BPO DIAL LIST

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Some of the text below is taken from:-

TELECOMMUNICATIONS INSTRUCTION, C MARKETING, INSTALLATION, 3 Internal, M0015, Issue 1, Sept 1971.
&
ENGINEERING INSTRUCTION, TELEPHONES, Automatic, B1003, Issue 1 & Issue 2, Oct 1955 & May 1965.
&
P.O. Engineering Dept. Technical Instructions XXV. Part 24 - Description and Adjustment of Dial Mechanisms, May 1928.

DIALS, AUTOMATIC

Scope of Instruction
This Instruction describes the dials in current use and gives general information relating to their operation, use and maintenance. Details of the dials are given in Table 1 . Dials stocked by the Supplies Dept. and the installations at which they are used are shown in Table 2 .

General
For a long time all the dials used by the P.O. were of the slipping-cam type which were superseded by a trigger type. However, a large number of the slipping cam dials are still in use and will remain so for a number of years. Dials of the trigger type are fully interchangeable with their counterparts of the slipping-cam type. The arrangement of the terminals is similar but there are slight differences in physical dimensions. The electrical characteristics are identical. Dgms. N 581 to N 584, N 588, N 610, N 612 and N 613 show the terminal numbering and spring-set connections.

TABLE 1

NOTE: Change-over unit, nominal operate to normal ratio of 3:2 within the limits of 56-64% operate.

TABLE 2

The Rate Book number of each dial is followed by characters which indicate the type of number ring and pulse wheel fitted. The first letter indicates the number ring, namely, "B" special dial for the Brighton area, "F" has figures only, "L" letters and figures, "C" has neither letters nor figures.

B	F	L

The second letter and any following number refer to the type of pulse wheel, shown below. Dials stocked by the Supplies Dept. and their use at various installations are shown in Table 2 .

• Type A = 66.6/33.3 - used on normal GPO Strowger exchanges.
• Type S = 33.3% break for Grimsby, Stockport, Southampton, Hurley and Ramsey (Siemens Bros.)
• Type W = 18.5% break plus final break of not less than 92.5% of the total impulse period, for Dudley and Darlington (STC Rotary).

A	S	W

Description of Dial, Automatic, No. 10LA (slipping-cam type)
The Dial, Automatic, No. 10LA, consists of a circular case in the centre of which is a brass bush in which the main spindle and gear-wheel turn. On the main spindle, at the rear of the dial, are mounted a main spring in a spring box, a pulse wheel, a slipping cam between washers, a spring washer and an off- normal lever. The outer end of the main spring is anchored in the spring box, which is keyed to the spindle, and the inner end of the spring engages in a slot in the brass bush. When the spindle is rotated clockwise, as viewed from the dial front, the spring is wound up. The dial mechanism is held in its rest position against the tension of the main spring by a screw in the main gear-wheel engaging with a post in the case. Removal of the screw allows the main spring to unwind. Within the case are mounted a centrifugal governor and worm on a common spindle, and a gear-wheel which drives the worm. A pinion on the gear-wheel shaft meshes with the main gear-wheel on the main spindle so that the governor spins rapidly when the mechanism runs. The governor weights move outwards under centrifugal force and, at the required speed, bear on the inner surface of the governor cup to exert a braking influence on the spindle. This keeps the speed of the mechanism within the required limits.

Original Dial No. 10 front view	Later Dial No. 10 front view	Dial No. 10 rear view

The governor gear-wheel assembly includes a simple 'free-wheel' device consisting of a coiled spring coupling which slips when the dial is wound up, so that the governor does not resist this motion.

The front of the case is enclosed by the number ring, which is held in the case by a spring ring.

The finger-plate assembly is attached to the main spindle boss which just protrudes through the number ring. This assembly comprises the finger plate, label-holder, and a backing plate to give rigidity to the finger plate.

The pulsing mechanism consists of the pulse wheel and slipping cam on the main spindle and the pulse lever and spring-set mounted on the rear of the case.

The spring-set assembly also contains two auxiliary contacts which are normally held open by an insulating bush on the off-normal lever. When the finger plate is moved from its normal position, these contacts make in sequence and re-open in the reverse order on its return

.

Operation of the slipping-cam mechanism
The pulse wheel is keyed to the main spindle and the slipping cam is frictionally coupled to the wheel by steel washers under the pressure of the phosphor-bronze spring washer. Movement of the cam is restricted by its two lugs engaging a forked stop mounted on the dial case.

