Home:  Subscriber Apparatus: GPO TRIMPHONE (DELTAPHONE), 1960's


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The 'Deltaphone' was introduced in 1963 by STC and won a design award in 1964. Adopted by the GPO and called the 'Trimphone'. The Transmitter is placed near the receiver and connected to the other end by a tube, causing some problems since holding the mouthpiece to provide privacy did not work, and its light weight made the whole thing unstable.
It also introduced a chirping sound instead of a bell,

Your comments:

  • The STC team involved Argue that the sliding across the table problem was caused by the GPO insisting upon the use of their standard handset cord rather than the longer one originally specified. Similarly they insisted upon their standard uk sourced dial rather than the original design spec lower torque Italian dialer unit. Both issues were to simplify spares availability in the GPO. Sod the customer.
    .......... Peter Carr, Harpenden , 7th of October 2022

  • As a linesman in the early seventies when these phones came out my biggest problem was reports of "no incoming ring".. the fitter hadnt bothered to adjust the screw on the ringer volume control and the "sub"...oops "customer" had turned the volume down and gone into the mute zone. If memory serves..we could only carry 6 Trimphones at one time and it is true that birds could imitate the ring tone. I took the ringer out of one and fitted it in a 700 series at home...our budgie twigged it very quickly and used to cause no end of chaos when I was dozing...
    .......... Rod Blunt, wolverhampton UK, 5th of January 2015

  • The 'phone was very light-weight, and could easily move across the desk or table when you turned the dial. The pushbutton form did not suffer this problem but it did not look so good.
    .......... Richard Truscott, Cambridge, 14th of January 2014

  • The distinctive chirping ring of the Trimphone was a sound that some birds could replicate very accurately, causing much confusion to human users!
    One practical problem with use was that the telephone was much lighter than standard models. If you pulled the handset away in use, the telephone had a habit of following you.
    .......... Andy Jackson, Sanderstead, Surrey, 7th of August 2012

  • The luminous dial was energised by Tritium, a radioactive isotope of Hydrogen with a half-life of about 12 1/2 years. This means that dials are pretty-much "dead" nowadays. I don't know the details but there were rules as to how many of these could be carried at a time in the fitters' vans, as well as those kept in Stores.
    .......... Ant, Ellesmere, 26th of December 2010

  • I seem to recall that the microphone was in fact located right alongside the receiver (ear-piece)and it collected the sound from the mouthpiece end of the handset via a plastic tube. Hence when you covered the mouthpiece so that the mother-in -law couldn't hear what you were saying about her she received every word in perfect clarity!
    .......... Geoff, Tutbury Staffordshire UK, 15th of June 2010

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