Section 4
REPAIRING AND RECONDITIONING RINGING MACHINES
by Ian Jutting

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Ringing Machines, Tone Machines or more correctly, Ringing Dynamotors were made obsolete with the introduction of digital switching. They generated the intermittent ringing current and the various tones required by large mechanical telephone exchanges such as Strowger and Crossbar types. They varied in size from 15 watts ringing output (75 volts, 0.2 amps RMS) up to 300 watts ringing output (75 volts, 4 amps RMS). It was usual practice to provide a mains driven machine with a battery driven reserve machine except on the smallest installations where both were usually battery driven. There was usually automatic changeover in the event of mains or machine failure and they were housed on the Alarm Equipment Rack.

Background

Simple Maintenance

Number Unobtainable Tone (NUT) 400 Hertz (Hz) continuous;
Engaged Tone (ET) 400 Hz, interrupted 0.75 sec on, 0.75 sec off;
Equipment Engaged Tone (EET) 400 Hz, interrupted 0.375 sec on, 1.125 sec off;
Dial Tone (DT) 50 Hz continuous;
Ringing Tone (RT) combined 400 Hz + 450 Hz interrupted 0.4 sec on, 0.2 sec off, 0.4 sec on, 2 sec off.

Many of the tones have now been replaced by voice messages on digital electronic exchanges and the new high pitched dial tone has replaced the old low dial tone. There are also `Interrupted outputs', the common ones being:

Interrupted Earth: 0.75 sec on, 0.75 sec off and Flicker Earth: 0.2 sec on, 0.2 sec off.

The latter outputs were used for common control applications within the exchange, such as various alarm conditions and for the `A' & `Z' pulse generator which was used for the forcible release of equipment such as held first selectors. Clock pulses were generally produced by long cased electric pendulum clocks such as the Type 36 Master Clock by Gent Ltd of Leicester UK, for other alarm functions as well as providing 30 second pulses (a short electric pulse every 30 seconds) for the slave clocks housed around the telephone exchange. Metering pulses for pricing calls were supplied by a separate `Meter Pulsing Machine'.

The Alarm Equipment Rack (AER) which housed the Ringing Machines of mechanical automatic telephone exchanges was equipped with a switch to manually start either ringer, a rotary switch for selecting the tones for testing and a watch receiver type ear piece on a short cord. The engineer was expected to listen to the tones daily.

The mainshaft (running at 1400 RPM on the Walter Jones type 17700/2 - PO No 44A) houses the windings for the motor as well as the ringing current generator and tone windings. At one end of the mainshaft a worm gear engages with a spur wheel which turns a camshaft at one revolution every three seconds. The camshaft, which is located perpendicular to the mainshaft, carries cams with suitable steps which operate interrupter spring sets to produce the above tone and ringing current interruptions. On main/public exchanges, the machines ran constantly, only dropping out of service for maintenance or under fault conditions. On small public and Private Automatic Branch Exchanges (PABX) the ringing machine would only start up when needed, for instance when any call was being routed through the equipment. It would drop out of service when the call had been successfully connected and no new calls were being routed.

In service they were hard working machines but with regular maintenance few problems occurred. Now they are only likely to be encountered in telecommunications historic collections on demonstration mechanical exchanges. They are relatively easy to service and maintain. Only the correct spare parts should be employed. It is getting much harder to obtain genuine spare parts. My experience is based on refurbishing two Walter Jones & Co. Type 17700/2  (P.O. No 44A) 15 watt battery driven tone induction ringing dynamotors.

The electrical output may then be tested. It became standard practice on the British public telephone system to provide a 50 Volt DC positive earth power supply for mechanical automatic exchanges. All earth points on the ringing machine terminal plate should be bonded with 1.5 mm² solid copper cable to a good earth point. A 1000 ohm or 4000 Ohm magneto bell in series with a 1.8 mFd, 250 VAC capacitor may be connected between fused battery (ie negative battery rail) and continuous ring, then each of the three phases of intermittent ring supply in turn. There is usually a fuselink placed in circuit between the continuous ring output and the common intermittent ring bus which supplies the interrupted ring camsprings. Failure of continuous output may be due to wrong connections, brush wear on the slip rings or an open circuit armature winding. Failure of all intermittent ringing phases suggests failure of the fuselink. Failure of only one phase suggests camshaft springset problems. On one machine it was found that the springset contact had fallen off and had to be redressed onto the appropriate spring.

Tones are tested by connecting the primary of a line isolating transformer (Maplin part no: BK57M) between earth and the appropriate terminal. An earpiece receiver 4T is connected across the transformer secondary. The following should be heard:

Dial Tone (DT), Number Unobtainable Tone (NUT) Engaged Tone (ET) and Equipment Engaged Tone (EET). Finally, Flicker Earth and Interrupted Earth can be tested by connecting a 50 volt No 8 lamp between the appropriate terminal and fused battery feed.

