[Milsurplus] Crystal checker

[Bob kb8tq]

Hi

The “original” designs for these crystals used a pair of metal pressure 
plates, a spring, simple “flapping” electrodes, and a crystal blank with no 
electrodes plated on to it. Later “improved” versions came along and used 
plated blanks, but typically retained the poorly sealed enclosure. 

The improved designs are not likely to live forever. Industry learned a bit
about the need to seal plated blanks. ( yes those sealing techniques got better
and better over the years …). These rubber gasket sealed plated designs sorta / 
kinda went away ( or should have …. ). Given the demand for same / same 
replacements and the problems with doing them the original way, the plated 
parts lived a longer life than one might think. 

If you have un-plated blank versions, tossing them out probably isn’t a great
idea. With some effort, they can be brought to life. The exception would 
be parts that have been dropped and the blanks cracked …. no fix for them.

One thing that killed the original crystals back in WWII and still kills them is
surface contamination on the blank. The fix they came up with back then still
works today. Yes, it involves chemicals that will dissolve quartz. It very much 
is *not* for folks who have any doubts about their ability / training in dealing
with chemicals that can cause problems. ( Note: one can overdo this process
and nuke the blank. Even done carefully, the frequency will go up a bit …).

Since the electrodes in the original designs are just metal sheets, they *could*
be replaced if they have failed in some way. Maybe not a quick fix. Certainly
a “basement” kind of task. 

Bob

> On Jul 30, 2022, at 11:28 AM, Hubert Miller <Kargo_cult at msn.com> wrote:
> 
> I was thinking about the LF FT-241s for selectivity crystals in receiver circuits, either as add-ons, scratch built, or replacement, but I learned those crystals are not to be relied on
> and they are ALL approaching the end of the road. Perhaps there may still be a vanishingly small demand among vehicle collectors for the crystal sets for just display?
> -Hue Miller
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