PXML No. 81 

User Guide

Installation Guide

Sales Brochure - CP/MS492

The Minimaster 3 is a processor controlled PBX with a maximum capacity of 2 exchange lines and 10 extensions. The central control unit is wall mounted and houses the single circuit board and power unit. There are no dedicated system phones, extension telephones may be any approved 2-wire, 10 pps LD pulsing telephone or dual signalling telephone switched to LD), telephones do not need `recall' facility as recall is by dialling the digit 1.

A Central Bell may be provided in place of Extension 23 to signal incoming exchange calls.

The Minimaster 3 CCU measures approximately 385 x 280 mm x 80 mm and weighs approximately 3.5 kg.

Made by Lake Electronics Ltd.

Introduced around 1986.

No longer available from BT since July 1991, was supplied on sale and rental terms. Also available from third party suppliers new and second-hand.

Approved as described in the PXML which is updated as further enhancements are approved. Only modifications and facilities specified as approved in the PXML Approved for direct and for indirect connection (LD signalling) to the PSTN. Minimaster 3 does not provide recall signals to host systems.

Approved software levels for Minimaster 3 are:-
SA3089-1 (6.10) - only on systems made before week 48 of 1984.
SA3089-2 (6.11) - all systems
SA3089-3 (6.12) - all systems
SA3089-4 (6.13) - all systems
Software level as labelled on the plug-in EPROM IC2.

6.9 software was found to have 2 bugs:

  1. There is no shuttle facility on internal calls.

  2. Option 3 - If extension 22 is engaged on an internal call, no tone is heard indicating an incoming call.  The call moves directly to extension 23.

CCU nominal power consumption is 30 watts. There is no standby power, exchange lines are switched through to predetermined power fail sockets.

Functional (Telecommunications) Earth must be connected to the termination on the PCB and the mains supply must also be earthed.

Cover in place
Cover removed


Description BT item Code
Minimaster 3 CCU (new) 374573
Minimaster 3 CCU (refurbished) 940866
Fuses (internal) Fuse 72/0.315  
Plug Top (13 Amp) fuse 3 Amp fuse
Minimaster 3, 6.13 software 661632

Minimaster 3, 6.13 software is designed to overcome the following problems which may occur with earlier software:-

  • Exchange lines 'double switching' (i.e. both exchange lines appear to be in contact), this may be caused by mains borne interference.
  • Exchange lines being put into hold due to 'gravity switch bounce' when extensions replace the handset.
  • Overhearing other extensions dialling out (during dial tone).

During 1988 there was a manufacturing error involving the Ringing Pattern Option Switches on all Minimaster 3's produced between weeks 0188 and 4588. When the switches are set for Option 3 (extns 21, 22 and 23 to ring in sequence) Option 4 is applied (ringing stays at Extn 23). When set for 'Ringing Options' 1, 2 or 4 it operates correctly.  Stocks returned to the manufacturer for rework to correct the error are date marked 4888.

System lock-up can occur in lightning prone areas (this can be restored by powering the system down), this may be avoided by applying mains protection.

The following is a list of the facilities Provided by the system:-

  • Internal calls from all extensions.
  • Selection of defined exchange line.
  • Hold.
  • Enquiry ring internal and external calls.
  • Shuttle between exchange line and third party extension.
  • Transfer.
  • Call diversion.
  • Three party conference between one exchange line and two extensions, or between three extensions.
  • Secrecy on all connections.
  • Secretarial override of call diversion.
  • Last number redial.
  • Alert tone to indicate incoming calls, if the designated receiving extension is busy.
  • Any extension can answer an incoming call.
  • Different ringing cadence for internal and external calls.
  • A choice of incoming ringing sequences to designated extensions.
  • Night service.
  • Mains fail connection of two designated extensions to the exchange lines.

Central Control Unit
The central control unit (CCU) is a single wall mounted unit. This unit houses:-

1. The power supply for the system, the extension line feed and the ringing generator.

2. A single printed circuit board.

3. Insulation displacement termination (IDT) for the exchange lines and extension lines terminations.

4. A screw termination for an earth cable.

A wide variety of terminal equipment can be connected to the system. Earth recall is not required by the system. Terminals must comply with the safety specification BS6301 and with the specification for simple extension telephones BS6317.

