[Milsurplus] Crystal Surprise

[David Stinson]

Spring Project: A Bendix "Skipper 430" fishing boat
AM transceiver, the "big brother" of the Bendix Skipper 35
I revived last year.  The receiver is solid-state, while
the transmitter is a 12BY7 oscillator driving a pair
of 6146s.  Several of our members sent me crystals
for the project, for which I thank them.
https://photos.app.goo.gl/PkHLjc13s6qv1XvY6

Most were not exactly on the freqs needed, but I've
gotten a bit "handy" with grinding, etching and "penciling"
crystal blanks to get them on frequency- even HC-6U.
Sometimes it even works :-D  When I'm done, the set will
have two 160M channels, four 75M AM channels,
a SWBC channel and WWV 5 MC.  It's currently operational
on 3870, 3880, 3885, 3890 KCs and the WWV monitor.
Still a "work in progress."
https://photos.app.goo.gl/xrkBaAR4d221ZSp97

The 3880 Receive crystal, a 4335 KC DC-34,
would not oscillate. It's a "large blank" crystal.
These BC-669 crystal which are in the brown holder
often have "metal fatigued" broken contacts inside
the case, which are easily repaired.  No such luck
this time; I opened the case and found
a cracked crystal blank.

Got to thinking about it and, on a hunch,
decided to see if I could "salvage" the blank.
I had read somewhere that, during WWII, crystal
makers had starting making four blanks out
of these larger crystals and that each one of them
would oscillate at the same, original frequency.
This seemed to beg credulity
(translation: I didn't believe it),
but curiosity won-out.

Using a hemostat, I carefully knapped-away most of
the corner containing the crack.  Probably should
have gone the other direction but too late to
back-track.  Left a small part of an adjacent
corner and worried that asymmetry might be a problem.
Once the material was removed, I used 800 grit wet
sandpaper to sand the new edge straight, then beveled
the three edges in the illustration.  Used alcohol
to clean the blank before reassembly (make sure
your blank is absolutely dry before putting it
back in the holder).
https://photos.app.goo.gl/wZxE7x29rXufNWgK7

As you know, the electrodes that contact the
blank in most crystals made from WWII onwards
have elevated corners on one side, so the blank
is contacted only at the corners.  In this case,
I worried the missing material might cause a
short between electrodes at the corner where the
material was missing.  Turned both electrodes
around so the large central flat area would
contact the blank.  Reassembled and plugged
it into the receiver crystal bank.

The crystal came right up and oscillates well.
The frequency of oscillation?  4335 KC.
I don't explain it; I ain't that smart.
I just report it.

Thought you might find that kinda cool.
GL OM ES 73 DE Dave AB5S

P.S.  Didn't take pictures of the crystal
as I was fiddling with it because
I didn't expect it to work, and now I'm afraid
to take it apart for photos because it might
not work again. :-D



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