[Milsurplus] Question for Metallurgically-Smart People

[Richard]

It doesn't take much.  I once used a pencil eraser to clean oxide off a 
crystal because it seemed like a good idea, and it moved up sharply.

Richard, AA1P

On 5/22/20 8:23 AM, David Stinson wrote:
> Thank you all for the lively discussion.
> The reason I ask:
> Off-and-on, I've been working with crystals, etching them to freq
> or "pencilling" them down as needed.
> Some crystals have a silver plating on the blank.  This isn't for
> connectivity; it's to add mass and inertia to the blank in order
> to bring the crystals oscillation frequency down.
>
> I discovered this when trying to etch these crystals.
> The blanks were in "regular" holders; i.e. between two
> metal plate with raised corners- no direct contact
> with the metal coating.
> In the etchant, the metal coating bubbled vigorously.
> To my surprise, etching that metal coating moved the
> crystal's frequency up rapidly and by a large amount.
> I didn't make notes at the time, but I estimate I etched it
> for a couple of minutes, then became concerned at how
> quickly the metal was dissolving.  I removed the blank,
> cleaned it and tested, finding the crystal had moved a
> huge amount- I didn't write it down, but it was tens
> of KCs!  Way past my intention  IIRC, the blank started
> at something below the AM window on 80 meters and,
> after the short etching, ended above 4 MC.
> That was hard to imagine, so I tried another on a
> useless freq and the result was the same.  I did manage
> the use some of these crystals, but the etching time
> was in seconds!
>
> This is the same principle as "pencilling-down" a crystal,
> but with the uniformity and fine control needed to get
> the crystal spot-on and still active.  One reason "pencilling-
> down" often fails is because the random distribution of
> material on the face of the blank can cause it to stop
> ringing properly.  The metal coating is uniform, so it adds
> to the inertial moment of the blank without the random
> irregularities of "pencilling" that can kill oscillation.
> If there were a practical way to deposit this material
> on crystal blanks, there's just a world of now-useless crystals,
> too high in frequency, which could be "re-tuned" to usefulness.
>
> If I live long enough for another project,
> I'll play around with this some more.
> Be a while- still have parcels to ship and
> radios begging for attention ;-).
>
>  GL OM ES 73 DE Dave AB5S
>