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The BPO 2 + 6 (introduced circa 1961), 3 + 12 and 4 + 18 type of PMBX switchboard consists of a dark grey key console
mounted in a light grey moulded plastic case. The panel mounted variant, 3 + 12 and
5 + 25,
were installed in a customer provided unit which normally also housed other equipment.
700 type telephones with recall are fitted to extensions and all internal extensions are
connected to the switchboard by 4 wires. External extensions, inter PBX
lines and private circuits are normally connected by 2 wire wiring via auxiliary apparatus
A connection box and power unit, both wall mounted, were provided for each PMBX of this
Made by various manufacturers.
Distributed, installed and maintained by BT.
PMBX's are hardwired "Grandfathered" equipment
Wiring diagrams for PMBXs and associated equipment can be found in the "N"
series, N1003 through to N1887. SA and SAW diagrams that have been identified for PMBX
equipment are also listed below.
Main diagrams for PMBX 2+6 (2/2A)
SAW 71630 & SAW 71631
Main diagrams for 3+12 (2/3A)
SA 7164 & SAW 71640
Main diagrams for 4+18 (2/4A)
N 1104 & N 1128
SA 7165 & SAW 71650
Main diagram for Panel Mounted 3/- PMBX - SAW 7169
The principal TI's relating to installation and maintenance of PMBX's are as follows;
PMBX 2/2A (2+6), 2/3A (3+12), 2/4A (4+18) MAINTENANCE E5 D1351
PMBX 2/2A & 2/2B - INSTALLATION C3 E4001
PMBX 2/3A (3+12) - DESCRIPTION AND INSTALLATION C3 E4002
PMBX 2/4A (4+18) - DESCRIPTION AND INSTALLATION C3 E4003
PMBX 3/5B - FRAMES AND MOUNTING C3 E4035
Maintenance was offered until 30th September 1994.
Customers were sent a letter by post on 17th September 1993 giving them 12 months notice
PMBX 2+6 (Item Code 410217)
PMBX 3+12 (Item Code 395910)
PMBX 4+18 (Item Code 377424)