|Bakelite covered base unit, complete with switch unit, buzzer, two push
buttons and indicator flap. Used as the main instrument on extension plans 5 & 7 and has to have a Telephone
No. 248 fitted on top.
Bellset No. 39 as supplied
There were two variants:-
Bellset No. 39A - for use on Direct Exchange Lines.
Bellset No. 39C - for use on Shared Service Exchange Lines.
The press buttons and labels are store inside the Bellset and fitted as
appropriate on installation.
There is no magneto bell fitted inside the Bellset for incoming calls. The
incoming call bell was
always fitted externally.
Bellset includes (1956):-
1 x Block, Terminal, No. 20/12, Black.
1 x Cover No. 13.
1 x Buzzer No. 24A, Unmounted.
1 x Filter, Suppression, No. 6A.
3 x Capacitors, M.C., No. 99.
1 x Indicator, No. 3900A.
1 x Cord, Inst., No. 9/10H, Brown, 54".
1 x Resistor, Coil, No. 15 - 300 ohms.
1 x Tray, D62076, Black.
Introduced in 1938 and superseded by the Bellset
No. 44 by 1954.
Made by GEC - click here for more details
Succeeded the Bellset No. 20.
Superseded by Bellset No. 44.
The Bellset No. 39 differs from the Bellset
No. 44 by having the indicator to the right of the sliding switch, a Buzzer No.
24A and chromed press buttons.
Diagram for 1 extension working (Plan 7) - N4310.
Diagram for 2 extension working (Plan 5) - N4308.
Circuit diagram for Bellset No. 39 - N539.
Click here for the difference between a Bell and a
How to convert your Bellset 39 to work with
plug and socket
|Bellset No. 39 with Telephone No. 248
(shown with no pushbuttons installed)
|Bellset No. 39 - with two buttons in
|Bellset No. 39 - Internal View
Note the two push buttons in the unused (parked) position
(front left and front right)
P.O. ENGINEERING DEPT.
Issue 2, Nov 1947
SUBSCRIBERS' COMBINED SET (TWO-PIECE) FOR EXTENSION
PLANS 5 & 7 'MAIN' INSTALLATIONS
(Telephone No. 248, CB, minus base with Bell-set No. 39...
NOTE:- "Bell-set No. 39" has been superseded by "Bell-set No.
39A." The physical design of the new bell-set is identical with the "Bell-set
No. 39" except for a small circuit change to overcome over-hearing. This change,
which provides for Short-circuit of the receiver of the main telephone on exchange to
extension calls, is shown in the paste-in diagram fixed to the underside of the
tray-holder (see par. 7). The description of the "Bell-set No. 39" applies in
every respect to "Bell-set No. 39A."
To facilitate the wiring change, the "Telephone No. 248 CB, minus base"
will be fitted with a "Cord, Instrument, No. 8/10E, 8in." in place of the
existing, "Cord, Instrument, No. 6/31E, 8in."
1. Scope of Instruction
This Instruction describes the physical features of the Telephone
No. 248, CB, minus base and Bell-set No. 39 used for Extension Plans 5, 5A, 7 and 7A
subscribers' installations in C.B. and automatic areas. Installation and maintenance details
of the Bell-set No. 39 are given in D 3908 and D 5101, respectively.
The various matters are dealt with as follows:-
|Telephone No. 248, CB, minus base description
|Bellset No. 39. (see NOTE) description
|Layout of apparatus
|Indicator No. 3900A
|Buzzer No. 24A, unmounted
|Filter, Suppression, No. 6A
The combined set for the "main" on Extension Plans 5 and 7 installations
comprises two parts:-
(a) a telephone of the handset type (Telephone No. 248, CB,. minus base), which is
(b) a base containing a switch, two press-buttons and a buzzer (Bell-set No. 39).
The Telephone No. 248CB, minus base and Bell-set No. 39 are stocked as separate items
and, when in use, the telephone is fixed to the bell-set by means of two screws passing
through the baseplate of the latter.
