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Introduction
Hi Friends. After my quick dabble with space wound cylinder coils in my #70 set,
I am firmly back in the familiar territory of the spider coils. Would you expect anything
different from me? While this set is much like my #66
, 68 and 69 radios in looks, there are some developments that occurred after the earlier sets were built. Some were based on the components available to me
at the time of construction, some were little improvements that makes this radio even nicer
to operate. Since I'm in a hurry to show you this set, I'll move right along.
The Idea
This radio came to being because of a request by a friend for one of my sets. He wanted it
"just like your #66 set". This one turned out to be about like my #66. The difference is
due to the parts availability, improvements thought of since the last sets and the fact that
since these are built with the plans in my head, There are no measurements around to make
another. I just hope each time that everything is placed right and something doesn't bump
into something else on the opposite side of the chassis. You can see my layout drawings on
the masking tape covered HDPE and garolite.
I'm in love with my spider contra coils!
However, not all variable capacitors are in love with those coils. That is, the capacity isn't
optimum for the best coil size, as well as the tuning ranges and dial spread. If the capacitor
range is less than about 15-300 pF, you won't be able to tune the entire band very well with
a contra coil. This is taking into account having a reasonable amount of dial spread. Dial
spread is the ability to have the frequency ranges you desire to cover as much of the capacitor
tuning range as possible.
Once the variable is much over 400 pF, then it can be a problem getting a good
dial spread. However by placing a higher fixed or variable padder capacitor in series with
your main variable, then you can end up with an effective capacitance in the range that
is best for contra coil operation. The perfect value is about 350 pF.
Circuit Description
The Antenna Tuning Unit (ATU) provides a way to transfer RF energy from the antenna to
the detector, and at the same time provide some selectivity. Dual tuned circuit sets are
always more selective than single coil radios. I won't describe this circuit much as it
is well known by DX crystal radio builders and is shown and described
elsewhere on my site. You will see an extra
padder capacitor in the pictures but not described on the circuit. This is because I used
an unusual 3 gang variable. The values were such that I put in a padder to adjust the
dial spread of the ATU. This isn't that important, but just something that makes my
radios just a little nicer.
The detector section is somewhat like some of my other dx sets shown
on my pages. The detector board starts with a
contra spider coil. This is a specially wound
coil with two windings. They are wound in such a way that they can be series or parallel
connected and still maintain a lower distributed capacitance than if the coil was wound
in a conventional manner. There are other nice features of this coil that you will find
on my contra coil pages.
Tuning this radio is achieved by the main 475 Holy Grail type high quality
variable capacitor. Since this capacitor value is too high for use in a good contra coil
radio, I have reduced the value of this capacitor by use of a 400-1200 pF padder type mica
capacitor. Since this value wasn't high enough, a 470 pF fixed mica capacitor was added
in parallel. You may want to use a 680 pF fixed capacitor for a more middle of the range
adjustment of the padder. Since I was able to adjust the padder the way I wanted, I left
the 470 in there.
Over the years I collected a lot of screwdriver adjust small value trimmer
capacitors. I would get them as part of eBay deals or whatever. I was wondering what
I would do with these and contemplated selling the bunch on eBay. I never thought that
these would be useful in crystal sets. Oh how I was wrong. This set ended up with three
of them.
The biggest one is a 100 pf trimmer. This is used to adjust the dial spread range.
If your coil is calculated and constructed correctly, this gives the final little adjustment
to put your tuning range right on target.
After the main tuning system is a type of selectivity enhancement circuit (SEC). This is
known as the trademarked Hobbydyne circuit. This is a circuit consisting of a
differential capacitor, an optional trimmer, and a 27 mH rf choke. The idea is to reduce
the tank loading and provide some impedance matching. More information can be found
on my DX Notes page.
The differential provides a "capacitance divider" circuit that feeds the diode.
But there is some capacitance from the detector that cause the main tank to
be detuned. A small trimmer capacitor is added to one leg of the differential capacitor
to compensate for this. This is called "tracking". Unfortunately it isn't perfect but
works across about 2/3 of the differential tuning range. This cuts the need to adjust
the main tuning so much when the selectivity adjustment is made.
There are some cases where you don't want to use the SEC. If the interference is
low and you want more volume, then bypassing this circuit is advisable. The lower in the
band you listen, the less important the SEC is. I built in a feature do bypass the SEC,
if desired. This is built in to the band switch as two extra positions on each band.
