Vintage quartz crystals
Quartz cristals are used as a frequency standard in many vintage (military)
radios, spy radio sets, covert radios, and even in modern computers. Over
the years, a wide variety of crystals, with varying specifications and
enclosures, have been made. As the name of the shape of the crystal enclosure
is sometimes very confusing, we have listed the most popular ones on this page.
All dimensions are in mm.
One of the most popular series of crystal enclosures, is the so-called
HC-series. The name starts with 'HC', followed by a number (e.g. 25)
and an extenstion — commonly '/U'. Over time, new extension have been
'invented', some of which have become official, and some of which have not.
HC-6/U, HC-17/U, HC-33/U
One of the most popular crystal enclosures for vintage spy radio sets,
is the HC-6 shape. The basic variant — HC-6/U — has thin pins that can
be inserted in a suitable crystal socket. A variant with thicker (and
longer) pins is the HC-17/U, which is sometimes called HC-6/L. The
shape is also available with solder legs, in which case it is known as
HC-33/U or HC-6/W.
Note that the case segments of the HC-6/U is soldered together at the bottom.
If the case parts are welded, the shape is designated HC-48/U.
A longer variant of the HC-6/U is the HC-13/U, which is often used in
frequency standards. It is shown here aside the HC-6/U.
One of the most propular crystal shapes for military equipment of the
Cold War, is the FT-243, which used to be available in abundance in
surplus stores in the Unites States and in Europe, in the 1960s and 70s.
The case of these crystals is commonly made of Bakelite and consists of
two parts that are held together by screws. Furthermore, these crystals
can generally take a higher current that the later (smaller) ones, which is
particularly important with valve-based equipment.
The FT-243 shape consits of a bakelite holder with a lid at the front.
The lid – which is either metal or bakelite – is generally held in place
by three screws. The FT-241 shape has the same outer dimensions, but
consists of a top and a bottom part, held together by one or two screws.
In many cases it will be possible to substitute FT-243 crystals by
HC-17/U ones — also known as HC-6/L — as the pin-distance (12.34 mm)
and pin-diameter (2.35 mm) is identical. There are even adapters for
fitting a HC-6/U crystal in a socket for an FT-243 crystal, as shown below:
Most of the allied spy radio sets of World War II, used crystals with
the case specifications shown below. There are many variations to the
size and shape of the bakelite enclosure, but the contact pins always have a
diameter of approx. 3 mm and the distance between them is 19 mm.
They are commonly made of bakelite and are usually black or
brown, but they may have a metal panel.
The image above shows a USA/UK WWII crystal aside a later USA FT-243
crystal. In some cases, radio sets had a double-socket which accepted
both versions. This was particularly the case with the Cold War
spy radio sets of Western countries. It allowed them to use various
In addition, some WWII spy radio sets used low-profile circular crystals,
such as the ones shown above. They have the advantage that they occupy less
space, which was necessary for spy radio sets like
The rightmost example shows a similar crystal with thicker (4 mm) pins.
A variation to the above themes in the DC-34 shape, which was commonly supplied
with the BC-669 transmitter. These crystals are sometimes found with spy radio
sets as well. They are smaller than the common American crystals shown at the top of this section, but have 4 mm thick pins. Adapters were available for
using this type with devices that only accept crystals with 3 mm pins.
During World War II, the German crystals were slightly different from
the allied ones. The pins were 1 mm thicker and the distance between the
pins was 1 mm larger. Furthermore, the shape of the enclosure was
different — often cylindrical. The drawing below shows a
The Bakelite enclosure is usually (dark) brown or black.
The case hight might be lower than 38 mm, as shown in the diagram below,
but in most cases the overall hight remains close to 38 mm.
Crystals like these were commonly supplied with German wartime equipment,
like the Radione RS-20M transmitter
and the various agent radio sets that were
used by the German Abwehr.
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© Crypto Museum. Created: Sunday 06 September 2020. Last changed: Wednesday, 28 October 2020 - 08:51 CET.