The set is not available in standard or non-standard colours (i.e. available in black
only) and is for use in C.B. and auto. areas only, on Extension Plans 5, 5A, 7 and 7A
installations where a table switching unit is required (see D 3908). In addition to the
combined set, a separate Bell-set No. 26 and batteries
(two R40 cells 1.5v) are
required at the "main" in every case. When an external extension on an Extension
Plan 7 installation is required, a second Bell-set No. 26 and a Generator No. 26 AP are
Facilities provided by Extension Plan 5 and 7 type installations are summarised as follows:-
Extension Plans 5 and 5A provide for two extensions in parallel (internal only)
Extension Plans 7 and 7A provide for one extension (internal or external) Extension Plans
5 and 7 "main" can overhear extensions Extension Plans 5A and 7A extensions are
secret from "main"
The telephone and bell provide for:-
(a) receiving calls from the exchange
(b) making calls to the exchange
(c) extending incoming exchange calls to one extension (Extension Plans 7 and 7A) or
one of two extensions (Extension Plans 5 and 5A)
(d) making calls to the extension or extensions
(e) holding exchange calls whilst making inquiry of the extensions)
(f) obtaining exchange calls and extending them to the extension or extensions or,
alternatively, switching the extension to exchange to allow the extension to call the
In all cases, the exchange calls the main on a magneto bell (Bell No.
(a) When used for internal extensions, signalling between main and extensions, and vice
versa, is by press-buttons and trembler bell or buzzer in both directions.
(b) For external extensions (Extension Plans 7 & 7A), a separate hand-generator is
provided for calling the extension, in which case the press-buttons will be blanked out
by dummy plates. The extension calls the "main" on an additional magneto bell (Bell-set No. 26).
6. New circuit features
Provision has been made for :-
(a) Preventing a permanent hold from remaining on the exchange should the switch be
left in the SPEAK TO EXTN-EXCH HELD position (Posn. 3).
(b) Disconnection of dialling circuit of the "main" when switch is in EXTN TO
EXCH position (Posn. 4), on Extension Plans 5 and 7.
Neither of these features is provided in a Bellset No. 20.
The circuit for Telephone No. 248, CB minus base is shown on Diagram N 348. The circuit for
Bell-set No. 39 is shown on Diagram N 539. The wiring diagram (panels 1 and 3) is reproduced
on the underside of the directory tray-holder.
Extension Plans 5 and 5A circuits are shown on Diagram N 4308, while Extension Plans 7 and
7A are given on Diagram N 4310.
TELEPHONE No. 248CB, MINUS BASE
Telephone No. 248, CB, minus base differs from the standard Telephone No. 1/232,
CB in the following respects only:-
(a) a base is not provided
(b) two additional springs (fitted to the cradle switch spring-set) make contact when
the handset is removed from its rest. The additional springs control the "hold
exchange" feature and eliminate the possibility of leaving a loop oil the exchange
line when the bell-set switch is inadvertently left in the SPEAK TO EXTN-EXCH HELD
position after the handset at the "main" has been replaced.
The anti-side-tone induction coil remains exactly as in the Telephone
No. 1/232. A Cord, Instrument, No. 8/10E, 8 in. is provided as a means of
connection between the telephone and the bell-set, the cord passing internally into the
latter. When used in auto. areas, a Dial, Auto. S.S. No. 10 and Cord, Instrument, No.
5/09E, 3 in. should be requisitioned and fitted locally.
BELL-SET No. 39
The Bell-set No. 39 has been designed on the same lines as the standard moulded bell-sets. All the apparatus is mounted on a metal base, over which is fitted a moulded cover. This
cover is removed in the following manner:-
(a) loosen the screw on the switch extension arm.
(b) lift and slide the extension arm, to disengage the slot from the screw.
(c) withdraw the switch extension arm through the slot in the moulded cover.
The cover may then be, carefully lifted off. The front of the cover is fitted with the
necessary labels for the switch and press-buttons and, when issued, the latter are blanked-out
by dummy plates. The metal base is fitted with a standard directory tray and four rubber
feet for table mounts.