When a digit is selected, the pulse wheel and slipping cam rotate together at first and the raised sector of the cam moves under the set of the pulse lever. The pulse contacts are thus kept closed. Then, the longer lug of the slipping cam strikes the forked stop to prevent any further rotation of the cam. The pulse wheel continues to rotate and the cut-away sectors are exposed from under the cam. The number of sectors that appear is determined by the digit selected.

When the finger plate is released, the mechanism rotates back to its normal position with the governor controlling the speed. During the first part of the return motion, the slipping cam and pulse wheel move together, so that the pulse lever slides over the cam. Thus, the pulse contacts remain closed for 220 milli-seconds (at 10 p.p.s.) to give the minimum inter-digital pause. This period ends when the slipping cam has rotated sufficiently for its raised sector to clear the set of the pulse lever, which then drops on to the lower sector to open the pulse contacts. Further movement of the slipping cam is prevented by its shorter lug striking the forked stop. As the pulse wheel continues to rotate, each 'tooth' raises the pulse lever and closes the contacts until the contacts are finally closed at the normal position of the dial.

Parts for Dials No's 10 & 11

Dial, Automatic, No. 11
This dial is the same as the Dial, Automatic No. 10, except that it has an additional set of springs that are operated by a cam fitted to the dial spindle.  These are used in Coinbox installations.  The cam has a number of drillings that will accept a rod.  The rod is moved to correspond with the digit that operates the springset.

Description of Dial, Automatic, No. 12LA (trigger type)
The general construction of this trigger type dial is similar to that of the slipping-cam type.

The trigger dial differs from the slipping-cam dial on the following major points:-

(a) An articulated trigger assembly is used for the pulsing mechanism. This assembly has two main parts: a swinging lever which pivots about a screw in the main case, and a trigger which pivots on. a pin at the free end of the swinging lever. A flat spring, which is part of the spring-set assembly, bears on the trigger to ensure its set follows the contour of the pulse wheel.

(b) The pulse contacts comprise a lever spring tensioned against a stiff buffer spring and they are moved apart by an insulating bush on the trigger striking an extension of the lever spring.

(c) The dial mechanism is held in its rest position by a spring stop plate which is fastened by the screws that hold the bearing bracket for the governor gear wheel. A lug on the stop plate engages with a pin on the underside of the main gear-wheel when the plate is sprung upwards by a screw inserted through the back of the case. Slackening the screw allows the stop plate to spring away from the gear-wheel and let the main spring unwind.

(d) The number ring has a smaller hole in the centre so that it is more difficult for dust to get into the dial mechanism.

Dial No. 12 front view	Dial No. 12 rear view

Operation of the trigger mechanism
When a digit is selected, the end of the trigger engages with the first tooth of the pulse wheel and is moved to the position shown in Fig. 11. Further rotation of the pulse wheel causes the end of the trigger to ride over the 'teeth' and the number of teeth that pass equals the number selected.

When the finger plate is released, the mechanism rotates in the opposite direction and the trigger is moved back to the pulsing position. This movement occurs m approximately 240 milliseconds, giving the minimum inter-digital pause. Having reached the pulsing position, the trigger rides over the teeth of the pulse wheel and the insulating bush breaks the pulsing contacts at each tooth.

The number of pulses sent equals the number of teeth over which the trigger passed when the dial was wound up.

Purpose of the slipping-cam and trigger mechanisms Both mechanisms provide the minimum inter-digital pause before pulses are sent out so that:-

(a) If a subscriber commences to dial without waiting for the receipt of dialling tone the delay in sending the first pulse provides additional time to allow a free outlet to be seized.

(b) the dial mechanism is able to attain its governed speed before it sends out pulses.

Neither of these advantages could be achieved with P.O. dials if the pause occurred after the pulse train, instead of before it.

Advantages of the trigger mechanism

(a) Its operation does not rely on frictional contact between surfaces, as in the slipping-cam design. Frictional contact can vary considerably during the years of service without attention that are expected of a dial, and departure from the correct value can cause false pulsing or stalling.

(b) The position to which the finger plate must be rotated to select the digit required is less critical than with the slipping-cam design, in which a small variation can cause a shortened first pulse or a false one. With the trigger design the correct number of pulses will be obtained if the finger plate is moved short of, or past, the finger stop by half the diameter of the finger hole.

(c) Wear of the pulse contacts and wear of the trigger set or pulse wheel affect the pulse ratio in opposite ways, so that any changes tend to neutralise one another.

With the slipping cam wear tends to increase the break to make ratio of the pulses.

Description of Dial, Automatic, No. 13FA (trigger type)
Dials for some types of coin collecting box circuits require auxiliary contacts which break when certain digits are selected and remain open until the pulsing stops.