If a revolution counter is available then the mainshaft speed can be checked against the specifications (1400 RPM on the P.O. 44A).

A spare set of brushes should be kept available for comparison. Badly worn brushes bounce in their holders causing sparking and eventual damage to the holders, springs and commutators/sliprings. Pitting and ridging can ultimately occur on the surface of the latter two which can only be corrected by having the commutator/sliprings professionally turned on a lathe or even replaced, a very expensive process. Only brushes of the correct type should be used. It is unwise to attempt to make brushes from spare blocks of carbon. Copper bearing carbon brushes should never be employed. If old brushes are reused they must always be returned to their original position to prevent transfer of ridge patterns to other parts of the commutator/sliprings. If the latter are cleaned, turned or replaced, new brushes are mandatory.

B) Springsets

Springset contacts should be cleaned regularly using a small quantity of switch cleaner. Heavily oxidised contacts can be cleaned with a piece of chamois leather. Abrasives and the use of fine contact files should not be used unless there are serious contact pitting problems. It may even be best to replace such contacts and establish and rectify the cause of their deterioration. The buffer blocks should be kept clean and free of debris to prevent low resistance faults. The springset tensions can be checked using appropriate tension gauges.

C) Lubrication

Lubrication required is minimal. Excess lubrication tends to find it's way into the various contacts causing high resistance faults . It also attracts dust which can lead to fouling of moving parts. Sealed bearings as located on the camshaft require no further lubrication. Half shielded bearings as located on the main shaft need repacking with high melting point bearing grease (e.g. Castrol MS3) every 1-2 years depending on machine use. Clean grease should be forced into the bearing on the open side at the centre to displace the old  grease at the edge. The bearing should be finally left two thirds full of new grease. The spur wheel, being made of nylon normally requires no lubrication.

D) Cleansing

The casing should be kept clean and grease free both inside and outside. The brush chamber is particularly prone to carbon dust. This may be cleaned using clean cotton cloth moistened with white spirit. The commutator/sliprings can be cleaned by first removing the brushes and using clean cotton cloth moistened with * carbon tetrachloride. Both agents must be used with care in a well ventilated room.

* Carbon Tetrachloride. Handle as a carcinogen. Consider an alternative chemical if possible. (Note that, because of health risks, large-scale production of this chemical is being ended; in many countries it is now be impossible to buy it without a permit.)

he brushes should be removed and marked appropriately if they are to be reused. The cam-shaft cover is removed by withdrawing the securing bolts. The springset mounting plate securing bolts are loosened in rotation and finally withdrawn. The plate is swung down, supported by its's wiring. The worm gear grub screws are loosened using an appropriate AF hexagonal wrench. The gear should NOT be removed at this stage. The Right-hand camshaft bearing retaining spring is carefully removed using an old screwdriver and light hammer. Some models have a nylon pressure pad held against the right-hand camshaft which acts as an anti-vibration pad. This can now be withdrawn. If new camshaft bearings are fitted, this pad will no longer be required. The camshaft can be brought forward on the right-hand side bringing the worm gear off the mainshaft. The camshaft can then be pulled to the right bringing the left-hand camshaft bearing out of it's housing so freeing the whole camshaft which can be set aside. One end of the camshaft should be marked to ensure that it is replaced the correct way round. Worn cams or the spur wheel will require replacement. The spur wheel has a concave track which is normal.

The camshaft bearings should rotate freely without grating and virtually no side play which is checked by trying to rock the outer races. If doubtful, the bearings should be replaced as a pair. They can be removed squarely using a small bearing puller. The shaft should be cleaned well and the new bearings applied squarely ideally by press fitting but can be gently tapped into position. The shaft is held vertically, resting on a block of waste wood and the new bearing placed squarely on the uppermost shaft shoulder. A tube of the same diameter as the inner race is selected, without fouling the shaft, such as an appropriate sized socket spanner and placed on top of the bearing. A rubber mallet is used to tap the bearing gently into place. A bearing which does not move easily into place usually signifies that it is out of alignment with respect to the shaft and it will have to be withdrawn and the process repeated. At no time must the outer race be struck or forced as this will cause damage to the bearing with early life failure. The cams and spur wheel should be securely attached to the shaft, the grub screws being gently tightened as necessary. If it is intended to remove the mainshaft then the camshaft should be put aside, otherwise refitting is a reverse of the above procedure. The casing should not be further disassembled as it will prove very difficult to realign the casing and additional shims may need to be fitted.

