[ARC5] SCR-274 calibration Xtals

[k1uhy at comcast.net]

In post war 40's and 50's ham magazines that dipping was a well publicized method to raise the frequency but AFIK the history wasnt mentioned and came across as a brand new idea! Maybe Bliley hams started it. 

I used it on several surplus FT-243 rocks to move into the Novice bands or move the frequency a bit above the QRM pack on those common frequencies. Later it was used on various 8mc rocks for 6 and 2M. 

Your explanation explains why the process ranged from excellent to barely noticeable movement..thanks! 

----- Original Message -----

From: "Dennis Monticelli" <dennis.monticelli at gmail.com> 
To: "Bill Cromwell" <wrcromwell at gmail.com> 
Cc: "ARC-5 Maillist" <arc5 at mailman.qth.net> 
Sent: Friday, August 19, 2016 4:47:16 PM 
Subject: Re: [ARC5] SCR-274 calibration Xtals 

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: 

<blockquote>
On 18 Aug 2016 at 14:57, J Mcvey via ARC5 wrote: 


<blockquote>
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, 


<blockquote>
Is this typical? 

</blockquote>
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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