Dismantling a
| |||||||||||||||||||||||||||||||||
|
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 General
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?
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. Casings The Case
The Base
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.
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 Assembly
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.
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
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.
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.
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 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 Handset Drawer and Notice Holder
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.
References |
|||||||||||||||||||||||||||||||||
Last revised: February 03, 2023FM | |||||||||||||||||||||||||||||||||
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.
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.
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.
