Auto-CB and LB Table Telephones for
'Plan Number' Working
The new Post Office table telephones described in Bulletin No. 11 of July 1937 have been further developed to cover a variety
of methods for working extensions, known as 'plan number' working.
In this article therefore, the modified instruments, the apparatus required and a few plan number schemes are illustrated and described.
A telephone fitted with the maximum of three push buttons is shown in the
picture to the right. The buttons are situated between the cradle wings immediately in front of the micro-telephone. They are made of metal with a bright chromium finish, and are free to move up and down in
counter bored holes drilled in the body, in which they are retained by a washer and screw on the inner end of each button.
A metal designation label is fitted below the buttons to indicate their functions, that shown being for three extensions. The label is of the chemically engraved type with bright chromium characters and black background, and is fixed by means of two screws which engage with metal bushes inserted in the telephone body. When less than three buttons are required the vacant holes are fitted with bright chromium finished metal dummies. Bodies drilled for one and three buttons are standardized. This type of telephone
is a standard auto-CB as described in the previous article but fitted with a key unit, described hereafter, and has a magneto bell.
For some of the plan number workings the extensions are fitted with a D.C. trembler bell, and a view of the underside of this type of chassis is shown in
the picture to the right. The bell movement is insulated from the chassis frame, and four terminals for connecting the bell in series or parallel are
mounted on the base of a suppressor unit which is fitted for the purpose of reducing to a minimum, interference with radio.
All key units have spring sets of the make-before-break change over type, known as 'K' combination.
A key unit with one set of K springs is illustrated
on the left where is it seen fitted in place on a chassis, the operating push button occupying the central position on the telephone.
Telephones so equipped are primarily for use in connection with a private branch exchange (P.B.X.), so that, when a call is extended to a subscriber on the public
exchange, the P.B.X. operator's attention may, if required, be attracted by momentarily pressing the button on the telephone, thus flashing the supervisory signal on the P.B.X. switchboard. The operator then breaks-in and ascertains the requirements of the extension.
This instrument can also be arranged as a main station with a single extension on the lines of plan No.
For other extension schemes the telephones are fitted with key units having a number of K spring sets, the standards at present being 4 and 9 sets.
A key unit with 4K spring sets is illustrated in picture below, and is arranged so that the push button on the telephone operates respectively (left to right)
2K-1K1K spring sets. Each of the 12 contact springs is connected, consecutively from left to right figure 6, to the terminal block where the four sets of terminals are numbered I to 12 downwards.
The plungers for operating the spring sets are restored by means of a phosphor bronze leaf spring, bearing on the bottom of each plunger which also has a projection that engages with a pivoted latch bracket for locking the plunger in the operated position,
see the picture above right.
The latch bracket is cut away opposite the centre plunger so that this key is always non-locking. On the inside of the bracket there is a differential plate which can be fixed, by means of two screws, in any one of eight positions,
the plungers labelled A, B and C.
On referring to these diagrams it will readily be seen that the lock and non-lock of plungers A and C and the trip and
non-trip by plunger B are controlled by the position of the differential plate and therefore with the standardized multiple springset key units a variety of extension schemes may be set up as required. Furthermore, for some extension schemes it is necessary to release locked plungers by the replacement of the micro-telephone on the cradle. For this purpose there is supplied with each key unit a T shaped plate which, for convenience is normally attached to the side of the key unit as seen in
the picture above, on the left hand side. When the extension scheme necessitates micro-telephone release, as stated above, the T plate is removed from the key unit and fixed to the cradle lever as shown on a standard auto-CB chassis, so that by engaging with a projecting limb, 'X'
on the latch bracket, the plungers are released. These can be seen on the
The picture, above right, illustrates the 9 K spring set key unit mounted on a chassis. The contact
springs are connected to the terminals in the same way as for the 4 K unit, the end terminal numbered 28 being spare. The spring sets are arranged so that the three plungers operate
4K-1K-4K sets respectively. This key unit is designed to allow for the more elaborate plan number schemes which may arise from time to time.
All contact springs on the key units are made of phosphor bronze and twin contacts are fitted. The spring sets can be correctly and accurately adjusted and tensioned before the unit is mounted on the chassis. Finally to make these key units as complete as possible, they are provided with captive type fixing screws.
It should also be noted that the chassis of the standard auto, CB and LB instruments are arranged for the key units
and T plate described above to be screwed in position, so that, by having the body drilled for the push buttons, the
standard instruments can be adapted for plan number working if required.
