(BPO Type 746)
This automatic telephone N2160A (BPO type 746) meets the most
up-to-date requirements for an instrument of compact proportions, advanced design and
economic cost. Based on the BPO type 706, developed by Plessey
in conjunction with the British Post Office, but with new and improved components, it
combines simplified construction with functional efficiency. Its high-quality transmission
and reception are based on CCITT recommendations.
Main Design Features
Modular shock-resistant construction
Printed-circuit board with miniature components
Stain-resistant, high-impact shell mouldings
Built-in carrying handle
Easily visible dial characters
Automatic line regulator
Optional press button for operator-
recall, shared service or bell on/off operation
Optional amplified handset
Standard or tropical finish
The cover is a one-piece ABS copolymer moulding of high-gloss finish and high-impact
strength incorporating a recessed carrying handle. An aperture, provided above the dial to
accommodate a press-button, is closed by a dummy button when not required.
The dial is the conventional trigger type. A clear-plastic finger plate covers the
antique-silver number plate, making the black numbers clearly visible even in poor light.
The clear-plastic number card protector can be removed with a simple rubber suction disc.
Dial markings can be provided to most specifications.
The terminal block is of identical material to the main instrument and can be secured to
any flat surface.
The lightweight handset is the same as that used in the type 706 telephone, the mouthpiece
and earpiece being screw fitted. The cord is bayonet clamped and the transmitter and
receiver inserts are connected by screw terminals to give good connections. Both inserts
are adjusted in manufacture for optimum sensitivity and frequency response.
An amplified handset can be provided as a direct replacement for the standard handset. It
is suitable for use by persons with impaired hearing or in locations where reception
is difficult. this now jumps to cords etc.
Cover, Handset and Terminal Block
ABS high-impact plastic.
Pressed-steel with overall passivated zinc-plate finish, supplemented by an underside
coating of crinkled black enamel.
Standard: black, ivory or two-tone grey.
Special orders: red, blue, yellow or two-tone green.
PVC covered copper-tinsel conductors in p.v.c. sheath. Spade terminations.
Handset cord: 216 mm (8.5in.) coiled, 1780 mm (6 ft) extended
Line cord: 1780 mm (6 ft) standard, with other lengths available.
Microswitch BPO type 20A.
BPO type 21 FA trigger dial.
Black on antique-silver back ground to most specifications.
Dial Speed and Ratio
1 0 p.p.s. and 2:1.
Other ratios can be supplied.
Receiver insert: Rocking armature replaceable capsule BPO type 4T
Transmitter insert: Carbon granule replaceable capsule BPO type 16
20 dB maximum gain.
Both transmission and reception zero line sensitivity attenuated by 5 dB. Attenuation
decreases with line length to 0 dB at 4000.
Type: Unicoil, BPO type 79A.
DC resistance: 1000 ohms
Impedance at 25 Hz: 3000 ohms
Impedance at l kHz: 10k ohms
Ringing current: 15 to 30 Hz
Double gong; harmonising tones.
Both line and handset cords have four copper tinsel conductors with p.v.c. insulation. The
outer sheath is of pliable p.v.c. The handset cord is the coiled extensible type.
The pressed-steel baseplate rests on four chloroprene non-stain feet to prevent the
instrument from sliding even on a highly polished surface.
All interior parts of the instrument, cradle switch bracket, ringer and gongs, dial
assembly and printed-wiring board are mounted on the baseplate.
A printed-wiring board, on which all incoming and outgoing connections are terminated,
accommodates all speech circuit components except the transducers. The cradle
microswitch has a low operating pressure and requires no maintenance; its reliability is
greatly improved compared with previous types of switch. The regulator, which operates on
conventional principles, maintains constant speech levels, irrespective of line
length. Unregulated instruments can be provided for large-quantity orders.
The cover is easily removed, being secured by a single screw at the rear of the
instrument. The outer dial ring, close fitting to the dial and a floating fit on the
cover, facilitates cover removal. A manually operated latch locks the cradle switch in the
handset-on position, allowing calls to be received during instrument inspection.
All cord conductors have spade terminations, allowing easy connection to the instrument
wiring board and the terminal block. Ringer, bell gongs and printed- wiring board are
readily accessible and easily replaced.
Dial assembly contacts are protected from dust by a clamped-on plastic cover.
Robust construction and reliable components ensure trouble-free operation over long
Standard colours are black, ivory and two-tone grey, but demands for special colours (red,
blue, yellow and two-tone green) can be met for instruments ordered in sufficient
In two-tone colours, the handset, cords and terminal block are in the darker tone.
Dimensions and Weight
height: 133 mm (5.25in.)
width: 254mm (10in.)
depth: 216mm (8.25in.)
Taken from the Plessey Telephone Catalogue (1971)
Publication No. 7281