for the Telephone No. 80.
Engineers were demanding
a dial on their test phones as automation was steadily covering
country. To this end the No. 280 was introduced.
With a rubber covered metal body and smaller than usual dial
test phone was in universal use throughout BT until the mid
1980's when electronic
test telephones entered the market due to the requirement for DTMF
dialling. The Telephone No. 280 was robust and could be easily
The dial came with numbers only or with letters painted onto the
dial finger plate.
Trailed circa 1954, the phone was then introduced in
the switch hook button was non locking, but in 1959 this was
changed to a locking
There was a Mark 1 and Mark 2 versions.
The picture to the right shows a typical used Telephone No.
280, with tape around
the body and a collection of different plugs on the cord.
See also Plessey Model No.
1 x Dial, Automatic No. 17LA.
1 x Capacitor No. 8005.
1 x Press Button W.
1 x Receiver, Inset No. 4T.
1 x Transmitter, Inset No. 13.
1 x Resistor, Carbon No. 1, 120 ohms.
1 x Resistor, Carbon No. 2, 82 ohms.
1 x Cord, Instrument No. 2/95F Black 48 inch.
Circuit diagram - N380.
P.O. ENGINEERING DEPT.
Issue 1, 1/5/59
TELEPHONE No. 280
1. Scope of Instruction
This Instruction describes the Telephone No. 280 which is an
exchange maintenance telephone incorporating a dial. TOOLS &
TRANSPORT, Hand Tools, A 0040 gives the basis of provision of
exchange maintenance telephones and A 3102 in this sub-division
details the procedure to be adopted during exchange transfers.
The Telephone No. 280 is a telephone designed for the use of
exchange maintenance staff when a dial is desirable. The
telephone is made of rubber to obtain the necessary
robustness and uses a small lightweight dial to reduce the
overall size and weight. A press button is provided in the
telephone for completing the calling loop and in the
un-operated position it leaves a monitoring circuit connected.
The circuit of the Telephone No. 280 is shown in Dgm. N 380.
It is similar to that of the
Telephone No. 80, Mark 5, but component
values differ due to the substitution of a Receiver-inset No. 4T
for the Receiver-inset No. 1L or No. 2P, and the circuit is
further modified to include the
off normal and pulsing springs of a dial.
The case is moulded in black natural rubber and consists of
three parts: the body, earpiece and mouthpiece (see
above). Both the earpiece and mouthpiece are “force-fits” in the
moulded body. Other features moulded with the body
(a) A lug on the receiver end of the body for hanging up the
(b) A hole for the press button in the side of the body
(c) A lead-out grommet for the cord in the transmitter end of
(d) Four inserts moulded into the transmitter end of the body
for cord terminations and strapping
(e) An identification code is moulded on the body beside the
The earpiece and mouthpiece can be levered from the body in a
similar manner to the removal of a cycle tyre from its rim. Care
should be taken, when replacing, to ensure that the key on both
mouthpiece and earpiece engages with the keyway on the body.
A tubular aluminium frame is inserted in the hollow handle
of the telephone to act as a stiffener and as a mounting chassis
for the capacitor, induction coil and press button. A clip is
riveted inside the frame to secure the capacitor, and a plate
and block are fitted to the receiver end to screw the dial
mounting to the frame.
6. Transmitter and receiver
A Transmitter inset No. 13 is
fitted into the rubber mouthpiece and held in position by a
metal ring and two rubber lugs. Connexion is made by means of
two wander leads, one connected via an 82-ohm resistor to a tag
on the metal ring and the other terminated on a sprung pin (Tag.
Part 1/DTA/153) which is inserted in the connecting hole in the
back of the transmitter.
A Receiver-inset No. 4T is fitted and secured in the rubber
earpiece by means of a metal ring and two rubber lugs. Connexion
is made to the receiver by two wander leads terminated with ring
tags secured under the receiver terminal slotted nuts (Part
To release either the transmitter or receiver from the
mouthpiece or earpiece, the two rubber lugs should be eased
outward from the metal securing ring and the inset then pushed
out by pressure on the outer face of the mouthpiece or earpiece. When replacing the transmitter in the mouthpiece, care should be
taken to ensure that the cut-away portion of the securing ring
is under the lug on the shallow side of the mouthpiece.
7. Induction coil
The Coil, Induction, No. 29 is mounted inside the frame on a
cut-out lug. It is a
non-anti-sidetone induction coil consisting of two windings of
36 and 135 ohms.
8. Dial and dial mounting
Dial, Automatic, No. 17LA fitted in
the Telephone No. 280 is a development of the
Dial Automatic, No. 12 described in
B 1003, differing only in a modified number plate, case, finger
plate and stop. It is mounted and secured by three screws to a
cast aluminium alloy ring which is a force fit in the rubber
To remove the dial, first remove the rubber earpiece and
receiver. Next release the dial cord terminal screws and remove
the dial cover to expose the rear of the dial. The dial can then
be removed from the telephone by releasing the three screws
which pass through lugs in the cast ring into tapped holes in
the dial case.
9. Press button assembly
The press button is mounted in the side of the handle in a
convenient position for thumb or index finger operation. The
latest supplies of the telephone are fitted with a modified
button assembly which includes contact “follow-through”
and locking devices. This type of press button is available as a
Rate Book item and is coded Part 1 /DBU/ 14. Early pattern
Telephones No. 280 without the locking type of press button may
be modified locally by ordering Part 1 /DBU/ 14 and fitting it
in place of the existing part.
To release the button assembly from the body it is only
necessary to unscrew the hexagonal bush through which the button
protrudes and the button assembly can be withdrawn from the case
10. Cord and plug
The cord fitted to the telephone is a Cord, Instrument, No.
2/95F, Black, 48 in. The plug for terminating the cords should
requisitioned separately according to local requirements, the
types of plug available being detailed in
11. Replacement parts
The following parts are available if replacements are
Dial, Automatic, No. 17LA
Transmitter-inset, No. 13
Receiver-inset, No. 4T
Tag. Part l/DTA/153
Button, Press, Locking, Part l/DBU/14 (Part of Press Button W)
Nut, Hexagonal Slotted. Part l/DNU/87
Telephone No. 280
- Receiver and transmitter - these are both removed by prize
out carefully from rubber shroud.
- Remove connections.
- The dial must now be removed
- Remove centre screw and finger plate, this gives access to
small screw into the handle framework.
- Remove this screw.
- Once there is access to the rear of dial unit - remove all 4
connections to dial tags.
- Now remove the 3 screws retaining the rear cover to dial and
- Remove 2 further screws connecting dial housing to handle
- Push out entire dial housing through the front aperture.
- To remove handle internals - firstly remove switch button.
- This is easily done by unscrewing with a suitable box
spanner - a 13mm box spanner can be made to fit. Be
careful not to turn the whole switch.
- Service can now be carried out to the switch contacts (this
switch can be a source of trouble).
- Beware of losing the spring and the silver disk can become
- The handle assembly can now be withdrawn into the dial space
and out of the handle, but will probably be a tight fit after
- Unclip the capacitor from the top end and withdraw.
- Remove the screw retaining the induction coil.
- It is now possible to withdraw the inards sufficiently, from
the dial end, to access the induction coil, but to remove
completely you will probably have to disconnect the capacitor.