Discussion in 'Ham Radio Discussions' started by KF4SCI, Jul 9, 2017.
Has anyone tried regrinding crystals to another freq? If so... any sucess?
Many years ago, when novices were required to be crystal controlled, they could change the resonant frequency of a crystal only a few KHz (Kc back then)
You could raise the frequency by rubbing it on an abrasive surface (VERY fine sand paper?) or lower it by coating it with (graphite?) from a pencil. Crystals were plentiful in the years after WWII but they were often on the wrong frequeny for the novice.
I remember doing that as a novice with no money to buy lots of crystals. Quickly learned how to not 'over-shoot' the novice band limitations and correct the inevitable errors. But then, we built our own transmitters too
Yes, I did many of them, with fair success. I found a source of inexpensive 40 meter crystals in FT243 holders that I could move.
The grinding method used Comet cleanser as the grinding agent, and a plate of glass. The Comet would be put on the glass in a small pile, and water added to make it a thin paste. The crystal would be moved around in the Comet, with considerable finger pressure applied, in a 'figure 8' pattern. The objective was to maintain the geometry of the crystal and get a smooth, consistent grind, while being careful to keep the faces of the crystal parallel to each other. Crystals that were not parallel would either not oscillate correctly, or in some cases, oscillated on more than one frequency. Higher frequency crystals were harder to do, because the material was already quite thin.
A much better solution was to etch the crystals with hydrofluoric acid. I used Whink Rust Remover, which contains a very weak concentration of Hfl. You must use the proper kind of plastic for tools and vessels to contain the liquid - take a hint from the bottle the Wink comes in - Hfl will dissolve glass! I'd test anything you intend to use, first. Be extremely careful of the Hfl - it is a very dangerous substance and a systemic poison. Here's the Material Safety Data Sheet on the nasty stuff, which you can buy almost anywhere - http://www.whink.com/cmssites/ws081...SDS/Moonglum/Rust Stain Remover 10062008.pdf
The etching process is usually very slow. I strongly advise checking the crystals frequently, so you don't overshoot the desired frequency. Etching depends on temperature so the warmer the solution, the faster it will etch. I advise against trying to calculate the time required - I ruined a whole batch that way once, when the etching suddenly started going faster. I used this method on any crystal where I could remove the crystal blank from the holder, regardless of type.
The crystals in metal cans with metal electrodes directly on the quartz can be lowered slightly in frequency by rubbing pencil lead on them. FT243 cases can be modified to incorporate a threaded hole and screw in the face plate, so you can vary pressure on the crystal from the outside, which will also change the frequency.
Yes. But it is hard to find a good pencil now a days.
Here is what you need!
These kits were sold from after World War II until the mid-1950s.
Actually, there are a number of things that can be used to grind crystals including Ajax Cleanser and rubbing compound used with paint.
You need as flat a surface as possible upon which to do the grinding. A sheet of glass, like for replacing a small window or from a picture frame, works very well and are readily available.
The best way to work the crystal blank is in a figure "8" motion on the grinding surface. One has to clean the crystal often and check the actual frequency. As one gets closer to the desired frequency, the more often that the frequency needs to be verified.
If the crystal gets "sluggish" or even quits oscillating completely, grinding the edge of the blank, at an angle, often gets the crystal working again.
Plated crystals, i.e. HC6/U types, can have the frequency moved a little bit. Of course, one has to un-solder the the main housing from the base which contains the actual crystal. To raise the frequency, a pencil eraser can be used to remove some of the plating material. To lower the frequency, putting "dots" from a #2, or softer, pencil on the plating will work. However, the frequency, of plated crystals, can only be moved a small amount before the crystal stops oscillating.
I have, over the years, ground FT-243 type crystals and have "moved" the frequency on HC6/U crystals.
Yes - I did a number of them myself last year. But never more than a few KHz of change. I was just filling in some holes in my "Quartz VFO" (I have over 60 rocks that cover almost all of 40m with big chunks of 80 and 20 as well) And I killed a couple along the way in the process. There's lots of tutorial info on the web if you Google for it.
It's a fiddly, delicate art but rewarding when it works.
As a transmitter engineer with TWR africa, I raised some FT crystals made for the 7MHz band for a maximum of 5 kHz by grinding them on glass, using a mixture of toothpaste and copper-polish. Usually the Q goes down too much if you try to go higher. The lower Q can be helped by increasing the gain of the oscillator. By redesigning the oscillator you can "pull" the crystal about one or 2 kHz further, specially if you replace the tube oscillator with field effect transistors. A pencil stripe lowers it somewhat. Too much pencil work stops the oscillator but that is reversible.
I have a home-brew, tube type, VXO using 12 MHz crystals that I use for 2-meter and 70 cm (432 MHz range) transmitters. All the crystals are HC6/U types. Depending on the individual crystal, the VXO can "warp" the fundamental frequency from about 6 kHz to over 15 kHz. The basic circuit is from an older version of the ARRL VHF Manual.
One can also "warp" the frequency of a non-plated crystal by varying the pressure on the crystal. There were, still are, crystal holders that have a screw through the front that allows the pressure to be varied which, in turn, varies the frequency.
Such arrangements can vary the fundamental frequency from a kHz, or two, to maybe as much as 5 kHz depending on the characteristics of the individual crystal.
Used to do it a lot....especially when I was a novice. Had about 100 crystals ground.....just about a week before they gave us VFOs. O well.