|
I call my #6 set the El Grande.
This is an update of my fifth receiver.
I found the knobs, the tap switch points and arm at
Play Things Of Past.
Gary has lots of old radio parts there and at
reasonable prices! Because of the nature of his inventory, it does
take some searching and fiddling to find what you want, but you are
richly rewarded.
I tried to make this radio look authentic. I am sure that those
H.H. Smith binding posts weren't around in the twenties, but I like
them a lot. Otherwise someone might think that this is truly an
old crystal set. (The diode on the inside gives it away)
This crystal set has two coils, two variable capacitors, making
up two tuned circuits. The coils are
wound on a two inch mailing tube available at Staples. Before winding
the coil, I sprayed some Parks brand shellac over the coil form. This
will keep out the moisture and gives the coil a nice shine. I bought
the shellac at Lowes. The antenna side
coil has 120 turns with taps about every 13 turns. The 8 taps and two
ends are connected to the tap switch points. This coil is a little over
three inches (75mm) in length on the seven inch (175mm) coil form.
This coil is wound with 23 gauge wire.
The tap switch has a 1-1/4 inch arm. So the contact points are also
1-1/4 inches from the arm pivot. I laid out the switch by first putting
masking tape on the surface. Then I measured for the arm pivot hole.
I took a protractor and measured the angles. The contacts are at 15
degree increments. By drawing a vertical line, I made the top two
contact points 7-1/2 degrees from center. You might want to practice
on a spare piece of garolite before you make the big cut. My first
switch had the "Dave Let terman tooth gap" in the center.
Since I live out in the sticks, far away from the big 10-50kw stations,
I decided to go with tight coupling from the primary to the secondary
windings. The detector coil starts about a quarter inch (6 mm) away
from the antenna coil.
The detector coil is wound with my ole favorite litz wire. As before, my
litz wire is 40 strands of 44 gauge wire with a nylon covering. There
is a lot of wire in a one pound spool, so you will see me using a lot
of this! There are 65 turns wound on the form with the diode tap at
50 turns.
There are four thumb nuts located just below the four binding posts.
By using jumpers, you can configure the antenna tuner so the capacitor
is in parallel or series with the antenna coil, or you can take
the variable capacitor out of the circuit. The settings of the jumpers
will depend on the antenna you are using and the frequency that you
are tuning. Use the trial and error method to find which is best
for you.
The two binding posts on the left are for the headphone connections,
and the next one is the ground connection, and the binding post on
the right is for the antenna.
Since I had the multi ganged variable capacitors, I decided that I
would parallel two gangs in the antenna circuit. This gives me a
larger capacitance range. To increase the tuning range in the detector
circuit, I put a 200pf fixed capacitor between the stator that
connects to the coil and a spare stator. This increases the maximum
440 pf somewhat to tune the whole broadcast band but doesn't make
the tuning impossible. The third gang of each capacitor is left
unconnected.
Speaking of more crystal sets... I want to build at least one shortwave
set. Also on the to-do list is a crystal set breadboard. This will
consist of variable capacitors, and diode detectors all tied to Jones
barrier strips. This will allow for easier experimentation.
So stay tuned... (pun intended)
|
