Heterodyne Crystal Failures Mode?

[Bob and Bettina Groh]

Good data, Jim, and good reasoning.  Excellent work.

To throw a bit of info your way, the crystals are certainly affected by 
'crud' on the plates and they (typically) are 'sealed' in their cases. 
Two of the mechanisms (of many) that affect the crystal blank are 
accumulation of mass on the blank (i.e. stuff precipitates out of the 
environment onto the blank) which should (theoretically) decrease the 
resonant frequency and the loss of mass (e.g. the minute vibration of 
the crystal in operation actually flakes off parts of the crystal) which 
should theoretically increase the frequency. It would seem that neither 
is the case here but it could be that crud was 'shunting' the crystal 
blank and greatly decreased the parallel resonance value  - the 
impedance goes, as you indicated, through (as the frequency goes from 
low to high) a series (zero) then  the parallel resonance.

And, yes, they do extract most of the air and then 'seal' the crystal 
can. The hole was there for that purpose. One source of the crud that 
contaminates the crystal blank is the solder used to seal the can - that 
is why the higher quality crystals have a hermetically sealed can which 
is welded on. Plus they use either a vacuum and/or a rare gas to create 
a clean environment for the crystal.

Anyways excellent work and nice data. It would be nice to have some more 
explicit data, before and after, of the series and parallel resonant 
points as well as the values of the impedance at each point. But, hey, 
that would be gilding the lilly!

73 de WA2CKY
Bob

Jim Flanagan wrote:
> Please excuse me if this accidentally was double posted....
>
> Hi guys..
> Well I spent some time this weekend looking into this xtal issue.  The
> HW-101 that I am restoring had three crystal that would not oscillate:
>
> 22.895 MHz ( 14 MHz Band)
> 37.385 MHz ( 28.5 MHz Band)
> 37.895 MHz ( 29 MHz Band)
>
> What I ended up doing is to take each one out, one at a time, and make a
> quick series resonant measurement using a sig gen and scope.  I could 
> tell
> that there was a 'slight' resonance on each that was somewhat 3-4 kHz 
> higher
> than it should have been.  I also know that these xtals used in a 
> parallel
> mode, but the series resonance was easier for me to check and once the
> series res was found then the parallel res would/should be about 1.5 
> kHz or
> so higher.
>
> Thus, verifying that the crystals *appeared* dead, I used a heat gun 
> to melt
> the solder around the base and remove the cover.  Under a jewelers loop
> there were no discernible fractures, etc.  I made a simple continuity 
> test
> on each leg and all looked OK.  Now the magic.  I used some 99.9% pure
> alcohol with a q-tip and gently cleaned each side of the quartz material.
> Using another Q-tip, dry, I went back over the quartz material to help 
> dry
> out the device.  Upon a recheck using my sig gen/scope, the xtal was 
> again
> functioning!  I carefully resoldered the case, checked again and then
> reinstalled the xtal back into the unit.  Worked like a champ!  Of 
> course,
> time will tell how long the fix is good for.
>
> I then repeated the same procedure for the other two xtals.  Each time 
> with
> success. So, the question is, WHY??  Are the xtals subjected to 
> residue that
> caused them to deteriorate?  When I cleaned them, there was no noticeable
> residue, etc.  I notice that there is a small pin hole on the side of 
> each
> xtal that is soldered shut.  Do they pull a vacuum on xtals?  If not, 
> what
> is the hole for?
>
> Well, there you have it.  I guess the next time a xtal fails, I will try
> this trick to see if I can bring it back to life.
>
> Your comments & criticism are welcome..
> Jim
> WB5KYE
>
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