HANDSET No. 3
No 3 is the standard handset used on most BPO 700 type telephones.
Originally the handset was supplied with the Transmitter Inset No. 13, but this was
replaced by the Transmitter Inset No. 16.
ETL Part - N9506E.
Original Handset No. 3 broken down into parts
How to Dismantle a Handset No. 3
This is a continuation of the article on dismantling a Telephone No 706. Handset No 3 was as much an innovation as the rest of the telephone.
The receiver and transmitter caps unscrew from the main body. Their designations are Earpiece No. 26A and Mouthpiece No. 21A. The caps may have become stuck with age or just over tightened. There was a GPO tool to aid their removal which works much like the tool seen on the right. The one I'm using in the photo above is a Baby Boa Constrictor which is sold as an aid for people with disabilities. It is a rubber strap attached to a handle which tightens the strap round whatever is being turned.
The two handsets I'm taking apart illustrate further changes. The left hand picture depicts the earlier Transmitter Inset No. 13. This is the familiar hat shape. Notice that it's fitted with a screw terminal and nut to hold the spade connector on the handset cord. A special plug with thread and nut pushes into the rear connection hole. The three raised tabs on the transmitter fit into the recesses in the housing prevent it from turning.
The right hand picture shows the later Transmitter Inset No. 16 which has 6BA screw inserts ready to accept the spades. Inside the housing is a spring ring to press the transmitter firmly against the cap. The transmitter has ridges to engage with the same recesses to prevent it from turning. Both type of transmitter use carbon granule technology
Transmitter Inset No 13 replaced by No 16
Transmitter Inset No. 13, the old hat-shaped variety had been in use for over 30 years by the time its replacement came along in 1965. Even the telephone No. 150 from the 1920s was fitted with Transmitter Inset No. 13.
A great deal of design effort and testing went into producing the replacement transmitter. Although increased sensitivity was not required, improvements were made to the frequency response and distortion, as well as mechanical improvement to the moisture seal and lower production cost. A POEEJ article states that the design life of the transmitter was 2 years. This has proved to be unduly pessimistic. Many are still in working order at over 40 years old.
Transmitter Inset No. 16 is not designed to be taken apart. It would be replaced entirely on failure. Total failure was comparatively rare, but degraded performance, such as frying and noise, were the main reasons for replacement. The transmitter was also prone to weak performance in severe frost when used outdoors in weatherproof telephones such as the Telephone No. 745.
Receiver Inset No 4T
The 4T is described as a rocking armature receiver. An armature, rocking on a central support, drives a diaphragm via a small rod. The armature is polarised by a central permanent magnet and driven by two coils wired in opposition mounted either side. The whole unit is hermetically sealed.
The receiver is a great improvement over its predecessor, the Receiver Inset No. 2P as used on the handset Telephone No. 164. Sensitivity is increased and the frequency response is smoother giving improved intelligibility. A fuller description of its operation is given in the GEC article about the 1000-Series telephone in the Document Repository. Like the transmitter, the receiver is sealed and not designed to be serviced.
Removing the Handset Cord
The picture also illustrates another point. In order to make the handset as light as possible, the handle is as thin as it can be made. This requires a largish core in the mould that is withdrawn through the grommet hole after moulding. The size of the hole required is too big for a grommet to sit snugly and so a small moulded block is glued into place which can be clearly seen on the left.
To refit the cord, thread the wires through the grommet hole, push the grommet into the recess and turn to allow it to slide in. The metal ring then springs round to lock the grommet in place.
The curly handset cord was a first with the introduction of Telephone No. 706 although some very early models had a fabric covered cord. The curly cord became available in 1959 in colours to match the telephone and handset body. The conductors inside are still of the tinsel type to give maximum flexibility. The new transmission circuit in the Telephone No 706 required a four-conductor handset cord (Cord Instrument No. 4/108) instead of the previous three-conductor cord.
Gone was the tie cord to retain the cord. In its place were moulded grommets glued to the cable itself. Gone were the fiddly wrapped loops for termination replaced by crimped spade tags. These are both plus points for reducing the cost of manufacture and actually make the cord easier to use.
Once the earpiece, mouthpiece, transmitter and receiver insets have been removed the plastic parts are ready for cleaning and reassembly. You can prise the spring rings out if you like, but they won't corrode from being washed and dried.
There are two other different features on the handset above. The receiver cap is an Earpiece No. 29A which was introduced in 1981 to make the handset compliant with safety directives. It has smaller perforations protect the user from dangerous voltages induced into the telephone circuit, whilst allowing the sound to escape. The transmitter is the newer, electronic Transmitter Inset No. 21. It contains an electret microphone transducer and an integrated circuit amplifier. The insets in this handset were both made by A.P. Besson of Hove, code DAE (now Hosiden-Besson).
Handset No. 3 is much lighter than its predecessor, Telephone No. 164. This is achieved at the expense of needing the extra block to be glued in the core exit hole.
The insets introduced with the Handset No. 3 are improvements in transmission terms over their predecessors, Transmitter Inset No 13 and Receiver Inset No 2P, as well as being considerably lighter.
Handset No. 3 was superseded by Handset No. 16 used on Telephone No. 776, the
Ambassador and Statesman. Initially the insets were the same but in later
telephones the carbon transmitter was replaced by a dynamic transducer.
Last revised: December 10, 2019