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by Geoff Jull

From an advertisement in a 1930 car magazine......

The Graham Carfone, 6 or 12 volt.
No induction coil needed. Price: 5 Guineas.

Recently I was asked to help restore parts of a Carfone that a friend found in his 1932 Rolls-Royce vintage car.....

The most visible item was a handsome brass microphone sitting in a bracket on the rear door pillar. Tracing the wiring unearthed a very neat retracting reel behind the trim to wind back the microphone cord after use. A further discovery was an interesting receiver mounted quite incorrectly under the dash. A previous owner had assumed it was some kind of a horn or warning device and had mounted it out of sight.

Unfortunately the microphone had been badly damaged by someone attempting to unscrew the face plate to repair the worn cord. Not realising the faceplate was threaded left-hand, the good old alternatives of cold chisel and then the gas torch had been applied to the case. The heat damaged the carbon diaphragm and the three felt cups that held the carbon granules. Hopefully I can adapt a handset capsule to replace the original (Figure 1).

The receiver is fairly conventional having no magnets, coils wound to a very low 4.5 ohms resistance and a metal diaphragm with provision to adjust the gap between the pole pieces and the diaphragm. Attached to the case is a long resonator tube. Unfortunately the original trumpet type of horn is missing but I hope to modify the horn from an old Ericsson operator's head-set (Figure 2).

Part of the wiring loom included two torch-bulb holders (bulbs missing). The only clues for the presence of these appears to be very low resistance of the receiver windings, the course carbon granules and the original capsule stamped 9 volts. We presume the bulbs were used as a dropping resistance when used with a 12 volt battery. Any thought as to why torch bulbs were used in place of a cheaper resistor would be appreciated.

I intend to mount the repaired receiver on the driver's door pillar or on the glass division screen behind the driver's head. Note that there was only one way speech, no chance for the driver to answer back!

From another 1920's Rolls-Royce I was shown an early carbon microphone, this one designed to be permanently mounted somewhere in the back seat area. This microphone had a large carbon diaphragm with a diameter of over 2 inches covering two separate and individually insulated carbon blocks each having three small depressions to hold very fine carbon balls and with no felts to retain the carbons. Each block had its own external terminals on the rear of the case. Was this to ensure fail-safe speech reliability or was it an early attempt at stereo speech? (Figure 3). Your comments welcome.

Thanks to New Zealand member Geoff Gull for this contribution, first published in the ATCS Newsletter, November 1995.

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