Describes the construction and operation of a preset point contact Josephson junction for use with submillimetre wave lasers. The device has good mechanical stability but cannot be reliably recycled between room and liquid helium temperatures. However, a continuous life of several weeks is readily achieved by using the device in a 251 liquid helium storage vessel. The device has been used as a harmonic mixer and frequency monitor for wavelengths primarily of 0.3 mm and longer, although operation has been observed down to 0.12 mm.