AF4K Early Radio Experiences
I used to work in a crystal factory and here is roughly how the process worked there:
I worked for Cathodeon Crystals in Linton, Cambs. England in 1970 (Gosh that was ONLY about 35 years ago!) and I worked there for a few months helping to make the precious rocks for the radio world!
I used to commute over 20 miles each way from Cambridge in a Morris series-Z "Half hundred-weight van" with a 900 c.c. sidevalve engine, that had a maximum speed of 40 m.p.h. (no tickets) and it had to use 40 weight tractor oil in the engine due to leaking oil.
The first thing that they showed me at Cathodeon, was a very impressive process for pumping air out of the hermetically sealed crystal units, and these were all either HC-6/U size or the little HC-25 and HC-49 ones that were in vogue in the 70s and 80s. What was cool was that they made SOME HC-6/U size units with a GLASS case! Yes, the entire case was made of clear glass. I don't know who was buying those glass xtals or what they used them for. One could only guess. I've never seen them since!
The fabrication of crystals at Cathodeon Crystals ran something like this:
Step 1 involved the receiving in of the raw quartz crystal, which as I remember was a man-made rock about 8-10 inches long, perhaps longer and about 1-2" in section. I think they got these from a supplier in Russia.
I think there was a step here that I was less familiar with in which the large raw crystal was attached to a laterally reciprocating machine that ground the top edge perfectly flat using a grinding wheel with a mysterious white fluid that ran over the wheel all of the time to keep it cool. It took about 5 minutes at a time to run this machine, and the operator sat on a chair watching it, and getting hypnotized by the motion. They would often fall asleep and we had great fun with these folks in the factory! One white haired elderly gentleman with a red bow tie(!) did this job and he was known to take 40 winks in the afternoons, so someone put a metal pan on the side of the moving table of the machine and it eventually fell off, making a terrible clatter and woke him up, much to the amusement of all the nearby factory workers!
The raw crystals were then attached to a base plate in a computerized saw after being x-rayed to get the ANGLE of the crystalline structure inside the rock. They were then lined up on the base plate in the saw, a carbide-edged rotary blade came down onto the rock and sliced it up like sliced bread! The individual slices were then measured with a micrometer to make sure that they were the right thickness that was needed for the order. They were also x-rayed once again and the angle to the nearest 1/100th of a degree was recorded on the paperwork that travelled with the batch of crystals.
The next step was a lot of fun. They would take the crystal blanks and put them in a device called a "lapping machine" - which had a set of cog-like wheels that ran around inside two concentric circular gears, the outer one rotating. These smaller cogs each had six holes in them that could hold the crystals. The cogs came in many different thicknesses depending on the desired final thickness and the beginning thickness of the blanks. Once the crystals were in place, they would be run FIRST in a slurry of thick grease that contained an abrasive component. The operators had charts for how many minutes to run them for a desired shift in thickness. The COARSE adjustment was made first, and you could soon make a large difference in those jewels! Operators were given a target thickness. Typically about 60 blanks would be whirring around in the lapping machine at any given time of the day. They had a number of these machines and they ran all day long. The final step in the lapping process was a set of specialized lapping machines that were connected to the ANTENNA TERMINALS of some EDDYSTONE receivers! As they turned inside the machine, the grinding process would set up enough vibration of the crystals that it generated NOISE In the receivers and you could HEAR the crystals as the frequency shifted. Of course they would not ALL be at the exact same frequency so you got a kind of wideband noise!
Another problem was with "outliers" - some of the crystal blanks would wind up being too far off from their target thickness at the early stages or too far off frequency near the end. Often these ones had to be tossed out. Anyone here live near a crystal factory?? he he!!
After that the FINAL fine tuning was done by spraying GOLD onto the crystals, and that is how this plant did their electrode attachment. They put gold on the corner of each side of the blank for an electrical connection, and if they needed to lower the frequency a tad, they just added a little bit more gold. Then the wirs were soldered on, the whole thing was mounted in a holder & hermetically sealed with the vacuum pump machine and out they went!
New note added March 2014 - for those who are interested in understanding how to measure ESR, crystal current etc. -
CRYSTAL DESIGN FACTORS
This an overview of crystal design and various factors. You can look at the rms voltage across a crystal or the load cap measured by an oscilloscope which sums net effect of parallel capacitance of holder, the crystal's series inductance and resistance (power lost in mechanical energy, heat etc.) and use this with Ohm's Law for AC to determine the resistance, current, etc. It is best to keep the smaller crystals down to 40 mA. They have typical ESR resistance of 200 ohms area, while the older FT243 crystals have somewhat higher ESR, and being also more rugged can handle 60 - 80mA. NOTE that the smaller, newer crystals will not actually fracture until you reach around 200 mA in most cases, but above about 50 mA you will start to see some heating and drift. Avoid the old 1955 "How to become a Radio Amateur" 6L6 Tri-tet circuit, as it is hard on most crystals. This is often known as the "Crystal Cracker" circuit!
Bry Carling, AF4K
BUY AF4K Amateur Radio CRYSTALS
CRYSTAL CLEANING AND REVIVING TECHNIQUES
MORE Crystal Information...
Crystal Lapping Machines -typical of the kind I used to work with at Cathodeon Crystals.