They are named "Wide" and "Narrow" (how creative of me!). You can see on the schematic
how the signal is routed through from the tank over to the detector diode, or is directed
through the SEC.
There is one downfall. That is the capacitance differences between the SEC and without
it. I used another small air trimmer to compensate for this. So one could say that the
two smaller trimmer capacitors only increase the pleasure factor
in using this radio, but does not pull in more stations.
After the diode is a fairly simple audio output system. I use a parallel combination
of a 180k resistor and .1 uF capacitor to equalize the dc resistance of the
transformer
winding and the it's impedance. This improves the selectivity during times of strong
signal reception by not loading the tank due to the high conductivity through the diode.
I use this RC network as a place to connect a milli-voltmeter to for measuring the
relative signal strength.
More information on this, and the contra coil can be found on
Ben Tongue's web site. Spending a lot of hours
there is very worth while if you want to learn how to design your own high performance
crystal radios.
The Coils
There are two large coils in this radio. The ATU coil is approximately 155µH on
a 6 inch (15 cm) HDPE spider coil form. There are 39 turns of 660/46 litz wire
to make this inductance. The hub is 2 inches (50 mm) in diameter.
The detector coil is a contra wound
style. This coil must be designed to work with the variable capacitors that you will
be using. But in my case (using a 350 pf variable equivalent as the base), the coil
is about 220 µH in series and 55µH in parallel. I used a 7 inch (17,75 cm)
HDPE form, also with a 2 inch hub. The thickness of the HDPE is 1/8 inch (3 mm).
The inside windings is 27 turns of 660,46 litz.
The outer winding is 17.5 turns of the same litz. Please read the contra coil section
concerning the mechanics of winding this type of coil and their advantages.
These coils are available on a
custom wound basis.
Construction
Below I have included some pictures of the construction phase of this radio. They
include the panels and chassis before they are cut and drilled. Masking tape attached
to these surfaces is a good way to mark your holes. After you are done drilling, just
pull off the tape. I used a scroll saw to cut the pieces. A small one inch wide sanding
belt finished the fine work on this radio. It took a long time to get it right but
the pieces fit very well.
There are two pictures of a sub panel I built. The main purpose of this sub
panel is to allow for a set back from the front panel of the differential capacitor
and band switch. If they were mounted close to the panel, your hand capacity would
detune the radio and make it difficult to use. The capacitor and band switch with
their corresponding styrene shaft extenders keep this from happening. It was handy
to mount the three air trimmers also on this panel, as two of them were connected
to the differential capacitor and band switch anyway. I pre-wired this little sub panel
as it was much easier to do without the main chassis getting in the way. If you use
switches like I used, then the pictorial diagram will be very helpful.
To cut losses, two of the contra coil leads are soldered directly to the
main capacitor ground and the top of the padder. Smaller wires then connect to the
band switch. It isn't too difficult to wire the switch, but sections C and D, which
are closest to the panel should be wired first.
I used 3 pair of thumb nuts with brass screws as the connection points for
the antenna and ground, the test point and the diode. I like this type of connection
and there is a bonus of a nice brass look. It is easy to experiment with the diode
since the connections are on top of the chassis.
The chassis is made from HDPE 12 x 7-1/2 inches (30,5 cm x 18,2 cm and
is 1/4 inch (6 mm) thick. There is a problem
with mounting the impedance tap switch as well as the 1/4 inch phone jack (and the
mica padder too). The panel is too thick for a nut to be put on the end of the
bushings. I solved that by cutting a circle on the bottom side using a large forstner
bit. Start by drilling a pilot hole and then drill about half way in with the
forstner bit. Be careful so as not to drill too far. Then drill the 3/8 inch
holes for the bushings to go through.
When making these chassis sets, you have to be aware of what is on each
side when you are deciding where to put the components. For instance make sure
the placement of the posts are far enough in and back from the edges of the chassis,
in particular at the front. I placed the holes 1 inch (24 mm) back from the front
and 1-1/4 (30 mm) in from the sides. This gives the shafts that go to the
differential and band switch enough from to miss each other. (Don't you just
hate it when two parts interfere with each other. It just ruins your entire
day)
The oak wood base is 14 x 8 inches (35,5 cm x 20,3 cm) and is 3/4 inch (18 mm) thick.
