Dismantling a
Telephone No. 162 or No. 232

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

Dismantling a Telephone No. 162 and No. 232
The Pyramid Telephone

The Telephone No. 162 was the first to be adopted by the British Post Office (BPO) in 1924.  It was based on a design from Siemens Brothers which they chose to call the 'Neophone'.  The model standardised by the BPO had minor differences and went by a more prosaic name of the 'Microtelephone'.

Telephone No. 162 CB Telephone No. 232

Apart from the differences in the transmission circuit, the Telephones No's 162 and the 232 differ only in cosmetic aspects.  Both required a separate bell set as they did not have an internal bell.  The 162 required a bell set with an induction coil, such as the Bellset No. 1 or the Bellset No. 25, both were used with its predecessor, the Telephone No. 150 candlestick telephone.  Telephone No. 232 was produced in 1934 and used the simpler Bellset No. 26, containing just a bell and capacitor.  Both these illustrated instruments were originally CB - without a dial.  The dial has been added to the 232 for illustrative purposes.

The design was quite revolutionary at the time with its clean curves, moulded entirely in the modern wonder material, Bakelite.

Why Bakelite?
What stimulated the change from traditional materials: steel, brass and wood?  It could be:-

  1. From a design point of view it was time to move on.  Using plastic gave the designers almost unlimited scope in the shapes and sizes that could be achieved.

  2. Mass production. Increased demand for telephones meant that the intensive processes needed for traditional materials were too slow.  Once a mould has been made, vast quantities of identical plastic parts can be rapidly produced.

In retrospect Bakelite has proved to be a remarkably durable plastic as parts produced eighty years ago are still serviceable.  Compare that with some more modern plastics which start to disintegrate after as little as twenty years.  Bakelite has a limited range of colours, it is true, but they don't tend to fade in the rapid way that modern plastic colours do.

Bakelite is dimensionally stable and very strong.  It leaves the mould with a shine requiring no further finishing. Its main disadvantage is brittleness which is very difficult to glue together once broken.


The Case
The main case moulding and various other parts are identical for the two models.  Because of the ready inter-changeability of parts, telephones that have been in service with the Post Office are unlikely to retain the distinct differences.  Originally there was a slight difference in the cradle rest between models but the examples here have identical cradles. The
Telephone No. 162 first came with a plain base and additional lead weight:  the Telephone No. 232 was fitted with the same base but without the lead weight, however the Telephone No. 1/232 was normally fitted with the drawer for instructions, as seen above.

Base without drawer Base with drawer

The Base
The base is held in place by 2BA screws held captive by spacing collars.  The
Telephone No. 162 and 232 have small fishplates (above left) to retain the case fixing screws which are held in place by 4BA domed head screws with square nuts on the inside.  The drawer cover provides this function on the 1/232 (above right).

The feet are held in place by special 4 BA screws with a thickened shank and nuts on the inside of the base (right).   The moulded foot contains an integral washer.  The screw tightens directly against the base without compressing the rubber foot.  Today we would accomplish the same task using a standard machine screw inside a tubular spacer with a washer under its head - all standard off-the-shelf parts.  The rubber perishes over the years and becomes crumbly (see the feet above), but rubber was probably the best material for the job at the time of design.

To remove the base - release the two captive fixing screws.  On the drawer-less base release the middle outer screws, whilst on the base with drawer, release the central screws.

In the left picture below can be seen a lead plate on the base to give extra weight.  It is believed that later models had a steel plate instead - even then lead was an expensive metal.  The Telephone No. 232 doesn't really need any extra weight because of the induction coil, the steel frame and drawer underneath.

Internal view of a Telephone No. 162 base (with Lead Plate) Internal view of a Telephone No. 232 base

Once the feet are off on the Telephone No. 162 and the lead plate lifted away (below, left) you can see how the fishplates are attached.  One of the square nuts was missing and has been replaced with a hexagonal nut as you can see on the right-hand side of the base.  The frame for the drawer looks like this (below, right).

