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.

What do the numbers mean?

The nunbers on the base of this phone also help to identify it as a field trial Trimphone

66-LAU-60

Identifies this as a 712 Mark 1 or 2.

AAB

The first letter can be 'A' or 'G'. It gives the climate rating of the phone. 'A' is for UK use, 'G' for tropical climates. I think the second letter refers to the type of dial. The third letter is for the colour:

• A Grey-white
• B Grey-blue
• C Grey-green

Circuitry

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.

722 trimphone

The 712 was succeeded by the 722, which is much more common. The 722 was not the finished product. It went through a number of changes before arriving at 2/722, which was the Trimphone of the 70s.

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