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Introduction
Hi Gang. Remember me? It's been a long time since I built my last radio. I have
been gathering ideas with my associates at the Secret Radio Ranch over the summer.
Now I'm back, fresh as a daisy (so long as you don't stand too close), ready to go again.
Enough of the Uncle Miltie comedy routines. Now on with the show!
The Idea
Last week, the topic of a band-pass or tracking circuit crystal radio came up on the
RadioBoard forum .
I recalled back to my #16 Miller Time, as well
as the great looking #52 radio that I had built in
previous years. I thought it might be time to do another set. But this time, the
set was to be built with dx radio features. These are:
A high quality variable capacitor with ceramic insulators.
Dual in line verniers for a s-l-o-w tuning rate.
Ferrite toroidal coils with 165/46 litz wire.
A variable "Benny" control.
The famous Überformer output transformer.
Any of these features can be downsized if the budget or desire doesn't allow.
Don't be expected to put litz on the coil if it is difficult to put food
on the table. A Bogen T725
transformer will work ok too, with reduced performance.
The Antenna Input Circuit Description
The circuit is shown below. Starting with the antenna input on the left, there
is a 5 position switch to select how far up the primary coil the antenna will
connect. This is a new feature, and perhaps useless to some. Since I haven't
built many projects with toroids, I felt this was a good way to judge the
best tap on the transformer to use. I also felt it is possible to get the
best match for the antenna that I or others have available. Come to find out,
this is a useful feature. It is like moving the coils in my dual spider coil
sets. As it turns out, having the tapped antenna input allows for
the best sensitivity on the low end of the band by moving the antenna up the
coil, and at the high end, crank the switch back down.
The Tracking Circuit
The 4 capacitors,two coils and rf transformer are part of a tracking tuning circuit.
This is also called band-pass tuning.
The main coils are wound on Amidon FT-114A-61 ferrite toroidal forms. The resonant
tuned sections are wound with 165/46 litz wire. Both tuned sections have 39 turns
of litz on them, having an inductance of 220 µH (with a 440pF dual gang
capacitor)
One coil (L1)has 3 turns of small wire with a switch tap at every half turn.
This adjusts how the antenna is coupled to the radio. Adjust the turns and taps
to your liking. (You don't really need 5 taps, but it went with the 5 tap
switch that I used.)
The secondary of this coil (L2) resonates with the two capacitors across it
for tuning. L3 just has one winding as the detector is capacitive coupled.
RF transformer T1 is an Amidon FT50B-61 with three turns on both the primary and
secondary. A smaller toroid can be used here, but you may want to add more turns.
The .025uF can be a .022 or .033uF without difficulty. I found the capacitor
helped the mid band sensitivity. Thanks to the members on the
The RadioBoard for this hint.
Of course it seems for every positive, there is a negative. The sensitivity
of this type of crystal set is poorer than with other radio designs.
But you get a single turning control radio with fairly good selectivity.
The important requirement is to have good frequency tracking throughout the
tuning range. If you don't get this right, the radio will fail to satisfy. When
two or more tuned circuits are ganged with a single spindle capacitor, tracking
is always an issue.
The tracking is the most critical at the top of the band. Therefore I
installed two small trimmers to align the top of the band tuning.
To adjust the bottom of the band tracking, turns of wire can be removed
from one coil or the other. This adjustment isn't as critical as the top end
of the dial. More on the alignment will be covered below.
After the Track-Tuning Circuit
Beyond the track-tuning circuit is another 7-45pF trimmer, 27 mH choke as well as
the diode and rf bypass capacitor. The trimmer and choke are part of a
Selectivity Enhancement Circuit (SEC).
I have used this on many of my dx radios.
Then comes the "Benny". The 250k variable resistor
is set approximately to the impedance of the transformer (200k ohms). This reduces
coil loading and distortion on louder signals. The .1uF capacitor allows for
easy audio flow to the Überformer. (If you use a Bogen T725 instead, please
lower the 250k pot to 100k and increase the capacitor to .22uF.)
