[ARC5] SCR-274 calibration Xtals

[Dennis Monticelli]

FYI.  I have worked with surplus WW2 crystals a lot and can say that it was
relatively common for the upward drift to occur.

 It was not until approximately late 1943 or well into 1944 that ground
crystals were put through the proprietary Bliley final etch process.  This
process was developed to improve frequency accuracy but incidentally cured
serious drift problems with early rocks put in service in the ET.  The
Signal Corps eventually forced Bliley to pass that secretive process to
other suppliers, though I don't think all makers applied it.

How it works:
Grinding produces lots of fine grit, some of which lodges in the roughened
surface of the blank and does not wash out.  As the crystal vibrates in
operation it works loose some of the grit, which reduces loading and moves
the crystal freq upward.  The final short HF bath easily removes the
embedded grit along with molecules that are weakly chemically attached to
the surface.  What is left behind is a well ordered stable crystal surface.

An experiment you can do:
Take a rock from 1943 or earlier and give it a dip in a weak acid for a
minute, wash thoroughly with distilled water and alcohol, and then
remeasure freq.  It will move upward.  Do this again for an additional
minute, then a minute more, etc.  You will observe a curve of freq change
vs time that drops sharply at first and then settles into a linear
relationship of slow change vs time.  You are observing the acid attacking
the "low hanging fruit" and then hitting the well ordered crystal.
Usually, activity will improve as well.

Maybe more than you wanted to know :-)

Dennis AE6C

On Fri, Aug 19, 2016 at 12:37 PM, Bill Cromwell <wrcromwell at gmail.com>
wrote:

> Maybe the crystal started out 70 years ago at 10 kc low <evil grin>.
>
> 73,
>
> Bill  KU8H
>
>
> On 08/19/2016 01:31 PM, Kenneth G. Gordon wrote:
>
>> On 18 Aug 2016 at 14:57, J Mcvey via ARC5 wrote:
>>
>> Just for fun, I decided to see how the magic eye transmitter calibration
>>> system
>>> worked in practice. It actually worked pretty well, but the resonance
>>> was at
>>> 3502 khz instead of 3500 khz ( marked on xtal) .
>>>
>> Gee!! That close? That is amazing for a crystal which is over 70 years
>> old...not to mention the other components in the circuit which could
>> effect the
>> resonance frequency,
>>
>> Is this typical?
>>>
>> No. It is far too good. Typically, crystals drift upwards a certain
>> amount per
>> year. By now, it should have been a lot further off.
>>
>> As others have mentioned here, you can tweak it exactly on to frequency
>> with external components. Parallel capacitance will lower the frequency.
>>
>> Ken W7EKB
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> --
> bark less - wag more
>
>
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