|
I hope that this crystal set is well received by all of you.
My #62 radio combines the features of some of my earlier radios. It combines
the one piece approach of my #48
set and the dual detector of one of my recent sets.
Most of my radios center around a theme, circuit or part. This one
centers around a the coil
The coil arrangement is a dual honeycomb coil, both mounted in an
clear acrylic tube. So the coupling between the coils can be adjusted, one
of them is on a pivot and can be turned with a knob. If better selectivity
is desired, the antenna coil can be rotated perpendicular to the main coil.
This reduces the coupling between the coils, making a sharper signal peak.
The coils are made with 165 strand, 46 gauge litz wire.
This circuit will also work with a spider or space wound coils. The litz can
have 44 gauge or 46 gauge conductors, depending on your budget.
The three variable capacitors are dual gang types with 270 pf capacitance
per section. These were obtained on the surplus market. I made them work
very well in this radio. For example, the antenna input capacitor is one
section at 270 pf. But at the top of the broadcast band, the adjustment
can be a little touchy. By adding the second antenna terminal connection,
which places the two sections in series, the maximum capacitance is cut in
half. The capacitor next to it is used in parallel with the antenna coil.
Both sections are tied in parallel for a maximum of 540 pf. The main detector
tuning capacitor has one section directly connected across the tank coil,
while the other section has 270 pf in series going to the coil. This gives
a maximum capacitance of 400 pf. I am able to tune the entire band
using the full range of the capacitor.
The capacitors also have a 2:1 gear arrangement. This makes the
tuning a little nicer. The main detector tuning has a
6:1 vernier drive
connected to the 2:1 gear, giving 6 full turns to turn the capacitor
180 degrees. What bliss!
There are two complete crystal sets in this project. The first
detector is connected to the antenna coil and capacitors. The second
detector is connected to the detector tank circuit. The audio outputs
are wired to a front panel selector switch and routed to the
audio output transformer.
The detectors use a modified Hobbydyne detector circuit.
A small trimmer sets the loading on the tank circuits. This improves the
match and the selectivity. I haven't actually checked it, but I believe
that the loss of signal is minimal. In my #61 set, I did disconnect the
detectors from the capacitor because I thought this would help reduce the
losses when using the other detector. I found this not to be a problem
because of the feather touch loading on the tank. So I left that feature
out of this radio.
There is a parallel combination of a 47k ohm resistor and
a .1uf capacitor. This is used to balance the dc resistance with the
transformer impedance. This is done so that when strong signals come in,
the tank won't be loaded due to the heavier conductance of
the diode. This little circuit used for this
purpose has been named the
benny. I used
a pair of FO-215
high performance germanium diodes in this radio. 1N34A diodes
will also work.
I also added a 4 point brass link switch to select the impedance
that closely matches the headphones used. This is in an out of
the way place as it usually gets set only once. If you change headphones
a lot, you might want to locate the switch on the front panel.
If you never change headphones, just pick the right tap and wire
it directly to the headphone jack.
The base is made from a 8x12 inch (20x30cm) piece of
garolite. the thickness is 3/16
inches or about 4,5 mm. The front panel is 4-1/2 inches or 11,5 cm.
I cut the front panel to give a little extra eye appeal to this
radio. It was suggested that I could add length to the coupling
adjustment and bring it to the front panel, but since I don't use
plans or drawings when I build, it would have made the project
a lot more difficult. I like how this turned out.
This radio is quite heavy and sets well on my table.
The reception is very good. I sure like testing these sets during
the winter. More signals. The selectivity is good, considering the
components.
I haven't mentioned this in a while, I made the labels
with a Brother P-Touch label maker that is connected to my computer.
I try not to over label. I put enough on so you know what the controls
are for. That's about it. If you have questions, please ask.
Good DX! Dave - N2DS
|
