So I came across a small treasure trove of WWII vintage channel marked radio crystals…
Almost the full set for the BC-604 transceiver that covered 20-28 MHz which was at home in armored vehicles of the time….most of them in their original packaging. These are quartz crystals in the FT-241-A holders. These were made in a couple of series, the 2 digit channel numbers and another series of 3 digit numbers that were intended for the BC-684 transmitter, which covered 28-38 MHz. These crystals are marked with their channel number and intended frequency. However, this frequency is of the channel as used in the radio set and NOT the actual fundamental frequency of the crystal itself. Therefore, the following may be of use if you also find a box full of these someplace:
TWO digit channel series:
Channel range– 0 to 79
Marked Frequencies– 20 to 28 MHz (MC for megacycles in older parlance)
Crystal Fundamental Frequency range– 370 to 516 KHz (.370-.516 MHz)
This fundamental frequency is equal to: (20+channel Number/10)/54
example: Channel 46, marked frequency is 24.6 MHz… (20+24.6/10)/54 = 455.5 KHz
THREE digit channel series:
Channel range– 270 to 389
Marked Frequency– 27.0 to 38.9 MHz
Fundamental Frequencies– 375 to 540.27 KHz
On these, the actual fundamental frequency is simply the marked frequency divided by 72 (or the channel number/720)
for example: Channel 327, marked frequency is 32.7 MHz, fundamental frequency is 32.7/72 OR 327/720 = 455 KHz.
Of note is that many of these, because they were near the 455 KHz range, ended up on the surplus market after the war and found their way into many HAM radio applications–especially building crystal filters for SSB exciters…..a browse through the early 1950’s ARRL handbooks and radio magazines of the era will yield a variety of projects that utilized these.
OK. So? Well, I listed the above information mostly as a resource for others. Just in case you run across some of these and do not know what they are about. Now you do. It is likely that this information exists elsewhere…afterall…I did find it before……..but in case it doesn’t anymore, there it is. But, I am a geek and an enthusiast for such things so naturally finding these reminded me of something else I had been working on, possibly as a presentation for our local rock and mineral club, the Chugach Gem and Mineral Society.
Now a little closer look…
Quartz is not a particularly rare mineral and it occurs naturally in a variety of colors and types and can be found around the world. Today, it is collected by rockhounds and those interested in it for metaphysical purposes. Many, however, are unaware that this common material is not only necessary for our modern, technological society, but that it helped win a war.
A very interesting thing about quartz crystals. They are everywhere today. In your clocks and watches. Your computer. Smartphone. Car. Damn near any piece of modern technology that you can point at has at least one tiny crystal in it someplace. These crystals are used most often in oscillator circuits, where they generate a fixed and stable frequency base for timing, RF transmission, etc……and in filters, where they allow only certain ranges of frequency signals to pass and block others. Their development marked a new era in electronics technology where a stable frequency source could be created that was very low cost and small enough to be carried.
Today, these crystals are grown in a lab under controlled conditions. Any kid who has ever played with one of those little crystal growing kits with the solutions that precipitate little colored crystals onto a string hanging in the mixture will understand the basic idea. Growing quartz in a lab was revolutionary. It resulted in materials of exacting composition, free of flaws, that could readily be cut and “tuned” for the needed frequency. Lab grown crystals made to order was a development of the late 1940’s, and was brought about by WWII. Prior to 1950, all the quartz used for electronics was natural quartz.
Much of that pure, good quality “radio grade” quartz came from only a few mines in Brazil and had to be mined by hand so as to not damage the specimens found. (a very labor intensive process that no doubt motivated those in the postwar world to find ways of growing synthetic replacements) In an era with a world war of a high tech nature, the need was great on all sides. Just like oil, steel, copper, and everything else, quartz was a strategic mineral and in demand by every major combatant. Good radio crystals insured good stable communications, radar contacts, and information flow…free from major frequency drift…..far better than what was possible with the simple tuned circuits of the teens and twenties–a temperature controlled quartz oscillator was “rock solid” by comparison–literally and figuratively so! …and that could mean the difference between winning and losing. Since the best quartz for electronic use came from only a few mines-all in Brazil, it is no great leap to envision cloak and dagger spies running around South America attempting to help their side gain control of this limited resource. Forget crystal skulls, this was the real deal. Perhaps another “Indiana Jones” movie? (drop me a note, Mr Spielberg and we’ll talk)
If you take a piece of quartz and cut it to a certain size, smooth and polish it….then mount it under pressure in a holder with electrical contacts, it can be made to resonate when power is applied. This is due to the piezoelectric effect of the material and is not unlike the reversed process used in the little push-button lighters on your gas grill.
There are a variety of cuts used to meet specific needs, in almost all cases the main goal is stability under a variety of operating conditions. During the 1930’s, RCA developed what they called a “V cut” This is a cut that was oriented away from the optic or Z axis and also not parallel to the X or Y axis of the specimen. This allowed for a low temperature coefficient crystal and also added thickness for mechanical strength and durability. The 1939 RCA catalog covering radio and early television equipment gives a good description of their V cut crystal lineup and some of the manufacturing steps involved. These pages are reproduced here:
RCA 1939 V Cut Crystals info_Doc – Jan 18 2017 – 15-45
During this same period, and on into the 1950’s, many HAM radio operators would cut and lap their own crystals or modify existing ones in order to tune them to a specific resonance frequency for their project. This was, needless to say, an exacting and time consuming process that required a high degree of skill. A second look through RCA’s described manufacturing process gives an idea of just what a feat it was to accomplish a similar task in one’s own home.
Crystal oscillators were first produced in 1918 and from 1929 until the advent of atomic clocks in the 1950’s they were used for frequency standards by the US National Bureau of Standards. These carefully made and maintained crystal clocks replaced the previously used high accuracy pendulum clocks and they could achieve a +/- 1 second per 30 years accuracy. Only the Rubidium and Cesium Beam atomic clocks of today can exceed this specification….and thus far, those are too big to wear on your wrist (in spite of the photo I found below)