Internal view of a Telephone No. 162 base (without Lead Plate) Drawer in retaining housing frame

Internal Assembly
Looking inside the two telephones the differences become apparent.  The
Telephone No. 162 (below left) has a Transformer No. 35A with 4 terminals fixed to the case, which is provided to reduce the sidetone, but does not form a true hybrid.  The Telephone No. 1/232 (below right) has a Coil Induction No. 27 with 7 terminals, containing balancing resistors, which does form a hybrid circuit both reducing sidetone and improving reception.  Terminals 7 and 8 (below left) are not required on Telephone No. 162.

Telephone No. 162 internals Telephone No. 232 internals

Looking at the underside of the Telephone No. 232 base you will see a stamp and this means that the Telephone No. 1/232 was refurbished in the Post Office Factory in 1962.  This explains the PVC wiring inside which would not be original.  The Telephone No. 162 is wired in solid copper wire with heat-resistant sleeving, which should be original.  The red and the white colours are still evident, but the wire that should be green is far from it.

It's time to take off all the cords before the phones are dismantled further.  The cords were either missing or cut off and have been replaced.  The handset cords are more modern and are terminated in spade terminals whereas the line cords use the traditional wrapped loops with cord whipping.

Before taking the cords off check them against the diagram to see which version you have.  The Post Office cunningly swapped some of the terminal functions around between models.  See the margin notes on diagram N332.

The cord retaining laces are tied to the chromed brass loop (see above).  They would normally be cut off to the right length.  Notice also in the picture the metal link which is only needed when the dial is not fitted.  This may be left in situ to avoid losing it - not normally present on a regular dial model.

The improvement in these models can now be appreciated over the messy terminal arrangement in the 150 candlestick telephone, as the terminals are all easily accessible.

The terminal block screws have a square head to prevent them rotating when the nuts are turned. A neat feature, allowing one-sided working in the workshop when changing cords. I suspect that today we would use hexagon head screws, as being standard components. The threads are 4BA, but the nuts are oversize and require a 3BA spanner.

Dismantling the body
First remove the line, handset and dial cords.  To do this - remove all the nuts from each screw, but  do not take the bottom nut off or the screw will fall out.  When re-assembling the sequence of parts is, from the head end of the screw:-

  1. Solder tag (where fitted),

  2. Washer,

  3. Nut,

  4. 2 x washer,

  5. Nut,

  6. 2 x washer,

  7. Nut.

The cords go between the pairs of washers.

From now on it will only be the Telephone No. 1/232 that is dismantled as the construction of the two models is identical.

With the cord out of the way it is now a good time to look at how the switch hook mechanism works.  It is very simple and robust.  The cradle plunger is Bakelite and is moulded onto a brass extension which forms a rod terminating in an insulated point (below, left).  It is lifted upwards by a coil spring around the outside and held in place by a split pin - simple and reliable.

To remove the dial or blanking plate remove the 6BA retaining screw (right) completely out.  Then turn the case so the dial is facing you,  twist the dial or dial blank plate slightly anti-clockwise and lift it away from the casing.

The next step is to remove the cradle rest and plunger assembly.  Whilst holding the switchook plunger, pull out the split pin (below, left) from the other end of the plunger with the use of a pair of long-nosed pliers.  Then slowly drop the plunger out from the casing ensuring that you catch the spring and retainer.

Close up of the switchook springs - on hook Plunger, spring and spring retainer

It is probably best to remove the terminal block and transformer/ induction coil together in one go.  Remove the two 6BA screws holding the transformer in place, then turn the case over and release the two 2BA terminal block fixing screws from the top (below left).  The screws on the 162 have been chemically blackened to make them less obvious, but these have no decorative coating.

View of the two 2BA screws that hold the terminal block View an the empty case

The main mouldings are now separated and empty, ready for cleaning.  Another advantage of Bakelite is visible on the main case and this is the ability to incorporate metal inserts in the moulding: the cord anchorage ring and the threaded nuts.

