Although the transistor was in use, the AMVF links to the radio stations used valves to modulate the telegraph signals onto the telephone lines. These valves varied from very large high power devices to the more normal types found in domestic equipment. Each Monday morning it was the duty of the apprentices in the Control Room to go round the equipment and measure the mutual conductance with the built in test meter. In this way the valves could be replaced before they failed.
At other times we would sit with the engineers on the desks where the schedules were set up. Since international capacity was expensive and limited many companies would book time to send telegraph messages. The duty of the engineer was to plug the customers line through to the radio station, tell the radio station to set up the call and tell the customer when the call was available. I made the mistake of telling the engineer I was working with that I did not like making phone calls - with predicatable results. Communicating with the radio stations was carried out either by phone or by morse. It was quite an education to see the engineer calling the radio station by key. One of the radio staions had the phone line connected to a speaker, the caller was expected to state his business and the receiver would then be lifted. I imagine quite a few apprentices were caught out by this system, amusing the radio station staff into the bargain! The telephone lines to the radio station were quite noisy, in the main I believe because they were phantom circuits
Electra House, The Overseas Telegraphs Main Office
The Control Room, described above, was on the third floor of Electra House. Below it, on the upper ground, first and second floors and on the fourth floor were huge rooms with tables of teleprinters tended by operators. Tucked away in a corner of these floors were the workshops. A good idea of what the floors looked like can be gained from here although the picture entitled "Rcvg on an early DC3-those water wheels lasted 50 years!" looks like what I knew as a direct printer. Perhaps a few words on the types of machines used might fit well here. The machine I have already mentioned used cable code and was used in my time solely on South American circuits together with autotransmitters. The punched tape used in the autotransmitters had two holes, either punched or unpunched, which carried the code. The DC3s had two motors, one to provide power and the other, phonic motor, to syncronise the machine. The drive for the phonic motors was provided from the Old Control Room at the front of the building on the same floor. The cable code machines were maintained in a small workshop adjacent to the Old Control Room. One machine, I recall, was lubricated by means of a wire loop that flicked oil from a sump up onto the clear casing from which it dripped onto the moving parts. Heaven help the apprentice who switched one on with the cover removed!
I was more familiar with the 5 unit, or Baudot, machines. The vast majority were Creed 7B or 11 with a sprinkling of 7E, some with reperforator attachments, autotransmitter 2 and perforator 45.
--to be continued
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AMVF (amplitude modulated voice frequency) a way of transmitting telegraph signals down a telephone line
phantom circuits A phantom circuit is one that uses the two pairs of a four wire telephone circuit to provide a two wire circuit. This is done by using the center tap of the coupling transfomers to connect to.
Circuit In this context the telegraph connection.
Cable Code A form of Morse code where positive and negative currents are sent instead of long and short signals
