Crystal penning

As referred in the previous post I was building a circuit with a Polyakov mixer, for that circuit I would need at least two crystals; 3920 and 4144 or 7840 and 8288 Mhz (double of the first values). Unfortunately when finding odd crystals their never in the junk box, so I remembered a page I have seen before on the subject of crystal penning, by Hans Summers.

The idea is to open the Crystal case and start "writing" on the crystal itself to lower it's natural frequency or as others have done start grinding the crystal to raise the frequency.

Opening the crystal case it's the hardest part because one has to have a little care, luckily for me I opened 3 of then with an hack saw and none got broke, expect the first one that got a little bit loose from the support but never the less it works.... the best technic IMHO is to cut first the thin (top's) part of the crystal case (both) and then gently start cutting the biggest parts until some holes start to appear, then slowly break the remaining metal. I took me about 3 minutes for each crystal, using a vice for support.


For the penning itself (I didn't tried grinding since my wanted frequency was lower) I used an "Edding 3000" pen like the ones used to make printed circuit boards, and just "wrote" on top of the crystal, sometimes it stooped working and it was time to let the ink dry. I also tried with nails varnish but had to clean part of it so the crystal would resuscitate, I guess nail's varnish is too thick for the job.

Now, here are my experiments: 

The one in the crystal tester is completely painted in black..


The 4 Mhz crystal, got to this:

I need 3920...

The 8Mhz version, almost 40Khz...:


I need 7840...

The 8.365Mhz crystal:

..close but not there to 8288 Mhz


....didn't managed to get further down on frequency, the crystals would stop working or maintain their frequency because the ink would not deposited any more, the pen would dissolve and clean the previous ink layer......so I will continue my search for the mentioned crystals, or build a PLL... :)

Anyhow it was fun!