Construction
Starting from the bottom up, the box is a bamboo storage container sold by Bed, Bath
and Beyond. They are cheap. I buy them when I have a discount coupon. The box
is 12x6x2.5 inches (30x15x6,3 cm). There is a ridge on the inside that is for
stacking boxes, but I use it to fit the black garolite chassis into. I do a
rough cut and then with my small bench sander, I sand until the garolite fits
nicely. The garolite is 1/8 inch or 3mm thick.
Then I start positioning the pieces on the top to make sure I have
room for everything and that it will look nice. I used more pieces of garolite
to make the front stand up panels. Four metal brackets hold the front panels
to the chassis. Two verniers are included in all this. The capacitor was
placed on standoffs so the height would be good so that I can use a 4 inch
(10cm) plastic tuning indicator dial.
Mounting the capacitor and verniers is the main task in this project. All the
other switches, jacks and other parts can be fit where they will work well.
Of course, this should be planned before the first hole is drilled!
Now a couple of words how the garolite chassis is mounted to the wood box. Since
my early projects I have used a small pieces of pine wood attached to the sides of
the wood box with a pair of screws. Then I measure and drill the holes to mount
the chassis. (Allow for the box edge when measuring the garolite.) Then a T-Nut
would be placed under the pine wood and screwed into place.
The only change is that this time I used wood glue to attach these two pieces
of pine wood to the box. That worked out very well! I will do this in the
future. It only took me 8 years to figure it out.
Before winding the toroidal coils, I wrapped the toroids with plumber's pipe
tape (Teflon). This is meant to improve the performance of the coils.
To keep the toroids away from the chassis, I used a scrap piece of HDPE. Not sure
if that is really necessary, but I'm not going to throw away Q on sloppy
construction!
After that, the wiring is completed. The wiring is easy on this set. The tracking
trimmers were placed on the bottom of the chassis and held in place with
thick bare wires. All the wiring is out of sight when the project is finished.
If I say so myself, this set looks elegant. The lady of your house may
disagree. Work it out, don't contact me about this.
Alignment
You should have an inductance meter for making sure the coils are the same
inductance. I recommend the one from AADE for
a hundred bucks as a kit. You won't regret it. Also a signal generator is
very helpful, unless you have strong signals in strategic positions on the band.
Start by setting all trimmers at the mid value.
Tune your signal generator to 1600 kHz and tune the radio to about 90% open
on the variable capacitor. Peak the trimmers. Then move to 600 kHz and check
the position of the capacitor (Do you think it will do down to 530?). You
can also adjust one of the trimmers and rock the variable tuning capacitor
to see if the tuned circuit is still peaked.
If it seems to be off, then you will have to adjust the windings on one
of the coils. Take a couple of turns off and resolder the wire on the
one toroid. Check and see if the tracking seems to be working, by first
going back up to the top of the band.
If the tracking is further off, then put the turns back on, and take two
turns off the other coil.
This may take hours to do, so please be patient. If you have aligned
superhet receivers before, you know the routine here and it should be
easier.
Operation
Ok, connect your antenna and ground. Pick out the best pair of your high
impedance sound powered phones, regular magnetic headphones, or a set of
Piezophones. Then start tuning
around. When you hear a station, then adjust the switches. If you get a real
loud signal, adjust the Benny until you hear the lowest amount of distortion.
Conclusion
At first, I wasn't so sure that this was going to be that good of a radio.
These band-pass track tuning systems have a way of losing signal. This radio
in a short time has exceeded my modest expectations. This was my best effort
attempt to make a simple to use but good quality crystal radio. I have
no regrets.
While first testing this radio,
I was pleased to hear a new station. It was a 237 watt station about 500 km
from this location. This is a good omen! Here is my entry in the
2009 Sprint Contest. I heard that
station again and was able to log it.
Thanks for stopping by for a visit.
73 and good crystal DX. Dave - N2DS
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