The general construction of the No. 13... dial is similar to the standard trigger dial but an auxiliary spring-set and associated parts are mounted on the back of the dial.

The contacts of the auxiliary spring-set are normally held in the 'made' position by a cam mounted on an extension of the main spindle. A small control cam, loosely coupled to the main spindle by a spring clip, is mounted between the contact cam and the off-normal lever, and its movement is limited in both directions by a stop plate which is fixed to the supporting bracket of the main spring-set assembly. A masking disk is fitted over the pulse wheel to reduce the effective height of every tooth except the first one, and the insulating bush on the trigger is longer than that on the normal trigger dial.

Operation of the coin-box auxiliary contact
The auxiliary cam is free to turn on the main spindle and has a light helical spring attached to it from a bracket on the auxiliary spring-set assembly. Two control pins are riveted into the cam and project on the underside so that the off-normal bush can strike them.

When any digit up to and including '8' is selected, the spindle moves independently of the cam, which is held in its normal position by the pressure of the auxiliary spring-set.

If '9' or '0' is selected, the insulating bush on the off-normal lever strikes the operating pin (that furthest from the anchor screw for the helical spring) and rotates the cam to allow the auxiliary contacts to break. During the return motion the contacts remain open because the helical spring is not strong enough to turn the cam and lift the lever spring up the cam step. After the last pulse has been sent, the off-normal bush strikes the resetting pin (nearest to the anchor screw) and resets the cam and contacts to their normal position.

When free dialling of digit '8' is required, another control pin can be screwed into the tapped hole in the cam.

Prevention of fraudulent calls
The control cam, the masking disk, the longer trigger bush and the light helical spring in No. 13FA dials function to prevent certain forms of fraudulent calls being made.

Dials, Automatic, Nos. 15... and 16.. .
These dials are for testers and are of the trigger type. Details of their speeds and ratios are included in Table 1 . The standard governor mechanism is used, specially adjusted to give the required speeds, and the special pulse ratios are obtained by the use of different pulse wheels.

The No. 16... dials are fitted with an extra spring-set and off-normal lever. The spring-set is similar to the auxiliary one on the No. 13FA dial and is operated directly by the additional off-normal lever which is mounted on an extension of the main spindle. The angular position of this lever can be varied so that the spring-set is operated and the contacts made at different positions of the finger plate.

It is thus possible to utilise the contacts as make contacts for any particular instant in the travel of the dial, or as break contacts for the duration of the operation of the dial.

Dial, Automatic, No. 17LA
This dial is used for the Telephone No. 280. It has the same pulsing mechanism as the Dial, Automatic, No. 12 described above but its mechanism is mounted on a smaller case than that of the No. 12 dial. The number ring fits closely over the case and is printed with figures only. The finger plate is also smaller than that used on the No. 12 dial and is printed with letters in positions corresponding to the figures on the number ling.

Dial, Automatic, No. 19FA
This is a trigger type dial having a similar form of construction to the No. 12 dial. It is fitted with one pair of off-normal contacts and. the pulsing action is effected with a change-over unit. This dial is used for signalling on automatic telegraph switching systems and the double pulse of the change-over contacts is adjusted to a nominal 3:2 (operate to normal) ratio.

Dial, Automatic, No. 20LA
This is a trigger type dial having a similar form of construction to the Dial, Automatic, No. 12. It permits free dialling of digits '1', '9' and '0' and is used on prepayment coin-collecting boxes in director areas. The operation of the auxiliary spring-set for the dialling of '9' and '0' is the same as that of the No. 13 dial. To give the free dialling of digit '1' the masking disk is also used as a cam which, in conjunction with an operating lever, controls the auxiliary spring-set assembly. It is arranged that the auxiliary contacts are opened, or remain open for the duration of the last pulse of each pulse train.

In the normal position of the finger plate, the auxiliary contacts are opened by the operating lever which is resting on its cam. As the finger plate is rotated clockwise, the cam releases the lever, and the contacts close. The operation of the auxiliary spring- set is as previously described for the No. 13FA dial until the make period of the pulsing springs immediately before the last pulse is transmitted. During this make period the lever will again come under the control of the cam. If the contacts have already been opened by the switching cam, when dialling '9' or '0', the lever will only increase the contact gap. If a digit other than '9' or '0' is. dialled, the auxiliary contacts will be closed, until they are opened by the lever during the make period immediately before the last pulse, and will remain open until the dial is re-operated.

