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    Gerry O’Hara

    The first transistors suitable for use in RF applications used the alloy-junction fabrication process. However, Mullard followed the US-led technology direction by adopting the alloy-drift fabrication technique. The first RF transistors manufactured by Mullard that were available commercially in quantity were the OC169, OC170 and OC171, introduced in 1959. This range was supplemented in 1961 with the AF117 to AF118 series. All these transistors were housed in a TO-7 case style that included, in addition to the base, collector and emitter connections, a fourth lead, connected internally to the transistors’ aluminium case to act as a screen.

    The internal construction of this transistor family is illustrated in the attached photo. The collector lead acted as the support post for the semiconductor substrate, with fine wire connections from the emitter and base leads to the substrate. The upper part of the can was filled with a gob of silicon grease (presumably to protect the internal assembly from moisture ingress and possibly to add some resilience against shock). Beneath the silicon grease is a small air space – ok, a bit crude but functional, but here is the weird part…

    Over many years, microscopic metallic filaments develop inside this air space (these are reportedly only 0.008mm across). These filaments are noted as being ‘tough, springy and electrically conductive’. Eventually one or more of these filaments contacts one or more of the leads inside the can, shorting it to the can – effectively ‘killing’ the transistor. This phenomenon (no explanation found) affects both used and new-old-stock (NOS) devices at random. A sharp tap on the transistor case can effect a temporary cure by dislodging the offending filament. Alternatively you can try cutting the screen lead.

    Download DSC00194 [1024×768].JPG. (Caution: This file may not be virus scanned.)

    Download OC171 Whiskers in Silicon.jpg. (Caution: This file may not be virus scanned.)

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