|
Let's see.... I finish a radio, then write
a web page about it. Ok got it! It has been a couple weeks since my last crystal
set and I had to make sure I hadn't forgotten the routine.
I am using the same cheap basswood box that I had used for some
of my other sets.But with that MinWax mahogany stain, this
box does look nice. Golden oak stain used to be my favorite then it was
red oak and now mahogany. I better check out the rest of their stains.
I also used the same 1/8 inch garolite but I did change how I cut it.
I bought some cheap (what else) forstner bits on ebay. These gave me
a better cut and less jamming and pulling than regular drill bits.
BTW, I did buy some tan garolite so don't be surprised if you see
a "blond" radio. Remember that furniture from the 50's?
Ok. There is a picture below on how I fasten the top panel to
the main box. I cut some 5/8 inch by 3/4 inch pieces of soft wood and
fastened them to the side of the box with #6 x 1 inch flathead wood screws.
Then I set the panel on top of the box, using masking tape to hold
the top to the box. I then drill (after measuring) the 4 holes. Since
I am using #8 screws on this project, the drill is 11/64 inches. As I
drill each, I drop a #8 screw down just to make sure that the top
remains steady. After the last hole is drilled, I then take out the
screws and take off the tape. I then drill only the wood holes to
3/16 inches to accept the t-nuts.
This set centers around two major themes. First the coil
is a honeycomb coil. This is a coil wound on a form to provide as much
air space between each winding. This gives it maximum "Q". The higher
this Q, the better the coil. A high Q coil will provide maximum
selectivity. It has been over 30 years since I have studied Q, but
even though I can't explain it, I know a good one when I see it.
This coil was in a box of coils that I bought at a hamfest. The
inductance measured about 320 micro henries. I carefully took off
enough turns to make a 240-250 micro henry coil. There was a another
winding of a few turns mixed in with the coil. I left this winding open.
The honeycomb coil is mounted to the panel with plastic
tyraps and two pieces of garolite
just over 2 inches long and 3/4 inches wide. I put in some notches
at the top so that the tyraps would stay in position. The brackets
are held to the front panel with 4 little metal brackets and screws.
The other bright spot of this radio is it's moveable
coil. The closer the coil is in line with the other one, the higher
the energy transfer. But you also ruin the selectivity. Having an
adjustable coupling coil has its advantages. As an experiment,
I took my octocap and
placed it in series with the antenna connection. Placing the coil
at maximum and changing the capacitance had about the same effect
as having no capacitor and just moving the coil. This was done
in the daytime with just one trial, so I will be doing more research.
The antenna coil is a 2 inch mailing tube 1-3/4 inches long.
It is wound with 45 turns of 40/44ga litz wire. Why this size, etc?
Because that is what was laying around. The 45 turns is what would fit on
the mailing tube. Not too scientific. I like using the litz wire as
the leads are very flexible and works well when one part is moving
and the termination is fixed. I cut a 1/4 inch dowel 3-1/2 inches
long and drilled a small hole in one end. I then drilled a 1/4 inch
hole in the mailing tube and a small hole opposite the first.
The dowel fits through and is fastened to the mailing tube with a
little wood screw. Be careful not to split the dowel, like I did.
A quarter inch bushing is put through the panel and a bar knob
is attached to the other end. The bar knob tells me what position
the movable coils is in at all times.
I found that the coil movement was just too loose, so a change
I made was to put a compression spring and a flat washer
between the top of the coil and the bushing. The coil now has
some tension and tuning is a little tighter.
Another addition is the toggle switch at the top.
The switch adds a fixed capacitor in series with the main tuning
capacitor to reduce it's overall range and thus allowing better
fine tuning at the high end of the broadcast band. This feature
is optional and has not been well tested as of this writing.
I just thought it would be a good idea as the top end of the
broadcast band is crunched up in a very small portion of the
variable capacitor.
How does it work? Don't know, been writing this web page.
Well, it seems to work ok. It operates somewhat differently from
my other sets. I can tell when I am over coupled and I can see the
selectivity change as the coupling changes. Building these crystal
sets is a very good way to learn electronics and physics. I would
recommend all dads work with their kids making these sets. Your
son or daughter will never forget the experience!
Best wishes from -- Dave N2DS
|