Speech paths
The system has two internal speech paths and two external speech paths, therefore two external calls and two internal calls can take place simultaneously. External and internal speech paths cannot be exchanged, so if only one external speech path is in used it is still only possible to make two internal calls simultaneously.

If an enquiry call is made during an external call, two speech paths are used; one external and one internal.

If a conference call is required between one exchange line and two extensions, two speech paths are required to set up the conference; one external and one internal. Once the conference has been established, only the external speech path is used, the internal speech path being made available for another internal connection.

Only one internal speech path is required for enquiry during internal calls and for establishing a conference call between three extensions.

Numbering plan
The following is a summary of the numbering plan:

0 - Spare

1 - Hold and Enquiry

2 - Internal Calls

3 - Designated Exchange Line Access

31 - Select exchange line 1

32 - Select exchange line 2

41XX - Call diversion (41 - Diversion off)

44 - Invoke Night Service to Extn 23

44XX - Invoke Night Service to specific extn

45 - Revoke Night Service

5 - Repeat Last Number

8 - Universal Call Answer

9 - Exchange Access

Exchange Line Access
A system equipped with only one exchange line must always have extension 21 equipped. A system equipped with 2 exchange lines must always have both extension 21 and extension 22 equipped.

Note: Only equipment complying with BS6301 and BS6317 may be connected to these extension ports.

Extension Wiring
All wired to Master Line jacks.

System Colour Socket
xA WHITE/Blue 5
xB BLUE/White 2
Where x is the extension number marked on the PCB

Setting the option switches
Four dual in-line (dil) switches are located on the top left hand side of the pcb. to the right of the mains transformer. These switches are used to select the required incoming call ringing sequence and to select one or two exchange line operation.
Switch 4 is nearest the top edge of the pcb. A switch is in the ON position when the protruding plastic used for operating the switch is pushed towards the left hand side of the pcb. If one exchange line is supplied, set switch 4 (top switch) to the ON position (Push switch to the left). Refer to the list below to set the desired incoming call ringing.
The sequence ends when the call is answered.

OPTIONS SW1 SW2 SW3 Incoming call ringing sequence
1 OFF OFF OFF Incoming calls only ring extension 21. This option must only be selected if only one exchange is fitted, because if extension 21 is engaged on an exchange call then there will be no indication of an incoming call on the second exchange line..
2 ON OFF OFF Incoming calls ring extension 21 for 20 seconds and then ring extension 22 for 20 seconds. This sequence is then repeated.
3 ON ON OFF Incoming calls first ring extension 21 for 20 seconds, then ring extension 22 for 20 seconds, and finally ring extension 23 for 20 seconds before repeating this sequence from extension 21.
4 ON ON ON The incoming call sequence is the same as for option 3 above (ON ON OFF) but the sequence does not repeat itself after ringing 23 for 20 seconds. Instead, 23 continues to ring until the call is answered or the caller abandons the call.
This option must only be selected if a central bell is equipped at extension 23.

Incoming call ringing
'Night service' can be set-up to announce incoming calls at any equipped extension position. The extension position need not be equipped with a central bell.

Physical specification
CCU dimensions: 385mm x 280mm x 80mm
CCU weight: 3.5 Kg

Power requirements
- Power supply: 240 Volts, 192 Volts minimum, 264 Volts maximum (fused at 3 amps)
- Power consumption on idle, all extensions on hook, nominally 16 Watts.
- Power consumption, all extensions off hook: 45 Watts nominally.

This system must be earthed (functional earth).  This will be a direct connection from the building earth point to the CCU earth.

Internal fuse is a 315ma slow blow type

Extension line voltage is 48v DC and extension ringing is 50 volts at 25hz.

Maximum extension resistance to CCU is 200 ohms or 1190 metres using 0.5mm conductors.

Loss within the CCU:-
Extension to extension - 7.0db at 800hz
Extension to exchange line - 1.5db at 800hz

There are 4 speech paths:-
2 for the exchange lines.
2 for internal calls.

The time taken for an extension to ring when receiving an incoming call from the exchange is within 5 seconds.

The time to stop extension or central bell from ringing after an incoming call clears down without an answer is 5 to 8 seconds.

There is more technical information at the end of the Installation Guide.


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Last revised: April 29, 2023