The cradle rest forks on the Telephone No. 1/232 are made of a different material from the Telephone No. 162, which are Bakelite.  This is a Cellulose Acetate material, which is more resistant to damage, but less resistant to temperature variation, humidity and other chemicals.  The forks have suffered the ravages of time making the surface wrinkled in places as a result.  If the cradle rest smells acrid then replace them as they are deteriorating and will eventually corrode the brass plunger rod.

Terminal Block and Switch Hook Springset
It's unlikely that the terminal block and spring set are dirty, but they can be dismantled further if need be. It is advisable not to dismantle the springset as reassembly can be difficult.  By removing the screws from the terminal block the transformer can be detached without unsoldering.  The wires are solid core and if one snaps off, it is suggested that this single wire be replaced with stranded wire, which will reduce the problem in the future.

Notice the improved construction of the switch hook spring set over the awkward design of the candlestick spring set.  A separate 6BA screw holds the set in place and the springs are held together by two 8BA screws.  The screws are insulated by close-fitting tubes.  This set the pattern for spring sets for many years to come.

The contacts can be cleaned in situ with a contact cleaning burnisher.  The springset is fiendishly difficult to re-assemble, but if you must take it apart (and you advised not to) then it is shown here with the individual layers in the correct sequence as a guide.

The contact springs originally only had one contact per spring but the design was changed so later springs had two contacts per spring (bifurcated).  Both sorts can be found and it does not matter which sort is in situ.

Other Parts

The handset, which the BPO quaintly named Hand Micro Telephone (HMT) No. 164, became the mainstay of Bakelite telephones until it's obsolescence in the 1950 - 60's and deserves an article on it's own.

Drawer and Notice Holder
The drawer and notice holder was provided as a convenient place to store dialling instructions and customer's notes of telephone numbers.  Dialling code cards were the correct size to fit in the clear flip-up holder when folded in two.  The holder is sprung so that the customer's notes can be gripped beneath it - a thoughtful touch, but pretty stupid as too much paper caused the holder to flip up after the drawer was closed, locking it shut.

View of the drawer with card holder in place Knob and face plate removed

The face plate is grooved to slide onto the steel base and held in place by the two 8BA countersunk screws which hold the knob in place.  You may find a jeweller's screwdriver is best to remove these screws.  The face plate and knob are made of a thermoplastic, possibly the same material as the cradle forks.  Earlier ones were Bakelite.

The card holder is made of a transparent plastic riveted along the sides - though not all were made this way - and fixed to a steel rod by crimps.  The rod is held by a bright steel pressing with a similar rod at the back.  This is coupled to the base plate by two flanges with holes and sprung at both end by torsion springs.  The steel base plate is painted on the upper side and has a riveted insert at the back with two hooks which engage in slots in the housing to stop it pulling out.

Apart from the face plate and knob, the drawer assembly is not designed to be taken apart.

Summary and Conclusion

The Telephone No. 162 started off the Bakelite revolution that set the pattern for telephone design right up until the introduction of the 700 series telephones in 1957.  Its clean lines and smooth curves give it a modern 'art deco' look.  The mechanical design is robust, simple and reliable with fairly few parts.  It is easy to replace cords and fairly easy to replace other parts with a minimum of tools.

It is disappointing that the Telephone No. 162 did not use a hybrid transformer in the transmission circuit when the principle was already well-known. Official publications of the time suggest that there was a need to use up excess stocks of Bellset No. 1 and with hindsight this may have been an unfortunate decision driven by financial rather than engineering considerations.  The Bellset No. 1 can be easily modified to remove the transformer from circuit, making a wooden cased version of Bellset No. 26.

The Telephone No. 232 remedies the deficiency, but the need for a bulky Bellset meant that it's days were numbered as the prototype for the self-contained telephone with a one-piece moulding was already in production by Ericsson's.  It's successor, the BPO Telephone No. 332 was released in 1936 just two years after the 232 came out.  However, when coupled with Bellset No. 39 & 44 (see picture below on a Bellset No. 44), the 200 series remained in existence as the means of providing extension plans 5 and 7 until the introduction of the Planphone No. 1, using Planset N625.

The Telecommunications Journal of Australia, Dec 1937

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Last revised: February 03, 2023