AMPLIFIER & LOUDSPEAKER No. 4

The Amplifier & Loudspeaker No. 4 required a PSU No. 73A.

Drawings - 92255, 942435 and 92436.

Circuit diagram - N4442.

Specifications - S649 and S882.

1. Introduction
Amplifier and Loudspeaker No. 4 provides amplified loud-speech reception of incoming signals from a telephone line where a higher volume than that provided by Amplifier and Loudspeaker No. 5 is required, or where the type of telephone installation is not suitable for Amplifier and Loudspeaker No. 5 (see F 1008). Amplifier and Loudspeaker No. 4 supersedes Amplifier and Loudspeaker Nos. 1 and 2. Loudspeaker Unit No. 3A is a matching, extension loudspeaker unit.

For sales purposes the system is named Speakerset 1.

2. Description and facilities
Amplifier and Loudspeaker No. 4 consists of a grey plastic unit approximately 6in. x 6in. x 4in., together with a grey 6-way cord. On the upper face is a plastic escutcheon plate on which are mounted a volume control and an ON/OFF push-button of square cross-section having an amber-coloured lens which is illuminated when the push-button is in the ON position. The loudspeaker faces downwards and the sound is propagated through a series of slots along the lower edges of the instrument. The unit may be either placed on a desk or table, or attached to a wall. The amplifier, which is a transistor type, delivers about 1.5 watts output, and is supplied with 12v d.c. from a Power-unit No. 73A connected to the public a.c. mains supply.

Loudspeaker-unit No. 3A is an extension loudspeaker unit, similar in appearance to the main unit. It way be operated in parallel with the loudspeaker in the main unit provided the 6 way cord is removed and permanent wiring used instead.

This is necessary because conventional cords introduce unacceptable resistive loss into the low-impedance extension loudspeaker circuit. Hence under these circumstances, it is necessary for both the main and extension units to be wall-mounted. The extension unit has an ON/OFF push-button which is not illuminated, and also a volume control which enables the extension volume to be regulated below, but not above, that of the main unit.

The main unit works in association with a telephone instrument. Transfer of a call to loudspeaking reception is made by depressing the ON/OFF push-button, which connects the line to the unit. The handset is then either replaced on the telephone or left on the desk. Restoration of the telephone circuit is effected by picking up the handset and again depressing the ON/OFF push-button.

The units are essentially for reception only and it is not possible for speech to be transmitted during use. When the main unit is switched on, the telephone is automatically disconnected, otherwise acoustic feedback from the loud-speaker to the 'handset microphone would cause 'howling' unless the output of the loudspeaker were drastically reduced. If bothway amplified transmission is required, it is necessary to use a Loudspeaking telephone (see F 1104).

3. Installation
(a) List of items
The items to be requisitioned are as follows:-
For the main instrument:-
1 x Amplifier and Loudspeaker No. 4
1 x Power-unit No. 73A
1 x Block, Terminal, No. 35A

For the extension instrument:-
1 x Loudspeaker-unit No. 3A

For wall-mounting of main or extension instruments:-
1 x Plate, Mounting, No. 3A
1 x Kit No. 130

(b) Connections
The connections to the main unit (Amplifier and Loudspeaker No. 4) are as shown in Table 1. The cord, line, telephone instrument and d.c. supply should be terminated on a Block, Terminal No. 35A, the cord being connected to the side numbered 7-12.

TABLE 1
CONNECTIONS FOR AMPLIFIER AND LOUDSPEAKER No. 4

A wiring sketch incorporating the above arrangement will be packed with each instrument (main or extension) in initial supplies. The sketch is to be used in lieu of Diagram N 4441 until the latter becomes available.

(c) Power supply
It is very important to observe the correct polarity of the d.c. supply. If there is any doubt, the polarity should be checked with a Meter before connecting. The d.c. supply should be obtained from a Power Unit No. 73A wired in accordance with Diagram N 674 as the preferred arrangement, although it is possible to operate the unit from secondary cells if required. For the latter arrangement it should be noted that on full volume the unit will draw approximately 0.5 amp; the quiescent current is approximately 100 ma. Power Unit No. 53A must not be used as an alternative to Power Unit No. 73A because it is incapable of delivering sufficient current. The power unit should be installed as close to the terminal block as possible and if Cable, P.V.C., No. 1, 2 wire is being used for the d.c. feed, as specified in Diagram N674, the cable length should not exceed 6yds.

(d) Locator, of the amplifier unit
The amplifier unit may either rest upon a hard flat surface or it may be wall-mounted. For wall-mounting it is necessary to requisition a Plate, Mounting, No. 3A and a Kit No. 130.

Kit No. 130 consists of a metal plate approximately 4in x 4.5in, together with associated screws, spacers, grommets, washers and locknuts. The four rubber feet should be removed from the base of the unit and replaced by the grommets. The spacers should then be pushed through the grommets from the outside and the metal plate secured to the base with the screws, washers and locknuts. The plate may be attached in alternative positions to allow the instrument cord to be fed from above or below the unit.

Plate, Mounting, No. 3A is an enamelled metal plate with keyhole cut-outs which is secured to the desired wall position by the 1 in x No. 6 screws provided. The three 5 in studs which are provided should be screwed home into the plate of the Kit No. 130 to enable it to engage in the keyhole slots of the Plate, Mounting, No. 3A.

(e) Extension loudspeaker
The Loudspeaker Unit No. 3A is wall-mounted using a Plate, Mounting, No. 3A and a Kit No. 130 in the same way as a wall-mounted Amplifier and Loudspeaker No. 4. The two connections from the extension unit must be run in permanent wiring to terminals 5 and 6 of the main unit using a gauge of wire such that the loop-resistance of the run does not exceed 3 ohms, otherwise. The sound volume at the extension unit will be noticeably lower than that at the main unit. Cable, P.V.C., No. 1, 2 wire should be used to a maximum distance of 18 yards. Due to this requirement, it is not possible to permit the main unit to be free-standing and wall-mounting has to be adopted; all connections to the main unit should therefore be in permanent wiring.

To gain access to the screw terminals of the main or extension units, remove the four screws from the underside of the unit and remove the base.

4. Installation tests
The installation should be tested as follows:-

(a) Initiate a test call, preferably to a recorded announcement. If a suitable service is not available locally, the assistance of the test desk should be sought.

(b) While speech is being received on the handset, depress the ON/OFF push button on the main unit. Speech should then be heard from the loudspeaker and at the same time the lamp within the button should glow. The handset will be cut-off. Rotate the volume control to the desired listening level.

(c) Depress the ON/OFF push-button again and note that the incoming signals are restored to the handset and the pilot lamp darkens.

(d) If an extension loudspeaker unit is fitted check the performance of the loudspeaker, the volume control and the ON/OFF push-button.

5. Maintenance
To change the lamp on the main unit, pull off the plastic ON/OFF push-button cover; remove the lamp with an Extractor No. 5 and replace with a new Lamp No. 2-6v. A lamp is not fitted to the extension unit.

To change the cord on the main unit, remove the four small screws securing the base and remove the base. The correct replacement is Cord, Instrument, No. 6/44AA, Grey, 72 in.

No attempt should be made to correct faults on the amplifier or loudspeaker; change the unit complete.

References:- F1008, F1104

Last revised: April 22, 2023