Dismantling a
Telephone 300 series


This information on this page is based on an article by Sam Hallas.  Click here for the original article.

Dismantling a Telephone 300 series

General
The 300 Series Telephone is the first British Post Office design to incorporate all components into a single body. The case design had a smooth modern appeal building on the sweeping curves introduced in its predecessors. The main body is a single moulding in the wonder thermo-setting plastic, Bakelite.

Fig 1 - Black Tele No 332 Fig 2 - Ericsson 1931 model

The 300 Series of telephones were the successors to the Telephone No 162 and 232, the pyramid-shaped phones introduced in 1929 and 1934 respectively.  Both required a separate bell set. However by 1931 LM Ericsson of Sweden were already marketing a one-piece telephone. In addition, their "telephone differed from earlier models in that . the handset did not rest in a moving cradle, as before, but in a fixed seating in the case" [Ref 1].  The case design was by Jean Heiberg, Professor in Oslo's National Academy of Arts. [Picure: LM Ericsson by permission]

In adapting the telephone for the British market the Post Office chose to use the already well-established handset, Telephone No. 164, rather than the Ericsson style.  Other changes in the Anglicising were a switch to rear-entry plaited cables, and the addition of an optional drawer for dialling instructions.  The dial was the standard BPO Dial No. 10.

The British Post Office was not entirely convinced that a single piece telephone was needed for domestic use.  It felt that the combined telephone and bell set would be more suitable for businesses.  It took the GPO until 1936/1937 to introduce the basic model, Telephone No. 332.  It has been suggested that the delay was to allow the various British manufacturers to set up their production lines.

There was a variety of models in the 300 Series for different applications (Ref 2).  The telephone in the picture is Telephone No. 332, the basic model for residential use.  I'll be looking at others in the range: Telephone No. 330, the model for Private Branch Exchange use with a recall button; Telephone No. 312, with a similar single button, but for shared service lines; and an Ericsson N1365CB, a model without a dial made for non-GPO customers, but with identical circuitry.


The Base, Feet and Drawer

Let's start by turning the telephone over and looking at the base (Fig 3).

Fig 3 - 332 Underside

The markings tell us that this is a Telephone 332 F - a residential model with an all-figure dial. (OK you'll have spotted that it had a dial with letters and numbers in the first picture: it must have been changed at some time).  The letter C and the number 57 tell us it was manufactured by GEC at Coventry in 1957 - fairly late in the life of the 300 Series.  It's a Mark 2A, which was the last model produced. As befits such a late model it has been fitted with plastic cords instead of the earlier plaited cotton-covered cords.  The desk cord is in fact the type used for the later Telephone No. 706, which may have been fitted in error.  I replaced the desk cord with a plaited one when I reassembled the phone.

The base is secured by the four captive 2BA screws in the corners through tapped holes in the steel pressing (do I need to remind you that I like captive screws?). Under the screw heads are shake-proof washers to reduce the risk of the screws coming loose in service.  The base has shaped indentations to hold the drawer with holes which I presume are to improve audibility of the internal bell.  The steel is painted a silver colour. Another in my collection is grey/ green.

Fig 4 - Drawer open

The upper side of the base has a circuit diagram pasted in the area covered by the drawer (Fig 4).  The drawer guides are also of steel spot welded to the main base.  On another phone I found the circuit had been silk-screen printed onto the painted surface.  The drawer is of identical construction to the one used on Telephones No 162 and 232.

The rubber feet are fixed in a similar manner to those on the Tele 232. They have an integral washer and the 4BA shoulder screws have plain sections of shank to stop them crushing the feet.  The screws are held in place by plain nuts, but, curiously, no washers.  A subtle touch is that the nuts are flat at the bottom and chamfered at the top.  The small plastic block at the rear applies pressure to the handset and desk cord to prevent them being pulled out. It's held in place by two countersunk 8BA screws. (Fig 5)

Fig 5 - Screws, feet, nuts

 

Internal Parts

Fig 6 - Inside the base
 
Fig 7 - Chassis removed

With the base removed the significant difference from previous telephones becomes obvious: the 300 series telephone has an internal chassis.  All the components except the dial are mounted on the chassis which is held in place by the three captive 2BA screws with shake-proof washers, centre top and bottom and just right of the capacitor in the picture.

The bell gongs are held by two 2BA screws and have a shake-proof washer between the gong and the chassis.  The gongs are slightly eccentric to allow them to be adjusted for optimum ring.  They have different tones to produce that characteristic British jangling bell sound. I took them off at this point to make the chassis more manageable.

Now is the time to disconnect the cords.  The dial cord is connected to the terminal block on top of the chassis with 4BA cheese head screws and washers through wire-wound loops (Fig 8).  With the bells towards you they go blue, slate, brown, pink orange from left to right.  The line cord goes: White on 1, Green on 2, Red on 9.  The handset cord goes: White on 4, Green on 6 and Red on 5.  The cords are anchored to the support pillars for the terminal block by their laces (Fig 9).