10. Layout of Apparatus
The layout of the apparatus provides fairly easy access to each individual component for
maintenance and adjustment. It is necessary, however, to remove the Condenser, M.C. No.
99 to gain access to the tension-spring adjusting screw on Buzzer No. 24A.
comprises the following components:-
Switch (with associated spring-sets)
Indicator No. 3900A
Buzzer No. 24A
Filter, Suppression, No 6A
Block Terminal, No. 20/12
Black Cord, Inst., No. 9/10H, Brown, 54 in. (When a longer cord is required, "Cord,
Inst., No. 9/10H, Brown, 66 in." may be requisitioned separately and fitted).
Note the left hand spare button is stored on the end of the switch and
spare button labels
Are fixed to the buzzer mounting.
The switch is mounted centrally on the baseplate and provides four switching positions,
(a) Speak to extension-spring - set No. 2 operated
(b) Speak to exchange - spring-set No. 1 operated (normal position of switch)
(c) Speak to extension - spring-sets Nos. 2 and 3 (exchange held) operated
(d) Extension to exchange - spring-set No. 4 operated
These positions have been arranged in the above order, reading from right to left on
the switch labelling. It is thus possible to extend an exchange call to an extension
without breaking down the connection, the "main" speaking to the extension with
the switch in position 3, thereby holding the exchange in the mean time.
A machined block of metal forms the main body of the mechanism and is fixed to the
baseplate by four screws. Inside the block are housed a solid brass cam and four standard
1in. dia. steel ball-bearings. The latter are forced against the cam by the spring-steel
buffers of the spring-sets, which are operated when the buffer springs are allowed to
force the ball-bearings into dome shaped cavities in the cam. Operation of the switch is
effected by moving the switch arm through an are of a circle. An extension arm, fitted
with a moulded knob, projects through the cover of the bell-set and the switching
conditions obtaining are shown on the label immediately below the knob.
The three switch spring-sets are mounted on two metal blocks, with the springs in a
vertical plane. Each spring is fitted with twin contacts.
Spring-sets Nos. 1, 2 and 3 have a common mounting block situated immediately behind
the switch mechanism, when viewed from the front of the bell-set. Spring-set No. 2 has two
spring units and these are mounted on the left-hand side of the block, one above the
other. Using the notation on Diagram N 539, spring unit 2b is mounted above 2a. Spring-sets
Nos. 1 and 3 have one unit each, No. 1 being mounted on the right-hand side of the block
above No. 3. Spring-set No. 4 is mounted on its own block to the left and in front of the
switch body, and consists of four spring units in two banks. The arrangement of the spring
units is as follows:- Spring units 4a and 4d on top, with 4a nearer to the mounting block;
4b and 4e below, with 4b nearer to the mounting block. Access to the lower spring-sets for
contact cleaning is gained through three holes cut in the metal baseplate.
13. Indicator No. 3900A
This is a 3000 type relay, having one change-over spring-set and flag signal attachment. The relay is mounted on the right-hand side of the switch on a special bracket and its
purpose is to give an indication when an extension-exchange call (or vice versa) has
terminated. The signal is visible through a glass window in the bell-set cover situated
above and to the left of extension No. 1 press button. The flag consists of a metal plate,
pivoted to the relay frame on two arms. The plate is divided vertically into halves, the
left-hand half being black and the right-hand half white. The right-hand half carries the
word ENGAGED in black vertical lettering. The operation of the indicator is as follows:-
(a) When the indicator is normal, the black section of the flag is immediately behind
(b) The indication is operated by the exchange current flowing through the extension
telephone. It is designed to operate with 30ma and to hold with 50mA when the extension
(c) When operated, the white section moves into position behind the window, showing
ENGAGED. The indicator cuts out the bell circuit at the "main", thus preventing
tinkling of the "main" instrument bell when the extension dials. A further
advantage lies in the fact that impulse distortion, due to the bell and condenser being
across the line, is eliminated.