Description of moulded body dial
The mechanism is essentially the same as that of the trigger dials previously described, except that it is mounted on a steel baseplate attached to a black moulding which forms the body. The material of the pulsing contacts has been changed to palladium to avoid high resistance contacts which have occurred on earlier dials. The number ring is located on two pillars between which its retaining spring is clipped. The finger plate is formed from rigid P.V.C. sheet in the colour range to match 700-type telephones. The label protector is moulded in clear polymethyl methacrylate. It snaps into position in the finger plate, and is held by three pips which are equally spaced around its periphery. A stainless-steel finger plate has been retained for some dials, e.g. those fitted at public call offices. The dial mechanism is held in the rest position by a screw passing through a lug on the steel baseplate and engaging with a pin screwed into the main gear-wheel.

Removal of the label protector The label protector should be removed by one of the following methods.

(a) Extractor No. 29. This is a rubber suction disk with a finger grip. The extractor should be firmly pressed on to the label protector and then sharply pulled away. It can be released from the protector by sliding or by lifting one edge of the rubber with the finger nail.

(b) An alternative method is to use a Screwdriver, Instrument, No. 1. This should be inserted radially between the finger plate and the number ring at the 3 o'clock position on the dial. The screwdriver will then enter a slot in the finger plate, and by turning the screwdriver beneath the pip in a thin metal plate which is under the label protector the protector will be ejected. Care must be taken to ensure that the end of the screwdriver is under the metal plate and not between the plate and the label protector. It is advisable to hold two fingers over the protector to restrain it during ejection.

Dial, Automatic, No. 21CA, (Colour)
This moulded body dial is for use with 700-type telephones. The number ring fitted is in matching colour and has printed arrow heads which appear beneath the finger holes. Some earlier Dials, Automatic, No. 21CA, (Colour) will have number rings without arrow heads.  Introduced in July 1959.  The dial shown on the right is a Dial 21CA Red.

Dial, Automatic, No. 21CA Steel
This dial is for use on Telephone No. 705 . It is similar to the Dial, Automatic, No. 21CA, (Colour) but is fitted with a stainless- steel finger plate. The number ring is as fitted on Dial, Automatic, No. 21CA, Green.

Dial, Automatic, No. 21LA
Black This dial is similar to Dial, Automatic, No. 21CA, Black with the exception that it is fitted with a number ring printed with figures and letters. It supersedes Dial, Automatic, No. 12LA.

Dial, Automatic, No. 21FA
The coloured finger plate was displaced by a clear plastic finger plate in the late 1960's.  All telephones No. 746 were fitted with this dial.  Introduced in June 1967 and an improved version in September 1971.  The introduction coincided with the nationwide introduction of all figure numbering.  This dial was also fitted to all factory refurbished Telephones that had a Dial No. 21CA.

Front view	Rear view

Dial, Automatic, No. 22FA
This dial has a stainless steel finger plate and a mechanism similar to that of Dial, Automatic, No. 13, mounted on a moulded body. It is fitted with a number ring bearing figures only. It supersedes Dial, Automatic, No. 13FA.

Dial, Automatic, No. 23LA
This has a mechanism similar to that of Dial, Automatic, No. 20, mounted on a moulded body. It is fitted with a stainless-steel finger plate and a number ring bearing figures and letters. It supersedes Dial, Automatic, No. 20LA.

Dial, Automatic, No. 24F
This dial has a similar mechanism to Dial, Automatic, No. 19, mounted on a moulded body. It is fitted with a number ring bearing figures only and a black P.V.C. finger plate. It supersedes Dials, Automatic, No. 19FA.

Dial, Automatic, Nos. 25LA, 25LT3 and 25LT4
These have mechanisms similar to Dials, Automatic, Nos. 16LA, 16LT3 and 16LT4 respectively, mounted on a moulded body. The dials are fitted with a number ring bearing figures and letters, and a black P.V.C. finger plate. These dials are used in testers and supersede Dials, Automatic, No. 16...

Dial, Automatic, No. 25CA
This dial is similar to Dial, Automatic, No. 25LA except that it has a grey P.V.C. finger plate and a number ring bearing arrowheads. It is for use on the operator's telephone of Switchboard, P.M.B.X., No. 2/... where inter-switchboard circuits are provided for dialling into P.A.B.X.s.