Fig 8 - Dial Terminal Strip
 
Fig 9 - Cord Anchorage

The dial can now be removed from the main case.  Notice the cunning groove at the front which allows a long screwdriver to reach the dial retaining screw (Fig 10).  With the dial terminals away from you, the colour sequence is the same as on the chassis terminal block, blue, slate, brown, pink, orange.  Take the dial retaining screw completely out, turn the case right way up and twist the dial slightly anti-clockwise to release it (Fig 11).

Fig 10 - Dial Retaining Screw
 
Fig 11 - Removing the dial

Turn the case over again and remove the cradle rest plungers by undoing the 1BA nuts holding them in place (Fig 12).  They are fairly inaccessible in normal use and so can benefit from a rub over with a nailbrush to clean them.  Each plunger is prevented from pulling out of its sleeve by a small circlip.

Fig 12 - Removing the cradle plungers
 
Fig 13 - Cradle rest plungers


Chassis, Cradle Switch and Other Components

Fig 14 - Chassis top
 
Fig 15 - Switch Hook Operation

Let's have a look at the chassis now (Fig 14).  The upper side has the cradle spring mechanism which comprises a rocking platform pivoted at one end on a steel rod with a torsion spring. The other end has a similar steel rod with a roller at each end, acted on by the cradle rest plungers.  The rollers prevent any tendency of the cradle mechanism to stick.  The rods are split at the end and spread to stop them sliding out.  Both sides of the platform have a protrusion which acts as a stop to limit travel.  One side has an insulated peg (Fig 15) which moves between the hook switch contacts, opening the circuit when the handset is on hook.  The hook switch springset is held together by the two outer 8BA screws and is fixed by the centre 6BA screw.  The springset is constructed similarly to that on Telephone No 162.

Early models of the 300 Series had a flat platform without the rollers or end stops and used differently shaped plungers.  The design was changed because of sticking. (Ref 3).

The dial cord terminal block is held in place by two 6BA dome head screws.  The left hand screw also holds a spring clip intended to restrain the dial cord so it doesn't interfere with the dial mechanism.  I have yet to see a phone where it's been used.  Just above the terminal block in the picture you can see the 4BA screws which hold the bell in place.  To the right hand side are the countersunk 6BA screws holding the induction coil, which are rather obscured by a cradle rest roller and the hook switch cable.

The tapped pillars, roughly central, are for mounting the accessory switches and rectifier element on other models.  The hank bushes at the bottom of the picture are for the bell gong screws.  I guess that these are needed because the thickness of the chassis doesn't provide enough thread depth to hold the bells tightly.

Fig 16 - Chassis Underside

The underside of the chassis contains all the remaining components and the terminal block which links them all together.  The design is remarkably compact, using composite components to good effect.  The induction coil (Coil, Induction No. 22 up to 1943 and No. 27C thereafter) contains not only the three anti-sidetone windings, but also the two non-inductive 30 ohm resistors.  The capacitor (Condenser, MC No. 97) is a double unit holding both the main 2 F bell capacitor and the 0.1 F auxiliary capacitor to provide radio frequency immunity.  The only other component is the bell (Bell No. 59A, unmounted).

All the components can be unscrewed.  We saw the screws for the bell and induction coil on the top of the chassis.  The tubular capacitor is held by metal clips retained by two 6BA screws.  Some models have a rectangular bodied capacitor.  The terminal block is held by four countersunk 6BA screws into tapped pillars. Sadly the chassis cannot be cleared of components completely without unsoldering some of the wires.  Not really necessary for the purposes of this article..

The terminal block provides various flexibility points of removable links for the common variations of what the Post Office called Plan Working.  Complete telephones with individual numbers were also issued for specific applications.

The handset is Telephone No. 164 which I'll describe in a separate article.

Summary

Despite the Post Office's initial scepticism, the one-piece 300 Series telephone became standard issue for over twenty years.  Many remained in service long after the introduction of the 700 series in 1959.

The 300 series telephone proved robust and reliable and was sufficiently flexible in its many variations to meet a large number of business and domestic needs.  It has ingrained itself into the nation's memory such that this is the model people think of when the phrase 'old telephone' is spoken.

References

L.M. Ericsson 100 Years, Vol III, Evolution of the Technology 1876-1976, Christian Jacobaeus et al, ISBN 91-7260-065-9
BPO Telephone No. 332 and variants, Rob Grant? http://www.britishtelephones.com/t332.htm
BPO 300 Type Telephone Info - Quick History, Rob Grant. http://www.britishtelephones.com/t300info.htm
The New Combined Hand Microtelephone and Bell Set, C.A.R. Pearce, Post Office Electrical Engineers' Journal Vol. XXXI, April 1938. (At URL in 3 above)

 

 
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Last revised: October 16, 2012

FM