A few extension schemes are described below. No figures are available,
even though they are mentioned.
PLAN No. 1
This is a scheme which provides for one main telephone and several extensions. Figure 11 is a diagram of the connections for Auto and C.B., and figure 12 a similar case but for magneto systems. The main instruments are fitted with from one to three push buttons and the differential plates on the key units are fixed
in the position shown in drawing 1 or 5 figure 8. When more than three extensions are required an additional key unit and push buttons external to the telephones may be used. The main sets have a magneto bell whereas the extensions are provided with a battery type trembler bell. The exchange line is connected to all instruments and outgoing calls can be made from any. Incoming calls operate the ringer in the telephone at the main station where the callers requirements are ascertained. If the call is for one of the extensions, the appropriate
button on the main instrument is depressed, thus applying battery from the exchange line to operate the trembler bell in the extension telephone where the call is then answered in the normal manner.
PLAN No. 1A
In this case, figure 13, there is one main and one or two extensions. All instruments have magneto type bells which are connected up in series. The extension instruments, only, are each fitted with two buttons one of which merely releases the other when required, and is not shown in the diagram. The differential plate on the key units is fixed in the position shown in drawing 3 or 7, figure 8, and a label marked 'bell on', 'bell off' is fixed in front of the buttons. The exchange line is connected to all instruments and outgoing calls can be made from any of the three telephones, but incoming calls normally ring the main telephone bell and can only be signalled
at the extensions by depressing the 'bell on' buttons, which remove a short circuit from the extension bells, and remain locked until released by the 'bell off ' button. It will thus be seen that incoming calls can be signalled at one, two, or all three stations as required.
PLAN No. 3
For one main and one extension telephone with secrecy at the extension when required. This arrangement is shown in figure 14. The main telephone is fitted with one push button in the centre position and the P.B.X. type of key unit as seen in figure 4.
The extension has two buttons one labelled 'secret'and the other 'normal', the latter being for releasing the former and is not shown in the diagram. The differential plate in this case is fixed as drawing
3 or 7, figure 8. The exchange line is connected to the main via the extension's key unit, so that normally incoming calls are signalled at and answered by the main. When a call requires the attention of the extension the push button at the main is depressed thereby ringing a battery bell fitted in any convenient position at the extension station. In replying the extension, if desired, can disconnect the main from the exchange line by depressing the ' secret ' button thereby connecting the extension instrument only to the exchange line, and while so connected incoming calls are signalled there on the instrument bell. When the extension again desires the main to answer incoming calls the normal
button is depressed.
It will be appreciated that should the extension require automatic release of the secret key after each secrecy call, so that the main shall answer all incoming calls this is done by fixing the T plate, mentioned above, to the cradle lever, in which case the 'normal' button is not required.
PLAN No. 8
Two exchange lines terminate separately on two main telephones and are extended to an extension which has access to both lines in accordance with the diagram in figure 15.
The main telephone chassis are equipped as shown in figure 4, while the extension chassis has a battery bell as in figure 2, and is fitted with one of the multi-spring key units, having the differential plate fixed as at 4 or 8 figure 8 and the T plate mounted on the cradle lever. There are two press buttons on the extension telephone and a label marked 'line
1', 'line 2'. The bell in this instrument is connected to line 1 and its main telephone, and another different toned bell is fitted near by for line 2 and its instrument.
When making outgoing calls it is necessary to listen to ascertain if the exchange
line is already engaged or not.
Incoming calls are answered at the main telephones and any for the extension are signalled thereto by pressing the button on the main instrument.
Normally the extension is connected to line 1 but is transferred to line 2 by pressing the button so marked.
The line 2 button when depressed remains locked until released by line 1 button, which is non-locking, or by replacing the microtelephone.
PLAN No. 11
This plan figure 16 has the exchange line connected to three instruments, a main and two extensions, so that out-going calls can be made from any of the
instruments. Incoming calls are answered at the main
and can only reach extension 2 after extension 1 has ascertained the nature of the call. In this way extension 2 is guarded against being troubled with calls except where absolutely essential.
The main telephone is fitted with a P.B.X. type of key for calling extension
1; extension 1 has a key unit, with the
differential plate fixed in any position 5 to 8 figure 8, for calling extension 2; and extension 2 has no key. The bell at the main is the magneto type while each of the extensions has a battery type.
From the foregoing and also the previous article, it will readily be seen that this new design of telephone can be universally used, and also adapted for numerous schemes of extension working.