HANDSET No. 3


Handset 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.

Colours:
Grey, ivory, black, blue, yellow, red, green and brown.

Handset No. 3A Black is used on BCC No. 725 (this variant of handset No. 3 is a weighted handset with a reinforced steel conductor cord and rivets through the ear and mouthpiece to prevent removal).
 
Handset No. 3 Handset No. 3A

Parts

Handset   Handset 3 (colour)
Mouthpiece   Mouthpiece 21 (colour)
Earpiece   Ear-piece 26 (colour)
Transmitter   Transmitter-inset 16
Receiver  Receiver-inset 4T
Transmitter spring  Part 1/DRI/50
Receiver spring  Part 1/DRI/50

Original Handset No. 3 broken down into parts
This is an early model with the Transmitter Inset No. 13


How to Dismantle a Handset No. 3
By Sam Hallas

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.

Introduction
Like the Telephone No 706 that it adorns, Handset No 3 was the first model using thermoplastic moulding on a standard GPO telephone.  It was based on a design which had been field-trialled as Handset No 1, with modifications to make it lighter.

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.

Transmitters inset No. 13 Transmitters inset No. 16

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
Handset No 3 was designed from the outset with the upgrade of transmitter in mind.  The plastic moulding is cleverly designed to accommodate both old and new designs of transmitter.

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 Receiver Inset No 4T (pictured above), although new to GPO mainstream telephone instruments had appeared already in the GEC and Ericsson 1000-Type Telephone and the GPO flameproof Telephone No. 702.

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 handset cord is held in place by a crimped metal ring.  Look at the shape of the hole (pictured above) it fits in, you can see that the ring needs to be turned to allow it to slide out.  I find that I can do this with a small spanner or a small pair of pliers.  Turn with the spanner at the same time as pulling on the cord grommet.  There is a little arrow head on the inside of the moulded block to show where the ring must be turned to.

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.

Other Variants
Handset No. 3A is weighted and with an armoured cord for use in call offices.
Handset No. 4 contains a small transistor amplifier in the handle to amplify incoming speech for the hard-of-hearing and was powered from the telephone line.
Handset No. 5 performs a similar function but required a battery to power it. Both had a knurled volume control wheel just below the receiver housing.
Handset No. 6 has a button just below the receiver to cut off the transmitter for privacy. No 6A-2 used the button to control dictation equipment.
Handset No. 7 is fitted with a neon lamp behind a window in the handle to flash in time with the incoming ringing current
 


The only one I have a sample of is the last, Handset No. 7.  Here is one sitting on a grey Telephone No. 746.  It has other features worth noting that differ from the two samples above.  The cord also has two extra conductors, coloured orange and black to connect the lamp to the bell circuit (see picture below).

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).

Summary
The one-piece moulding of the Handset No. 3 gives it exceptional strength. It is almost impossible to break the handle by bending the handset.  It needs no screws or glue that a two-part moulding would require.  However the one-piece construction makes it difficult to insert additional components such as the amplifier in Handsets No. 4 and No. 5 or the lamp in Handset No. 7.

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.
 

 

 
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Last revised: December 18, 2010

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