2 + 5 Keymaster
With Auto or CB exchange lines

n1676a.jpg (12355 bytes)The 2 + 5 'Keymaster' is a low cost system providing direct access from any one of five internal stations to either of two automatic or C.B. exchange lines, with intercom between stations and secrecy on exchange lines. It is designed for premises where all the stations are internal.

Exchange lines may be to a public exchange, PAX, PABX or PMBX.
Advantages of the 2 + 5 'Keymaster' are:-

  • Low equipment, installation and maintenance costs owing to simplicity of operation and cabling, and the absence of large auxiliary units and junction boxes.

  • Station telephones are small and reliable, being of Plan-Etelphone type. They incorporate 6V lamps for exchange-line supervision.

  • The system has a variety of applications and, if intercom code-calling for pairs of stations is adopted, there can be more than five stations.

Equipment Required
Apart from the station telephones, the only items required are a battery eliminator or dry batteries, and a small relay unit.

Line Limits
The system is suitable for connection to 1000 ohm exchange lines.


The following items are fitted in each station telephone:

For Exchange Calls
A Call/Answer button and associated 6V lamp with clear lens for each exchange line.
A Hold button and associated 6V red lamp for each exchange line.
A Cancel button (X) for each exchange line.
An a.c. bell and on/off switch.

For Intercom Calls
Four call buttons.
A dc. buzzer.

Exchange Calls
Direct access
All stations have direct access to both exchange lines.

Incoming calls are visually and audibly signalled
The call lamp flashes at ringing periodicity at all stations, then glows steadily when the call is answered and until the line is disengaged. Normally the bell is left operative at only one station, but all can be rung simultaneously if required.

Information calls
An exchange call can be held while making an information call to another station, or to, say, a PABX on the other exchange line. The Hold lamp glows steadily at the holding station during the hold condition. The information-call conversation cannot be overheard by the exchange party. At its conclusion, the original call can be resumed or transferred to another station.

Stations are normally connected in series to the exchange line; thus station 1 cuts off stations 2 to 5, station 2 cuts off 3 to 5, and so on.

Any station can transfer an exchange call to another station.

If a dialling error is made, the call can be cancelled and the line re-seized without replacing the handset, by pressing the X button of the exchange line concerned.

Simultaneous calls
While engaged on one exchange line, it is possible to answer a call on the other exchange line, then either clear or transfer the second call, or retain it and clear or transfer the first call.

Mains failure safeguard
Exchange line service is maintained during mains failure conditions.

Intercom Calls
Selective calling of all other stations by press button.
Incoming calls signalled by a buzzer.
Conference enabling all stations to converse together over the single intercom connecting circuit.
Buzzer Cut-off during exchange line conversations, by strap adjustment in the telephone.

Special Facilities
Call buttons on the intercom are operative whether the handset is off or on, in order to provide subsidiary signals if required. All other buttons are inoperative when the handset is on.

Operator recall, priority or a similar facility can be provided utilising one of the intercom call buttons. Should such a facility and five stations be required, two of the less important stations are code called.

Telephone N1676A (Auto) or N1677A (c.b.)
The general construction is similar to that of the Etelphone and Plan-Etelphone. Ivory or two-tone grey instruments with matching h.m.t. cord and grey desk terminal block can be supplied. The a.c. bell for exchange calls and the d.c. buzzer for intercom calls are in the instrument.

Press buttons and lamps are arranged as shown in diagram opposite, in which it will be seen that the Hold/Cancel (X) keys are of two-in-one type.

The Exchange and associated Hold buttons release each other, and the X button releases both. Any operated button is restored on replacement of the handset. To allow for intercom call-button numbering to be varied to suit the particular station, a fifth call button cap and a Recall cap are contained in the instrument.

Lamps are slide-mounted in a unit attached by a single screw to the equipment chassis and are easily accessible.

Exchange switching key springsets, consisting of six banks of comb-operated contacts in a single unit, are wired to three 6-way terminal blocks mounted on a frame which is secured to the rear of the chassis by a spring plunger.  The springsets, terminal blocks and lampholder are removable as a unit assembly.

The 22-way desk-cord conductors are crimped to spade tags which fit into the moulded desk terminal block. Cord and cable entries are at opposite ends of the block.

The common relay unit N24203A is a grey moulded plastic box with a cable entry on one side, and a metal base to which the cover is attached by two screws. The equipment includes a transmission feed coil for the intercom, two relays for ringing and lamp-signal purposes, and screw terminals for cable connection.

Power Unit N238018 is a battery eliminator incorporating a transformer, choke, fuses and terminal block in a grey enamelled metal case.  The smoothed output is 6V 1A d.c. for the lamps, and the input 100 to 125V in 5V steps, 200 to 250V in 10V steps 50/60Hz.

Alternative Power Supply
Batteries may be used if there is no local mains supply. In this event, a simple strap adjustment in the telephone renders the supervisory lamps operative only when the handset is removed, thereby reducing current drain.

The equipment has tropical finish. Connecting wires, including cord and cable conductors are insulated with p.v.c., which is impervious to moisture, fire-resistant and not subject to mould growth or insect attack.  Plastic covers are fitted over mechanisms where necessary, and foam rubber strip inserted between the base plate and cover of the telephone.

Installation and Cabling
Systems which are to be connected to exchanges under the jurisdiction of the British Post Office, are supplied and installed by them. Systems not in this category can be easily installed by the customer, since all cables connect to screw terminals.

Exchange line 1 has switching priority over line 2.  Therefore, if one line is more important than the other (e.g. if one is to the public exchange and the other to a PAX), it should be connected as line No 1 in the system.

The exchange lines are series-multipled into and out of each station in turn, normally beginning with No 1. Priority of access can be given to any station by connecting the exchange line(s) first to that particular station.

Telephone Types

Equipment   Item Code No.
Telephone (automatic)   N1676A
Telephone (C.B.)   N1677A
Relay unit   N24203A
Battery eliminator   N23801B


Model Number Colour
N1676A1T - Black
N1676A2T - Ivory
N1676A3T - Lacquer red
N1676A4T - Topaz yellow
N1676A5T - Concord blue
N1676A6T - Body - French Grey / Handset - Elephant Grey
N1676A7T - Body - Aircraft Grey-Green / Handset - Forest Green

Click here for blueprint

Taken from the Plessey Telecommunication Publication No. 7146 (1/69)

n1676a.jpg (12355 bytes)

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Last revised: October 07, 2022