 wires. There are two coils, L1 and L2. Each coil has a start and finish wire,
which is also marked. This coil is one that I specially wound for a research project.
The two coils have a space between them. While this is a good method, I found it hard
to build. But it does make a good visual example.
The two start wire ends are in the middle of the coil form. In the parallel configuration,
these two wires are connected together (and the outside wires are connected to each other).
Without going into too much of the theory of operation, I will just say that this is
the best way to connect two coils together for the high end of the band reception
(low losses).
The contra wound coil works because the windings at the center start winding
in the same direction but end up being opposite. That is, the two center start windings
head in the same direction, but L1 goes towards the center of the form and L2 winds
to the outside. Of course, to wind this coil, you start at L1f and wind out. When I'm
winding and get to L1s, I then start L2s by flipping over the form and continue
to wind in the same direction. If you look at my cylinder coil
on my contra page, that will show what is really going on here. (You can build this
radio with a cylinder contra coil if you desire.)
It is desirable to have both coils the same inductance. Jeff's calculator tells you
the amount of wire to use. Make sure to add about 10 inches (25 cm) of wire to the
coil, so the ends will reach the switch connections. Make sure to test the coil with
your inductance meter. You can also connect the coil to a variable capacitor, both
in parallel and series. Hold a portable radio near the coil and you will hear when
the circuit resonates.
Now that I beat that dead horse enough, now continue to my general construction
of my two radios.
Construction
The construction of these radios are fairly complicated. Many steps are required.
I will tell you now that this is not a beginners project! If you do build
this, maybe you have a friend close by that can help.
Just take it slow and easy. Take a lot of notes. Use logic to decide what comes
next. It is estimated that it takes me around 40 hours to build one of these sets.
The basic order I used to construct this radio is the following: Prepare the wood
bases, by cutting, sanding and staining the boards. A poly coating finishes
the preparation. I next drill the holes to fasten the posts between the chassis
and the wood base. The front posts should be at least one inch (24 mm) behind
the front edge of the chassis.
The most critical part to be mounted are the variable capacitors. After fastening
the front panel to the chassis, it is time to find the center hole position for
the capacitor shaft and vernier drive. To help me with this measurement, I took
short length of a 1/4 inch round rod and filed a point on the end. Then by adding
a shaft coupler, I mount this tool on the end of my capacitor shaft. I then jam
that point into my front panel material, making a mark to start drilling.
After the vernier holes are drilled,
the exact position of the variable capacitor can be found. This is the most difficult
part of the mechanical construction of these radios. To help me with my hole
location marking, I cover the top surfaces with painters masking tape. This allows me
to mark the hole positions. When I am finished, I peel off the tape and the
panel is perfect.
I hope this isn't too late, but make sure the capacitors you select for your
project have the same distance from the bottom mounting surface to the shafts
Most of my capacitors have a 1-1/2 inch distance (36 mm). Your set won't look
so wunderbar if the distances are different.
After the capacitor holes are drilled, you have a better idea of how much room
is left to mount the other components. Just watch the spacing and check if the
placement will interfere with another component on the other side of the chassis.
I do this quite well in my head. No plans are drawn or set. I just mark the panels
and imagine what will be on the other side.
After all the holes are drilled, everything can be mounted. In this project, I started
wiring the rotary band switch, starting with the wafers closest to the chassis
(C and D sections). After the switch and other components all the way out to the
headphone jack are wired, I then mount the variable capacitor. After the wiring
to that is done, I then attach the coil and wire it to the switch and variable
capacitor. Two of the coil wires can be wired directly to the variable capacitor.
The other two can go to the switch.
The white pieces are HDPE (high density polyethylene), which has been found to have
extremely low losses in the MW band range. The chassis size is 11-1/4 inches
(28,5 cm) long and 5-7/8 (15 cm) wide. Why not 6 inches? Well I can get 4 pieces
from a 24 inch standard width sheet of HDPE. By giving up an 1/8 of an inch, I
get the 4 panels, after counting the width of the saw blade. The same is true for
the front panel.
The coil form is 1/8 inch (3mm) thick
and the chassis is 1/4 inch thick. The front panel is 1/8 inch thick garolite.
Garolite is called a plastic and is a very tough material. It machines well. You
can see that nice curve I put on the bottom of each front panel. I used a large
pizza pan to trace the edge.
Be careful when soldering around the HDPE as it will melt. After the wiring
is finished and the radio is tested and aligned, then the labels can be applied.
I use a Brother P-Touch label maker. You
might find as I have that a well placed label will cover a mis-drilled hole. :)
This radio has a total of eight 4 inch tall (10 cm) standoffs. Delrin is the
material used, but just about anything you have will work.
My heartfelt thanks to Mike Peebles
for making them for me (along with the dial scale disc)
It is hard to make a centered hole in a dowel without a lathe.
Most all the hardware is brass. You can't beat brass hardware for just the
right look. Looks are important to me. You can see my radios, but you can't hear
them on the internet. BTW, I am more of an electronics person than a wood or
plastic worker. My shop is my kitchen (was my kitchen actually) with a half
dozen pieces of power tools.
Now that I confessed, lets continue.
The dials are also made from 1/8 inch thick HDPE. They are 3-1/2 inches (8,9 cm)
in diameter, which matches the panel height. I didn't want the panel to be
above the top of the chassis because of the top mounted band switch.
The brackets are made by Keystone. I used ten of them in this project.
I wired the small band switch using mostly 26 gauge Kynar coated, silver
plated wire. This is great wire for small places and the insulation doesn't
burn easily. The silver plating makes it very easy to solder.
I used Chinese imported
rotary switches in my project.
Wiring was highly simplified by using the pictorial version of my schematic,
which is shown below. If you use a different switch, draw a pictorial to match
your switch.
I can't think of anything else about the construction. If you have
some constructive construction comments, please pass them along to me and I will
add them here.
Alignment
This won't take a long time. It is helpful if you have a signal generator. If
you don't have one, a good knowledge of the MW band in your area is helpful.
Assuming you wound the coil correctly to match the capacitor you have, this
will be easy.
The idea is to have set the 100 pf air trimmer capacitor to a value
that will allow for a small amount of frequency overlap between the low and
high ranges. This will generally be around 1000 to 1050 khz. The low range
should tune higher than the high range tunes low.
One thing that you may notice (it took me a while) is the frequency of the
high range is exactly double the low band frequency at every dial setting.
It is possible to only mark the low range and just multiply by two for the
high range dial reading.
Next, the selectivity enhancement capacitor (marked 3-20 pf on my
schematic) should be adjusted. The best way to do this is to listen at the
high end of the band, and adjust the selectivity capacitor to a value you
like. You could put a knob on this capacitor for a user adjustable control.
Watch out for excessive hand capacitance if you do this.
Operation
It is now time to have fun using your radio. Myself, I think I have more
fun building the sets. If that is the case, sell that set and make another. :)
After connecting your antenna, ground and headphones, it is time to go for
the DX. The tuning capacitors on both units have to be tuned together. I
like moving one capacitor and the other, up or down the band
10 khz (9 khz in other parts of the world). Most of the time, I can
just rock my way from one end to the other, and know where I am at all
times.
With large litz sets and selectivity enhancement circuits, you
will notice a tonal difference between the sharp and broad switch settings.
The broad setting is HI-FI, but the sharp is quite restricted. This
position is best for DX listening.
Conclusion
I really didn't mean to lecture you on how to operate a crystal
set. If you built it, you should know how to run it. So I will stop my
chatter so you can begin planning your version.
73 and good crystal DX. Dave - N2DS
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