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The Main telephone has piano type control keys. There are Table and Wall models, each available in black, grey or ivory. The 10volt d.c. supply for signalling and transmission is normally obtained from the a.c. mains supply by means of a Power-unit No. 53B. If no mains supply is available, eight Cells, Dry, R40 in a Box Battery, No. 10 may be fitted.
The Table model is available complete as Planphone A, pre-wired to Diagram N 628 - see Vocabulary of Engineering Stores. The field of use of the Planphone A is given in TI C3 C0025. The table model can also be made up using a Telephone No. 706, 710 or 746 and a Plan-Set N625 Table. A Telephone No. 740 need never be used on a Plan-Set as the Telephone No. 746 can accept the necessary auxiliary gravity-switches for the most complex arrangement.
The Wall model must be made up using a Telephone No. 711 or 741 and a Plan-Set N625 Wall. When the complete model has been fixed to the wall access to the Plan-Set terminals is gained by pressing the slide bar (which is between the wall and the Plan-Set near the top left corner) with the blade of a screwdriver and moving the Plan-Set body to the right until the studs disengage and allow it to swing open. It is closed by swinging it shut, sliding it to the left and pressing the left hand side to lock the slide bar.
The Extension telephones are 700-type fitted with one or more press buttons.
The fourth key is non-locking and releases any locked key to establish the 'Main to extension' condition.
A white lamp is associated with the EXTENSION TO EXCHANGE key and a red lamp with the SPEAK TO EXTENSION EXCHANGE HELD key. On the table model the lamps glow through lenses behind the keys; on the wall model the lamps glow through the keys which are translucent. The white lamp glows while the EXTENSION TO EXCHANGE key is depressed and the extension handset is off the rest; the red lamp glows while the SPEAK TO EXTENSION EXCHANGE HELD key is depressed and the handset at the main is off the rest, indicating that an exchange line call is being held.
Signalling from the Main to the extensions is by means of the two non-locking keys below the four switching keys. Signalling from the extensions is by means of a non-locking button on each extension telephone.
Extension Plan 105 is a Main telephone and two extensions. Exchange calls from the Main cannot be heard at the extensions. Exchange calls from an extension can be heard at the other two telephones.
Extension Plan 105A is a Main telephone and two extensions. Exchange calls from the Main cannot be heard at the extensions. Exchange calls from an extension can be heard at the other extension but not at the Main.
Extension Plan 107 is a Main telephone and one extension. Exchange calls from the Main cannot be heard at the extension but exchange calls from the extension can be heard at the Main.
Extension Plan 107A is a Main telephone and one extension. Exchange calls cannot be heard at the other telephone.
Intercommunication is possible between Main and extensions and calling is by key at the Main and press button at the extensions.
Intercommunication is possible between extensions on Plans 105 or 105A. Internal extensions can call each other directly by buzzer. The assistance of the Main is needed to establish calls between external extensions.
Incoming calls ring bells except that, when the exchange line is switched to an external extension a buzzer sounds at the Main. On Plans 107 and 107A only, it is possible to arrange for the bell at the Main to ring in parallel with the extension bell if the customer prefers this to the buzzer. The buzzer may similarly be rendered inoperative if the customer requires that an audible signal should not be given at the Main when the exchange line is switched to the extension.
On a Plan 105 or 105A with internal extensions an additional press-button and switch may be fitted in one extension to render the bell inoperative if the customer requires that only one extension shall be rung on night service.
The plan-set is suitable for use on CB and automatic systems only.
External extensions cannot be provided on shared-service exchange line because it would not be possible for the equipment at the main to distinguish between the 'call main' and 'call exchange' signals. For the same reason an external extension cannot be fitted when the main is connected on the non-multiple station of a House Exchange System No. 3.
If the plan-set is connected to a PBX, external extensions from the main cannot be provided. because it is not possible to ensure that ringing and battery will be connected to the same wire. This is essential if correct signalling is to be provided between main and extension. Further, it would not be possible for the main to distinguish between 'call main' and 'recall switchboard' signals.
In Telephone No. 706 an additional gravity-switch spring-set and capacitor (Adapter, Plan-Set No. 1) should be mounted on the gravity-switch bracket of the Main telephone, with the spring-set opposite the existing gravity-switch spring- set, and the capacitor behind the bracket and regulator. In Telephones No. 746 a Switch No. 19D-1, and a Capacitor No. 7712-2 with a Clip No. 90 should be fitted. On all the telephones the blank grommet (Part No. 1/DBU/199) should be fitted in place of the recovered line cord in the telephone.
Using Plan-Set N625 Wall
Assuming that the terminal block is fitted at floor level, the maximum height at which the apparatus can be fitted is limited by the length of the cord. The keys on the plan-set should be at a convenient height but the customer should be consulted before the apparatus is finally fixed. Minimum clearances of 1 ft to the right and 6 in to the left are required.
In Telephone No. 711 an additional gravity-switch spring-set (Part No. 1/DSP/1501) and a Capacitor, No. 7712-2 in a Clip No. 90 should be mounted on the gravity-switch bracket of the main telephone. In Telephone No. 741 a Switch No. 19D-1 and a Kit 166A are used in place of Part 1/DSP/1503..
When required, the Converter Ringing, No. 9A (Diagram N626) should be mounted with the relay uppermost, adjacent to the terminal block.
If a Trimphone is fitted a Buzzer No. 20B should be fitted adjacent to the terminal block.
COMBINATIONS OF EXTENSION PLANS
Radio interference may occasionally be experienced on installations with external extensions. This is usually due to the diode in the Converter Ringing, No. 9A. Later issues have the diode bypassed with a 0.1 pf capacitor. Where this is not included by the manufacturer it should be provided locally.
If ringing is applied directly to the pair connecting an early version of the plan-set to an external extension, the transistors in the plan-set will be damaged. When an external extension pair is routed via the exchange the following note should be made in the remarks space on the fault card for the extension:- 'Transistorised equipment. Do not apply ringing to this circuit."
The Planphone A was a Planset N625 complete with Telephone No. 706. The Planset and telephone were always supplied as two separate items, but BT decided to amalgamate them in the BT repair factories and issue them ready made up for use. The 706 was used partly because the factory that was assembling the Planphones had refurbished 706s coming off an adjacent line, so two lots of transport between South Wales and Crayford could be saved.
There was also an issue about a carrying handle for the Planphone, which was not regarded as a "portable item". The plastic carrying handle added to Telephone 706's on refurbishment (to give the same facility as the 746) was not strong enough to lift a Plan Set N625 as well, so was not used.
The reason the Planphone A was introduced was to save field time in assembly and to permit factory testing of the combined unit. The Planphone was issued, initially, on new installation work to save time on building the unit out of separate parts, but was later released as a maintenance replacement (key switch breakage was the normal problem).
The Telephone 746 was designed so as to be able to use it on the Plan Set N625, but the introduction of the Planphone A made the 746 / Plan set combination more of a rarity.
Last revised: December 10, 2011