Crystal grinding: When electronics were REALLY hands-on

Reading Steve Leibson’s comment on the relatively recent use of crystals for electronics reference frequencies made me remember my lab partner at UC Irvine in the 70s, who was an amazing ham guy, saying that he adjusted his quartz crystals when they weren’t the right frequency by gently rubbing them in some toothpaste. I asked Steve if he’d ever heard of this practice –  – could you really pop the crystals out of their little can, do the toothpaste adjustment, put them back in their can, and have them work?

Steve, like me, was never a ham, so he asked Paul Rako, who knows everybody, if he could shed some light on crystal-tweaking. Paul’s not a ham either, but he ran it by several of his ham buddies, and Dennis Monticelli, a Fellow at National Semi, responded with this recollection:

"It was common back in the 30’s, 40’s, and 50’s to do a little crystal grinding. I did some myself in the late 60’s when I was a teenage ham. [Yep, that’s when my lab partner would have been tweaking.] There were many different home-grown methods. In the 30’s, the ham bought a crystal blank and ground it down to the target frequency using a slurry of grinding powder and water on a glass plate making little figure 8’s. Grit size was important and especially maintaining flatness. Typically, you only trusted yourself to grind one side so at least one side would stay flat. They would have killed for the sophisticated lapping machines we use in the semi industry to backgrind wafers.

During WWII crystal making took a leap forward in the US and turned into a big cottage industry supplying the wartime needs. There is a wonderful new book out on that subject that goes through the history and basically makes a "how we won the war" claim. Following the end of hostilities, the cottage industry collapsed and huge numbers of used and NOS crystals in screwed together non-hermetic holders flooded the market. Hams began grinding again and didn’t have to move the freq as much as before because the selection of starting material was huge. Hams could then use mild abrasives like comet or even milder like toothpaste powder. Today you could use wet silicon carbide sandpaper if you wished. I actually succeeded at etching them to freq using a very dilute HF solution when I was 15 and didn’t know how dangerous HF acid was. You could also lower the freq by loading down the blank with pencil lead. It only moved the rock by 1 or 2KHz and the Q went down, but, hey, it worked and it fit the budget of a teenage mowing lawns to buy electronics.

I still move rocks today for use for in my vintage equipment. I find old crystals at the swap and then etch them to freq at home using a dilute solution of buffered HF because I am a tad smarter than I was back then (still crazy, just a tad smarter).”

Bingo - sound exactly what my lab partner described. 

btw, Dennis says the book he refers to is: Crystal Clear: The Struggle for Reliable Communications Technology in World War II.

And Paul came up with these excellent crystal links:

A nice history of crystals: http://www.ieee-uffc.org/fc_history/bottom.html

Crystal grinding tutorial: http://www.subdevo.com:82/n4jvp/crystal%20grinding/CrystalGrinding.htm

A link-fest on crystals and old radios: http://www.af4k.com/Boatanchors_Directory/Crystals.htm

Thanks, Dennis, Paul, and Steve!