Dial, Automatic, No. 26LA
This is a modified Dial, Automatic, No. 10 fitted in a flameproof enclosure and is used with the Telephone No. 149. The flameproof construction requires it to have an additional spindle and to prevent this impeding the return of the pulsing mechanism the steel finger plate is coupled to the pulsing mechanism by means of a clutch. At the moment of release of the finger plate it is declutched from the pulsing mechanism and returns to normal very quickly under the action of an additional coiled spring. The return of the pulsing mechanism is therefore unimpeded. When the finger plate returns to normal it is locked until the completion of pulsing. Then when the mechanism returns to normal the finger plate is unlocked and the next digit can be dialled. The finger plate should be locked by repositioning the screw in the flyback mechanism when the Telephone No. 149 is used on C.B. systems.

Dial, Automatic, No. 28LA
This dial has a mechanism similar to that of a Dial, Automatic, No. 22 but modified By the addition of a cam so that the contacts of the auxiliary spring-set are closed during the pulsing of any contact other than '9'. It is used on installations where outgoing access is required to be limited to the '999' emergency service only.

Dial, Automatic, No. 30LA
This dial is similar to Dial, Automatic No 21 but has a fluorescent tube beneath the finger plate for illumination.  This dial is only used in Telephone No. 722 (Trimphone).

Dial, Automatic, No. 51A
Made by GEC this is an all plastic reduced cost dial.  Replacement for Dial No. 21.

Dial, Automatic, No. 54A
Later dial made completely of plastic.  Superseded Dial No. 21 for general use.

Front view	Rear view

Click here for more dial information

Dials for blind subscribers and subscribers with defective sight
The modification of dials for blind subscribers is described in Control C 1001. For subscribers having in operating the dial due to defective sight, the procedure outlined in M 0025 should be followed.

Quick list

DIAL No. 1              Automatic Electric, used at Epsom and the Official Switch (London).
DIAL No. 2              Variant on No. 1 used only on the test desk at Epsom.
DIAL No. 3              Siemens Bros, used at Grimsby, Stockport and Southampton.
DIAL No. 4              Never issued.
DIAL No. 5              Western Electric, used at Dudley and Darlington.
DIAL No. 6              Redesigned version of No. 5.
DIAL No. 7              Flameproof version of No. 3.
DIAL No. 8              Prototype of standard design. Four terminals, small dial label.
DIAL No 10		Standard dial, slipping cam type.
DIAL No 11		As No 10, Used on prepayment coin boxes, has extra contact set.
DIAL No 12		Standard dial, trigger type.
DIAL No 13		As No 12, Used on prepayment coin boxes, has extra contact set.
DIAL No 14		Used on testers.
DIAL No 15		Used on testers, has additional break contact.
DIAL No 17FA		Small figure dial for use on telephone No 280.
DIAL No 17LA		Small lettered dial for use on telephone No 280.
DIAL No 19		Telex dial, has changeover pulsing contacts.
DIAL No 20LA		Used in director areas to allow prepayment coin boxes to dial 100.
DIAL No 21CA		Standard dial with coloured finger plate and arrowed ring.
DIAL No 21FA clear	Standard dial with transparent figure plate and antique 
			silver ring with black numerals.
DIAL No 1/21FA clear	As 21FA but low inertia mechanism, fitted to telephone No 776.
DIAL No 21FA steel	As 21FA but fitted with steel fingerplate, used on 
			public call offices.
DIAL No 21LA black	Standard dial with black finger plate, used on Telephone 1/232.
DIAL No 22FA		Used on non director pre-payment coin box circuits, replaces 
			dial No 13.
DIAL No 23FA		Used on director pre-payment coin box circuits, replaces dial No 20.
DIAL No 24FA		Telex dial has changeover pulsing contacts, replaces dial No 19.
DIAL No 25FA clear	As 21FA but with additional break contact, used 
			on 2/.. switchboards with PW's attached.
DIAL No 25FT3		Used on testers, has 80% break and 12pps.
DIAL No 25FT4		Used on testers, has 50% break and 12pps.
DIAL No 26LA		Used on Flameproof telephones.
DIAL No 28FA clear	Extra contact set only allows digit 9 to be dialled.
DIAL No 30FA		Illuminated dial, used on telephone No 1/722.  The 
			fluorescent tube contains tritium, a radioactive gas. 
DIAL No 35FA clear	Used on testers, has 66% break and 10pps, has 
			additional break contact.
DIAL No 51		Standard dial of GEC design (alternative to dial No 21).
DIAL No 52		(alternative to dial No 21).
DIAL No 53		STC design (alternative to dial No 30 used on telephone No 722).
DIAL No 54a		Standard dial of EMI (Electric and Musical Industries formerly The 
                        Gramophone Company) design (alternative to dial No 21).

FA = Figures
LA = Numbers and letters

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