With the camshaft removed it is now possible to remove the mainshaft. The rear mainshaft bearing cover plate can be removed by withdrawing the three small bolts. The four large bolts are then withdrawn from the rear cover. It should now be possible to gently extract the mainshaft from the rear, supporting it from both front and rear to prevent damage to the stator or armature. If it will not initially move it may be necessary to gently tap the front of the shaft with a rubber headed mallet and wooden block.

The mainshaft bearings are tested exactly as the camshaft bearings and should be replaced with the slightest signs of wear. The front end bearing is easily removed using a three inch bearing puller. The rear end bearing may or may not have an external flange, depending on the serial number of the machine. It must always be replaced with the same type. The flange prevents end-play in the mainshaft. In machines where no flange is fitted, it will be found that there is a large washer retained by three small bolts which supports the front end bearing from within the camshaft box behind the worm and spur gears. If the rear end bearing has a flange fitted, it will retain the rear cover plate and the bearing puller cannot readily be applied. I found that using two large bladed screwdrivers inserted on opposite sides of the mainshaft under the rear cover, with the tips resting on the inner bearing race, the bearing can be pushed sufficiently far off the shaft to allow the bearing puller to be used. There is probably a special application tool available. A bearing removed this way should never be reused as damage to the bearing inevitably occurs. New bearings should be packed with high melting point grease on the open side and pressed to expel air, finally leaving the open side two thirds full. The bearings are fitted with the open sides facing outwards using the same techniques as the camshaft bearings. If a flanged rear bearing is fitted, the rear cover plate must be located onto the mainshaft first.

Before reassembly, the inside of the stator, brush compartment camshaft case and armature should be thoroughly cleaned. The exposed steel shafts should be lightly greased with high melting point grease to prevent rusting.

Commutator and Sliprings

The commutator and slip rings should be clean and bright. A few small ridges are permitted. The gaps between the commutator segments should be cleaned using wooden cocktail sticks. Simple cleaning can be conveniently carried out by using waste clean cotton cloth moistened with carbon tetrachloride in a well ventilated room. If more thorough cleaning is required, this can be achieved by using fine emery strip obtainable from motor repair agents. This is wrapped around the commutator then the slip rings and the mainshaft rotated in one direction only to ensure even abrasion of the contact surface. A small screwdriver blade is lightly stroked across the edge of each segment of the commutator to round it. A final light turn with the emery strip removes the swarf. Debris is removed from the grooves between the segments with a cocktail stick. Rounding the edges of the commutator segments in this way greatly improves the wear of the commutator brushes and also reduces commutator noise.

Reassembly

Reassembly is essentially a reverse of the above procedures, ensuring cleanliness throughout. Cams should be lightly greased. Testing should be carried out on the test bench prior to replacing the machine on the A.E.R..

-good quality screwdriver set
-Hexagonal wrench set, AF sizes
-Rubber faced mallet
-6 BA Box spanner
-Appropriate tension gauge
-foot long dowel, one inch in diameter smoothed at each end

For bearing replacement:

-two inch two arm bearing puller
-three inch three arm bearing puller
-2 x 1/2 x 1/2 inch pieces of wood
-steel tube or socket spanners to press bearings home

For commutator/slipring attention:

-fine grade emery strip
-cocktail sticks
-small bladed screwdriver

NOTE: Walter Jones and Company of Sydenham no longer exists. Their repair work was taken over by Airscrew Howden Limited, Fan Blower Mfrs & Suppliers, Unit 2, 111 Windmill Road, Sunbury-on-Thames, Middlesex UK, TW16 7EF Tel: + 44 (0) 1932 765822. They may still be able to help with parts and advice.

1) I.P.O.E.E.J. 1955 Vol 48 p159 `Developments in Exchange Ringing Equipment'

2) Atkinson, Telephony Vol 1, 1947 Ch XX p443 `Ringing Machines'

3) P.O. DGM SA 8209, `Ringer Rack Circuits P.A.B.X. No 3 using Dynamotor No 44A'

4) Walter Jones and Company, `Installation and Maintenance Instructions for Ringing Dynamotors type 13000' (Similar to P.O. No 44A but used in main exchanges, battery operated)

Acknowledgements

I am grateful to Mr Childs of MacAlarm for supplying the P.O. 44A ringing machines and the necessary circuit diagrams, also to Airscrew Howden Ltd for advice on fitting of bearings. Thanks are also due to Keith Brown of Manchester who provided a copy of the Installation and Maintenance Instructions (Ref 4).

This is a revised version of the article that first appeared in The Telecommunications Heritage Bulletin Issue15 (Summer 1990).

The comprehensive T.I's have many sections which refer to the maintenance and testing of ringing machines. Here are just a few (which may be available somewhere within the THG):-

Motor Generator 15-16
Dynamotor 23, 24, 41A, 41B, 44A, 45A, 50A, 51A

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