There are stick on rubber feet on the bottom. I also like to cut a 1/2 inch diameter
recesses so that I can use shorter screws and the screw heads are not sticking
the bottom. I prefer this method to a simple countersink as sometimes the holes
have to be "adjusted" to fit.
Both of the main tuning capacitors are connected to the knob through a 6:1 vernier
drives. A vernier is essential for operating high selectivity sets like this one!
The variable capacitors have their shafts at equal heights from their mounting base.
If yours aren't, then a shim could be made to bring up the height of the low one.
The shaft of the ATU has to be isolated from the vernier. I used a metal coupler
and a short piece of styrene rod to make the mechanical connection to the front panel.
The vernier has to be grounded to avoid the unwanted hand capacitance effects.
Those are the main building pointers I can think of. Much of what needs
to be done is covered by looking at the pictures and using that good ole' common sense.
(Builders in Washington D.C. may need some help there). :) If you can't get a handle
on something, send a short note and I will try to help.
Alignment
On the detector board there are 4 adjustment trimmers to be dealt with. Fortunately
some are not completely interlocked. Start by setting the differential and all the
air trimmers at their mid point. Adjust the mica trimmer a couple of turns from being
fully tight. Make sure you use the "Narrow" selectivity positions on the band
switch when starting this alignment.
Using a signal generator, see how the tuning is. On the low range, the goal
is to tune from 520 khz to 1050-1080 kHz. The high range should turn down to 1000 to 1030 kHz.
You don't really have much control about where the high end of the high band actually
will tune. Don't worry about that.
It is important that the high range tuning is able to tune lower than the
low range tunes high. I like about 30 khz overlap here. If you need more dial spread,
you can reduce the value of the 100 pf air trimmer. If you can't reach 520 kHz (gives
you a 10 Khz cushion over the North American MW band edge), then adjust the mica trimmer
a little higher.
I do this kind of adjustment in my sleep. Make sure to take the time and
get it so it is right.
Now that this is done, tune the receiver to 1700 kHz and try to adjust the
SEC tracking capacitor (on the leg of the differential) so as to keep the frequency
shift as low as possible while turning the differential capacitor.
The last adjustment is to set the SEC compensation control so there is no shift
between wide and narrow switch positions. Make sure your differential is set to mid
position. Make this adjustment at 1700 kHz. Adjust the SEC compensation trimmer so
that there is no tuning shift between wide and narrow.
Since there is a little bit on interaction between the capacitors, adjust the dial
spread capacitor as well as the mica padder so the ranges are just as you want them
to be.
Operation
This is a two handed radio. Both the ATU tuning as well as the detector
tuning adjustments have to be made in tandem. A signal generator can help you
spot the frequencies if you have trouble. I tend to rock the controls one station
at a time and work my way up and down the band.
A good pair of sensitive sound powered (balanced armature) headphones
will be just what you need.
Circuit Adjustments to match your junk box.
There can be a lot of substitutions to this circuit. You can leave out the two
small trimmers if you don't mind the tuning shift. You will need something like
the 100 pF trimmer for the dial spread adjustment. Try to use an air trimmer here.
Of course the main variable capacitors can be substituted to a certain
extent. Make sure that the coil you make will tune the entire band.
The ATU needs a minimum of a dual 365 pf capacitor. You might miss the
bottom channel or two (adjust for 530 kHz instead of 520 and hope). A dual 400
pf capacitor will give you leeway. You may need to adjust the coil size on the
ATU so, add a turn or two extra when you are winding. The end value is also
dependant on your antenna.
Conclusion
This radio marks a milestone in dx crystal set development, in my mind anyway. :)
Although some of the things I jotted down may have been shown elsewhere before, I haven't
seen it. So please don't e-mail me if you had thought of it first. :)
This radio is truly a Schmarder design. Here are the crystal set developments
included in this set:
Contra spider coil (I was first for this design shape)
An air trimmer to enhance the dial spread of the radio with the contra coil.
A main tuning padder capacitor to further enhance dial spread and allow for a
wider variety of tuning capacitor and contra coil values to be used.
Selectivity enhancement circuit can be switched in or out (4 position band switch)
Compensation added so that there was no tuning frequency shift when the
selectivity enhancement is out.
The Überformer
A padder in the ATU to further adjust the tuning range for improvement in dial
spread but allow for compensation due to different antennas.
The special look of a wooden base, with the chassis held up by posts.
Thanks for stopping by for a visit. 73 and good crystal DX. Dave - N2DS
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