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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.
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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. [Picture: 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).
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Fig 3 - 332 Underside |
The markings tell us that
this is a Telephone No. 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.
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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 Telephone No. 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)
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Fig 5 - Screws, feet, nuts |
Internal Parts
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Fig 6 - Inside the base |
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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).
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Fig 8 - Dial Terminal Strip |
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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).
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Fig 10 - Dial Retaining Screw |
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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. Please note that this
telephone has the improved switch hook assembly, which has rollers. The
earlier assembly used just a flat plate, but the other difference was the
plungers. The plungers match the chassis type and cannot be swapped.
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Fig 12 - Removing the cradle plungers |
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Fig 13 - Cradle rest plungers |
Chassis, Cradle Switch and Other Components
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Fig 14 - Chassis top |
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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 - this is the improved design and the earlier
design had a flat plate with no rollers. 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 switch hooks. (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.
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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
Coil, Induction 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 uF bell capacitor and the 0.1 uF
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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