THE CK722 CLASSIC

GERMANIUM TRANSISTOR MUSEUM

 

YOU WONíT BELIEVE WHATíS INSIDE THE CK722?

The transistor industry was very fast paced in the mid 1950ís, with rapid advances in transistor basic research as well as with manufacturing improvements.The hearing aid industry continued to be a major consumer of transistors, and so at Raytheon there was a real effort to produce physically smaller transistors - smaller transistors meant that more transistors could be fitted into higher performing, smaller hearing aids.By mid 1955, Raytheon had perfected the manufacturing processes necessary to produce a tiny hearing aid transistor less than 1/4 the size of the original CK718 hearing aid unit.The yield of these small metal cased transistors was still less than 100%, and it was possible to test and sort the transistors into different levels of performance, with the highest performing destined for use in hearing aids and the worst (although still meeting basic specs for gain and noise) ideally suited for the same hobbyist market as the CK722. There was a major problem though. These new units may have performed electrically the same as the CK722, but the physical dimensions were quite different.The tiny units were hard to work with manually and didnít have the same pinout dimensions to fit easily into the transistor sockets that were in common use for CK722ís.Also, from a marketing perspective, Raytheon had developed a huge name recognition for the CK722 and really wanted to use the same id for this new type.A very elegant solution was devised - the tiny metal cased transistors were to be mounted inside a larger blue metal package which had the same dimensions as the original black plastic CK722. In this manner, Raytheon was able to sell the thousands of new tiny hearing aid transistors which were not performing well enough to be used in hearing aids, but were more than adequate for the existing CK722.Double encapsulation at its best!!

 

 

 

The small blue transistor on the far right of this photo is an example of the small hearing aid transistor that was manufactured by Raytheon in the mid 1950ís.Compare it in size the to the CK718 original hearing aid transistor.Also in the mid 1950ís, the new blue metal cased CK722 replaced the original black plastic CK722, but, as can be seen in the photo, these were still the same physical size.If you take a look inside the blue CK722, youíll find a small metal hearing aid transistor.Since this is a destructive process, I donít like taking apart a CK722, so I use only damaged units.In the photo above, you can see the small transistor that I took out of a blue CK722, and, on the far left,you can see the small transistor that came out of a silver CK722.I have removed the back of a silver CK722 and you can see that a grey plastic spacer is used at the bottom of the metal case to force the small transistor leads into the standard CK722 pinout dimensions.Some type of white plastic ďpottingĒ compound is used inside the CK722 to physically mount the small hearing transistor.I suspect that all metal cased CK722ís have small units inside.

 

 

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