The 712 field trial Trimphone
The GPO took a while to perfect the trimphone. They started field trials in limited areas in 1965 and were gradually extended. The first trimphone was the ten millionth telephone. It was presented to a newly married couple in 1965 by the Postmaster General.
The first trial versions of the trimphone were all two-tone grey. The following year, two-tone blue, and green/brown were added.
The field trials hardly went well. In certain arrangements of extensions, it was possible to "ring" a trimphone when dialling out. This was of particular concern on shared service lines, when it was possible to tell when the other party was dialling out and listen in. The quality of the microphone in the handset could deteriorate quickly and need replacing. The trimphone finally became available nationwide in 1968. The phone itself went through several revisions before a final design was accepted in 1971. The phone illustrated is a field trial telephone from 1967. It still has the 'ABC' style lettered dial. Later models from the end of the 'sixties had just numbers.
This is a 712 mk2 field trial trimphone from 1967. You can clearly see the lettered dial. The other main visible difference between the 712 and the 722, which replaced, it is the absence of the plastic screw in the area where the handset rests. This was not needed, as the case was fixed by three screws underneath.
Inside the differences are clearer. For a sophisticated product, the original trimphone was remarkably old fashioned. Most of the circuitry is the same as the much larger 706. It was a tight fit!
The 712 was also much heavier than the 722. Some of the problems associated with the 712 moving when dialling were less evident in the 712.