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BPO 746 & 741 type

Auto, Table and Wall Telephone
BPO Types 746 & 741

bpo746.jpg (15705 bytes)These BPO approved table and wall telephones meet the most up-to-date requirements for instruments of compact proportions, advanced design and economic cost.

Transmission and reception are based on CCITT recommendations and full use is made of printed-circuit techniques for increased reliability and simplified construction.

Main Design Features
Modular shock resistant construction

Printed-circuit board with miniature components.

Stain-resistant, high-impact shell mouldings.

Built-in carrying handle (on table telephone).

Easily visible dial characters.

Automatic line regulator

Optional press button on table telephone for operator- recall, shared-service or bell on/off operation.

Optional amplified handset.

Standard or tropical finish
Cover - The cover is a one-piece ABS copolymer moulding of high-gloss finish and high-impact strength.

The cover of the table telephone incorporates a recessed carrying handle and a dummy button that can be easily replaced by a press button when shared-service operator recall facilities are 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 disk.

Dial markings can be provided to most specifications.

Terminal Block
This is for the termination of the table telephone and comprises a two-piece copolymer moulding that can be secured to any flat surface.

bpo741.jpg (15280 bytes)Handset
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.

Both line and handset cords have three copper tinsel conductors with PVC insulation. The outer sheath is of pliable PVC. The handset cord is the coiled extensible type.

All interior parts of the instrument, cradle switch bracket, ringer and gongs, dial assembly and printed-wiring board are mounted on the baseplate. Both table and wall telephones have baseplates moulded in tough, resilient polystyrene. Four chloroprene non-stain feet on the table-telephone baseplate prevent the instrument from sliding even on a highly polished surface.

Printed-Wiring Board
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.

Simplified Maintenance
Cover removal is achieved by the release of a single screw at the rear of the table telephone or the base of the wall-mounted model.

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 periods.

Colour Range
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 quantity

In two-tone colours, the handset, cords and terminal block are in the darker tone.

Dimensions and Weights
Table telephone
Overall height: 5.25 in (133 mm)
width: 10 in (254 mm)
depth: 8.25 in (210 mm)

weight: 3.25 lb. (147 kg)

Wall telephone
Overall height. 9 in (229 mm)
width: 10 in (254 mm)
depth: 4.75 in (121 mm)

weight: 3.5 lb. (1.59 kg)

Abridged Specification
Cover, Handset and Terminal Block ABS high-impact plastic.

Table telephone - Pressed-steel with passivated zinc-plate finish. Under-side coated with crinkled black enamel.
Wall telephone - Toughened, black polystyrene.

Standard: black, ivory or two-tone grey.
Special orders: red, blue, yellow or two-tone green.

PVC covered copper-tinsel conductors in PVC sheath. Spade terminations.
Table and wall telephones each have a coiled handset cord extensible from 1 0 in (254 mm) to 6 ft (1 829 mm). Desk cord for table model is 6 ft (1 829 mm) standard., other lengths are available.

Cradle Switch
Microswitch BPO Type 20A.

BPO Type 21 FA trigger dial.

Dial Characters
Black on antique-silver background to most specifications.
Dial Speed and Ratio 10 p/s and 2: 1.
Other ratios can be supplied.

Handset Transducers
Receiver: BPO Type 4T (rocking-armature type).
Transmitter: BPO Type 16 (carbon-granule type).
Amplified Handset: 20 dB maximum gain.

Both transmission and reception zero line sensitivity attenuated by 5 dB.
Attenuation decreases with line length to 0 dB at 400 ohms

Type: Unicoil, BPO 79A Type.
d.c. resistance: 1kohms
Impedance at 25Hz: 3kohms
Impedance at 1 kHz: 10kohms
Ringing frequency: 15 to 30Hz

Bell Gongs
Double gong, harmonising tones.

Part No's
617/1/02161/000 table telephone
617/1/02161/002 table telephone
617/1/02164/000 wall-mounted telephone

PUBLICATION NO. 7396, 1975

This telephone is more or less the GPO/PO/BT Telephone No. 746


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Last revised: March 25, 2024