This is the GECoPHONE Junior Crystal Set BC1700

vintage radio wireless crystal cat's whisker detector museum vintage marconi gecophone galena gec bc1700 junior

This is the GECoPHONE Junior Crystal Set BC1700. Built by GEC (UK) in 1925, this simple crystal set is housed in a polished mahogany case. Connection points for the aerial, earth and headphones are provided, and the controls comprise a tuning knob and a lever to adjust the detector. The set would have cost 15s 0d in 1925 with the 1600m Loading Coil costing an extra 7s 6d.

At the back of the set is the 'Entirely British Manufacture' BBC stamp.

For more information on the BBC Stamp and Post Office registration scheme that was in operation during the 1920s follow this link: BBC Stamp.

The circuit diagram for the set is shown below:

The aerial (antenna) is connected to either terminal 'A' or terminal 'B' dependant upon its length. Long aerials were connected to 'A' and medium length aerials to 'B'. A long aerial has significantly more capacitance (to earth) than a medium length aerial but the tuning control needs to have roughly the same tuning range for either. Taking the long aerial connection to a tap some way down the tuning inductor reduces its loading on the aerial tuning circuit to roughly the same as for a medium length aerial connected to 'B'. Hence the tuning range remains roughly constant. For MW operation a shorting link is fitted to the 'Loading Coil' terminals. The detector is of the galena/cat's whisker type. A picture of the detector is shown below:

galena/cat's whisker detector: click on image for a bigger picture

The galena crystal is held in the screw-cup to the left and the cat's whisker (a curly piece of wire) is held in a sort of pin-chuck to the right. In operation, the handle attached to the cat's whisker is carefully slid in and out and/or rotated so that the cat's whisker drags across the surface of the crystal. The crystal is covered in many contact sites that will allow rectification to take place. With careful adjustment the cat's whicker will fall upon one of these sites and a when this happens a detector is formed. Follow this link for more information on Crystal Sets. The audio output from the detector is connected to the headphones via terminals marked 'T' ('T' for 'telephones' as they were know then). Two sets of headphones can be connected in series by making use of a third, unmarked, terminal between the two 'T' terminals. Not that there is no RF decoupling capacitor connected across the audio output as was often seen on other crystal sets. No doubt the designer felt that the headphones themselves have sufficient capacitance for the task. For reception of the 15kW 5XX station on 1600 metres (broadcast from the Marconi works in Chelmsford, England) the 1600m Loading Coil could be plugged in.