|P.O. ENGINEERING DEPT.
Issue 1, 28.6.62
LAMP CALLING SIGNALS
ON 200 AND 300 TYPE TELEPHONES
Lamp Fittings No. 14A and No. 14B
This Instruction describes the Lamp Fittings No. 14A and 14B and their use
for providing a low-voltage lamp signal on a telephone. This lamp signal is
additional to the bell for indicating an incoming call, and the fittings are
suitable for mounting on 300 and 200 type table telephones respectively.
|Lamp Fitting No. 14A - Fitted to 300 type
|Lamp Fitting No. 14B - Fitted to 200 type
2. Lamp Fittings No. 14A and 14B (see Figs. 1 and 2)
Each consists of a moulded pillar with a screw lamp holder for a Lamp No.
26.. on a metal bracket for attaching to the back of a telephone so that the
lamp signal is visible from any angle. A transparent lamp cap, a cord and
terminal strip are included. They are fitted to 200 and 300 type telephones
- Lamp Fitting No. 14A on a 300 type telephone - The lamp fitting
should be slid down the back of the telephone case until the projections
of the bracket hold securely round the rear corners (see Fig. 3). The
bracket of the terminal strip is screwed to the sockets on the telephone
chassis which are used for Keys No. 303.. and No. 304... A suitable
instrument cord should be fitted with two of the longer conductors
connected to the terminal strip of the lamp fitting. When replacing the
chassis in the telephone case, care should be taken that the
lamp-fitting cord does not foul the gravity switch and that it is led
out through the cut-out portion of the chassis.
- Lamp Fitting No. 14A on a 300 type telephone which includes a Key
No. 303.. or 304... - The bracket of the terminal strip must be removed
and the terminal strip should be fixed on to the side of the Key No.
303.. or 304.. in place of the extension arm which is used when required
for gravity-switch release. The extension arm may be stored in a
suitable place elsewhere within the telephone. The terminal strip should
be fixed with the terminals downward (nearest chassis), and the cord
arranged clear of the gravity switch. This should be checked before the
mechanism is replaced in the case. To simplify the cord connexion the
lamp fitting terminal strip should be wired to spare terminals either on
the key or on the telephone terminal block.
- Lamp Fitting No. 14B on 200 type telephones - The U clamp is removed
from the fitting and is placed below the cradle (round the neck of the
telephone case) and the lamp bracket is then reassembled (see Fig. 4).
To enable the cord of the lamp fitting to be passed through the cord
hole in the telephone case, it should be removed from its terminal strip
and then reconnected inside the telephone case. The existing instrument
cord should be recovered, the terminal strip of the lamp fitting secured
under the nuts and washers of terminals 1 and 2, and the new instrument
|Lamp Fitting No. 14A - Shown fitted
|Lamp Fitting No. 14A - Shown fitted
3. The lamp signal
This can be of two types:-
(a) A discontinuous signal for use only with automatic ringing. The
signal lamp flashes in response to the ringing and is extinguished when
ringing ceases, i.e. when the called party answers or when the caller
abandons the call.
(b) A continuous signal for use with manual ringing. The lamp lights on
the first application of ringing and is extinguished when the handset is
4. Circuit description
- (a) Discontinuous lamp signalling - A relay is connected in series
with the bell and operates in unison with the incoming ring. A contact
of the relay completes the circuit to light a Lamp No. 26F from a local
24V supply. Dgms. N 1882 and N 1887 show the circuit schematic details
using Telephones No. 232 and No. 332 respectively, but other 200 and 300
type telephones may be used by making suitable modifications to the
- (b) Continuous lamp signalling - One coil of a double-coil relay is
connected in series with the bell. With the first application of
ringing, the relay operates over this coil and is held through its
second coil to a local 24V d.c. supply. The relay remains operated until
the telephone handset is lifted. A contact of the relay completes the
circuit to light a Lamp No. 26D via a dropping resistor from a local 24V
d.c. or a.c. supply. The 6V lamp is used to prevent overheating the lamp
fitting when the lamp is lit for long periods. Dgms. N 1883 and N 1886
show the circuit schematic details using Telephones No. 242 and No. 308
respectively. A Telephone No. 306 should be used if a d.c. bell is
required instead of a magneto bell.
The apparatus required is scheduled on the appropriate N diagrams. Complete
relay-sets are not stocked by Supplies Dept. and the components required
should be requisitioned separately and assembled and wired locally.
While stocks last it may be convenient to use a Bellset No. 41 which
contains certain of the components required; this could be used in place of
an extension bell.
6. Power supplies
The low-voltage power supplies required for the two types of signalling
- (a) Discontinuous lamp signals - 24V (normally a.c.) for lighting
the lamp (Dgm. N 1884 refers).
- (b) Continuous lamp signals - 24V d.c. for the locking circuit of
the lamp relays and 24V (normally a.c.) for lighting the lamps (Dgm. N
1885 refers). These power units should be constructed locally as
detailed on the appropriate N diagram. If a suitable power supply is
available locally this may be used instead of that shown on the diagrams
Provision should be made to enable the subscriber to disconnect the power
supplies to the continuous lamp signal during his absence, either at the
power unit or at each individual telephone.