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Old January 18th 16, 04:22 PM posted to rec.radio.amateur.moderated,rec.radio.amateur.homebrew
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Default [N2HTT] A complete digression.


73, de N2HTT

///////////////////////////////////////////
A complete digression.

Posted: 17 Jan 2016 11:05 AM PST
http://n2htt.net/2016/01/17/a-complete-digression/


Happy New Year.

Now that the holidays have passed and time has started to free up a bit,
Ive found my way back to the bench and started to resume experiments on a
space charge tube regen, And while I actually have some interesting
progress to report on that front, Ive been sidetracked into completing a
completely unrelated project by yet another random sequence of events.

This one started, as they often do, with an eBay purchase. I probably
shouldnt be telling you this, but I will sometimes bid on a lot of several
crystals, if there are at least a few I can identify as being on useful ham
frequencies. If the average price per crystal is attractive Ill go for it.

Out of band FT-243s are also useful because the holders can be filled with
modern HC49/U or similarÂ*crystals. I sometimes find crystals at frequencies
just below the ham bands. These can (in theory, I have never actually
attempted it) be ground down to raise their frequencies a few hundred
Hertz, to values that fall in-band.

Of course, I get a lot of fairly useless stuff as well. There may be a few
interesting holders, or items that might have antique interest, but for the
most part the rest is of no useÂ*and collects in a bin on a shelf in the
basement.

ThisÂ*particular lot contained a curious item: a sealed detector crystal for
a crystal receiver set.
The mysterious Philmore Fixed Crystal DETECTOR

These holders contained a hunk of galena and a cats whisker (short, stiff,
wire probe) sitting on a hot spot on the galena, potted in a plastic holder
to permanently secure the detecting action needed in a crystal set. This
was a modern convenience compared to the tricky business of finding a hot
spot that would detect. The really cool thing about these crystal receivers
is that they produce audio with no additional input power. All the energy
necessaryÂ*to produce the audio is captured by the wire antenna.

It was a curious artifact, and I put it on a shelf on my operating desk, as
a radio good luck talisman; but everytime I spied that crystal dector
sitting on the desk, I wondered to myself whether it still worked. I would
not have gotten much past the wondering stage were it not for the fact that
I happen to have a pair of high-impedance headphones a critical component
in any crystal receiver experiment.
High Impedance Headphones

These phones were given to me by my elmer, W2WTV Gordon (sadly a silent key
now for many years). I recall visiting him one Saturday while he was
cleaning up in his shack, and him handing these phones to me, saying
gruffly Here take these, you might want to build a crystal radio someday. I
held on to them ever since, not knowing when if ever I would want to build
a crystal radio, but they just seemed like something you shouldnt part
with, because theyd be difficult to get a hold of if the need ever arose. I
measured the DC resistance of these to be about 2200 Ohms.

So almost against my will, I started to research crystal radio designs.
Some of these babies can get very elaborate, after all at one point they
were state of the art. There are dozens of designs out there, and I started
to filter through them. Patterns began to emerge.

I decided on a variation of the simple oatmeal box receiver. Easy to build,
it requires a big air wound transformer (hence the oatmeal box) connected
to the antenna, a resistor, a variable capacitor, a diode detector, and
high-impedance phones.

I was all set, I had all that stuff on hand. I decided to go a little
uptown from the oatmeal box, and wound my coil on a piece of 2 inch PVC
pipe. Also, I used one of those cheap, ecologically responsible bamboo
cutting boards for a base. I think these things are great for any bread
board project, the small ones cost about five dollars in the produce
department of the local supermarket. Easy to drill, and good looking. Whats
not to like?

This past weekend, I got to work. Not that there werent about twenty more
pressing things I should have been doing, but by this point my curiousity
about the detector was approaching obsession. First step: recompute the
number of turns needed on my piece of pipe, by reverse engineering the
oatmeal box design.

Using an online air wound coil calculator, I estimated the inductance of
the oatmeal box secondary to be about 500 microHenries, and then calculated
the number of turns I would need on the pipe for the same inductance. Using
28 AWG gauge wire, it worked out to be about 100 turns. Sitting down to
binge watch old PBS shows, I started winding.

I wont bore you with the details of winding the coil. I know a lot of guys
dont like winding toroids this was far, far worse than any toroid. 28
gauge wire is impossible to handle. It was a nightmare.Â*Finally I got the
coil done, and measured the inductance. More than twice the calculated
value. This thing wasnt going to resonate any where near the AM broadcast
band.

Re-purposing some nice 18 AWG gauge enameled copper wire liberated from my
wifes studio, I wound a second coil of 60 turns, tapped as indicated in the
design. Much better experience all around. I dont recommend using teeny
wire for this kind of project.

Taking much more time than it should have, I laid out the components on the
cutting board. I didnt have the specified 47 kOhm resistor, so I used a 51
kOhm instead. Nothing about this design struck me as being all that
critical, so I figured close was good enough. With everything wired up, I
brough my creation up to the shack, and hooked to my antenna.
Attached to the antenna (new diode)

The antenna is a 135 foot doublet, fed with ladder line, with a bunch of
stuff between the wire and the shack, like a big balun and an automatic
tuner. But since rf does demonstrably get in through this pathway, I
figured it might work for the crystal receiver. I hooked the center pin of
th PL-259 connector to the top of my coil, and the shell to the ground
side. Hooked up the phones, and spun the dial.

Profound silence.

I tried all the taps. No good.

Okay, back to the design docs.
Schematic for my version.

After carefully reviewing the small schematic for the rig I realized I had
flipped the sense of the secondary.Â*Could phase matter? Well, just to be
sure I switched two soldered connections, and now my construction exactly
matched the schematic. Back to the shack, hooked up the alligator clips,
and. nada.

Okay, well it could be the phones I had never tried them in any other
circumstances. Or, it could be my antique crystal detector. I popped the
old unit off, and stretched a brand new 1N34 diode across the posts.
Connections hastily reconnected, tunedÂ*the cap on one tap, then the next
wait, what was that? Yes, faintly, distantly, but unmistakably salsa music!

I ran downstairs and got my son to come up and listen. He put on the
phones, concentrated for a moment, and said Sounds like a Spanish language
station?

Yes! Success! A soft, vague whisper, but reception nevertheless! My
receiver works, and the question answered: the antique crystal detector is
a curio only, dead as a doornail.

A little further experimentation reveals that the phase of the coils does
matter switching the antenna and ground connections completely killed the
signal. Playing with the receiver late in the evening, I was able to hear
four or five distinct stations, but none as strong as my Spanish station,
which turns out to be WEPN, ESPN Desportes on 1040 kHz AM in New York.

Switching between taps improves selectivity and reduces sensitivity as
fewer turns are selected. I probably could get better reception with a good
earth ground and a more direct connection to the wire, but those
experiments will wait for some other time. For the time being, I am at
peace with crystal radios.

Now about the space charge tube regen receiver. I have found an interesting
design that I think will adapt nicely to space charge tubes, and I have
prototyped the input RF amp with encouraging results. But this will be the
subject of another post. Until then

73

de N2HTT


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