14. Buzzer No. 24A, unmounted
This is used by internal extensions to call the "main". The buzzer is mounted
directly on the baseplate, to the left of the switch. The interrupter contacts are
insulated from the frame and adjustment is made by means of a screw, located in an
insulated pillar. This screw may be turned either by a screwdriver or a Tommy bar. Locking is effected by means of a screw, which closes a split in the top of the insulated
pillar. The tension of the armature return spring may be adjusted by means of the screw
and locking nut provided.
15. Filter, Suppression, No. 26A
This unit is mounted on a frame directly over the buzzer and consists of a capacitor and
resistor, mounted on a terminal board. The unit is intended to quench the sparking at the
buzzer interrupter-contacts and to reduce electrical interference with telephone circuits
radio receivers. Earlier supplies of Bell-set No. 39 were fitted with a "Filter,
Suppression, No. 6A", which is now obsolete.
These are standard Capacitors, M.C. No.99, 2uF and are similar in size to
Capacitors, M.C. No. 97 (2 x 0.l) uF as fitted in the 300-type telephones.
These are provided on the bell- set for calling the extension (or extensions), trembler
bells being fitted at the latter to give the necessary signal.
(a) On Plans 7 and 7A (internal extensions) one press-button is used, the other
being blanked-out with a dummy plate.
(b) For Extension Plan 7 or 7A (external extensions), see par. 5(b).
(c) On Extension Plans 5 and 5A (internal only) both press-buttons are used. The
press-button plungers are housed in moulded holders, which rest on the top springs of the
press-button spring-sets in the manner of a saddle. The lower spring is bent over at the
end and terminates in a fork, which acts as a clip for the plunger holder. A certain
amount of side, play is provided for the plunger holders, so that the plungers may project
centrally through the holes provided in the bell-set cover. To provide for slight
out-of-alignment at right-angles to the press-button springs, the mountings for the latter
are secured to the baseplate by one screw, allowing the whole spring-set to pivot about
this screw. The pivot action is restricted by the spring-set clamping screws, which
project through holes in the base plate. When the bell-set cover is placed over the baseplate, the conical tops of the plunger holders fit into two conical recesses on the
inside of the cover. The press-buttons are, therefore, automatically aligned centrally in
the holes of the bell-set cover.
18. Dummy plates
These are fitted over the press-button holes when the bell-set is issued, the plungers
being removed from their respective holders and stored in the bends of the lower springs
of the press-buttons. When it is desired to use one or both of the extension
press-buttons, the dummy plate or plates are removed and stored on the side of the
suppression-filter mounting bracket, a tapped hole and cheese-headed screw in the bracket
being, provided for this purpose.
19. Resistor coil
This is the standard Resistor, Coil, No. 15, of 100 ohms D.C. resistance. It is placed
across the exchange line as a hold coil in the switch is in the SPEAK TO EXTN-EXCH HELD position.
20. Connection strips
Two connection strips are fitted in the bell-set for cord terminations providing in all 25
terminals as follows:-
(a) Terminals 1 to 12 are accommodated on a curved synthetic-resin-bonded-paper
(S.R.B.P.) board, which follows the contour of the corner of the bell-set. The Cord,
Instrument, No. 9/10H, 54 in. terminates on this connection strip and is gripped by the
cord clamp fitted centrally at the back of the bell-set.
(b) Terminals 13 to 25 are accommodated on a straight S.R.B.P. board mounted on fixing
pillars. Connections to the telephone are made on these terminals by means of the Cord,
Instrument, No. 8/10E, 8 in (see par. 8). To see the numbering of the connection strips in
the correct order, the bell-set should be viewed from the rear.
21. Bell-set No. 39C
This model has been introduced specially for use on Extension Plan 7 on Shared Service
(Separate Metering). The left-hand press-button spring set has a change-over action to
enable it to be used as the "call-exchange" press-button and the connections are
brought out to additional terminals 26 and 27. A label engraved CALL EXCHANGE is supplied
and a Cord, Instrument, No. 10/05H, Brown, 54 in. is fitted. In all other details the
Bell-set No. 39C is similar to Bell-set No. 39A