Lamp Fittings No's 14A and 14B


P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
TELEPHONES
STATIONS
A 1052
Issue 1, 28.6.62

LAMP CALLING SIGNALS ON 200 AND 300 TYPE TELEPHONES
Lamp Fittings No. 14A and No. 14B

1. General
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 telephones
 
Lamp Fitting No. 14B - Fitted to 200 type telephones

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 as follows:-

  1. 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.
     
  2. 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.
     
  3. 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 cord connected.
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 lifted.

4. Circuit description

  1. (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 circuit.
     
  2. (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.

5. Apparatus
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 arrangements are:-

  1. (a) Discontinuous lamp signals - 24V (normally a.c.) for lighting the lamp (Dgm. N 1884 refers).
     
  2. (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 quoted above.

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.
 

 

 
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Last revised: April 09, 2012

FM