BELL ADJUSTMENT
(Magneto)

MAGNETO BELLS
Maintenance Adjustment Instructions
GENERAL

1. General
The purpose of this Instruction is to indicate the manner in which magneto bells should be adjusted to obtain the most satisfactory results.  Intelligent application of these adjustments should appreciably reduce the number of fault reports on these bells.

Subscribers' apparatus fitters and maintenance staff should ensure that the subscriber approves the operation of the bell, adjusting it, if necessary, within the limits of this Instruction.

2. Standard bells
Standard magneto bells in service and their uses are as indicated in Table 1.

TABLE 1

The subsequent instructions apply whether the bell is mounted separately or as part of the bell-set or telephone.

3. Ringing current
The standard frequency of ringing current is 16.66 c/s., and the adjustment detailed in this Instruction refer to this frequency.  At some P.B.X. installations, 50 cycle ringing is used; this being obtained from the public supply via a 'Transformer No. 92B'. Diagram N 1653 refers.

(Note: Only those bells or bell-sets which have a 'Bell No. 59A' movement are suitable for use under such conditions).

4. Consideration to be given to subscribers' requests
New bells are supplied with the largest permissible armature travel, which results in maximum loudness under normal ringing conditions.  If a subscriber requests that a bell may be made quieter, the armature travel should be reduced as specified in pars. 8 and 10, for 'Bells, Nos. 14 and 59A' respectively.  Similar action nay be taken if, as a result of special circuit arrangements, or extreme line conditions etc., the ringing is erratic.

5. Removal of bell or fitting of extension bell
It is probable that complaints may be made regarding the performance of bells, when the real cause of dissatisfaction is in the situation of the bell.  With experience, officers should be able to decide whether this is the trouble or not, and when it is, should explain tactfully to the subscriber how his needs may be met.  If the subscriber agrees that removal to a more favourable position is desirable or that an extra bell is necessary, the Sales Division should be informed.  Under certain circumstances, extension bells may be fitted free of charge on direct exchange lines (A 3113 refers) and on subscribers' installations fitted with coin-collecting boxes (call offices, A 3955 refers).

ADJUSTMENTS

6. General
When a magneto bell requires adjustment, the procedure detailed in the following paragraphs should be followed.

Before making the adjustments, however, any obvious physical or mechanical defect, such as cracked gongs or coil loose, should be rectified, if necessary.

7. Locking washers to be fitted
Certain older issues of bells, bell-sets and bells in telephones may not be fitted with locking washers.  Before adjusting any of these magneto bells, from which the locking washers are missing, the necessary washers should be fitted, and subscribers' apparatus maintenance officers should carry a small stock of washers for this purpose.
The appropriate items are:-
'Washers, Locking, No. 1210' for use with 'Bells No. 14 and 59A'
'Washers, Locking, No. 1211' for use with No. 534'
'Washers, Locking, No. 1218' for use with No. 674'

8. Adjustment of Bells No. 1A

1. Remove the bias spring, if fitted.

2. Loosen the bell gongs with a 'Spanner, Flat, No. 4', and move them well clear of the striker.

3. Check that the striker shaft is straight and does not foul the frame or wiring.  If necessary, adjust with 'Pliers, Wiring, No. 2'.

4. The armature should swing freely with a minimum amount of play at the pivots.  If necessary, adjust the pivot screw, using a 'Spanner, Flat, No. 2' and a suitable screwdriver.

5. Check the armature travel as follows:- Hold the armature in an operated position so that one of the residual studs is touching the core face opposite to it.  Then, check the gap between the other residual stud and the core face, by inserting gauges, as follows:- a 20 mil gauge should be loose, but a 10 mil and a 15 mil gauge, together, should be tight.
   If this condition is not met, re-adjust as follows:-

   1. Loosen the four positioning nuts on the spindles with a 'Spanner, Flat, No. 11.'

   2. Insert a 10 mil and a 15 mil gauge, together, between one core face and the residual a 10 mil and a 15 mil gauge, together, should be tight

   3. Press the armature pivot bracket towards both core faces so that the gauges are held.  Verify that the armature pivot bracket is square with the spindles

   4. Tighten the four positioning nuts.

6. Adjust the gongs so that the striker is just clear of the gong when the armature is held over by hand, first in one direction and then, in the other

7. Re-fit the bias spring (if any) and check that ringing is satisfactory.

If the ringing is too loud, rotate the gongs so that their edges are brought close together; the striking points on the gongs should still be equidistant from the central position of the striker.

If the ringing is not reliable, and the bell is part of a special circuit or operating under extreme conditions, the armature travel specified in (e) may be reduced, but not to a value less than 15 mils.

9. Adjustment of Bells No. 53A

1. Loosen the gongs with a 'Spanner, Flat, No. 5' and move them well clear of the striker.

2. The armature should swing freely on its pivots with a minimum amount of end play. Adjust the pivot, if necessary, by means of a 'Spanner, Flat, No. 3'.

3. Adjust the armature in the following manner so that it has a minimum residual of 10 mils and maximum travel of 35 mils.  Loosen the locking nuts on the two residual stop screws, using a 'Spanner, Flat, No. 3' and adjust the screws to give a residual of 10 mils. If the armature travel is now found to exceed 35 mils, tighten each residual screw equally to give an armature travel of 35 mils. This measurement can be made using a 20 and 15 mil feeler gauge together.

4. Adjust both gongs so that, when the striker is deflected to either of its two extreme positions, it has a clearance of 10 mils from the nearest gong.

On some 'Bells No. 53A' it will be found that, when residuals of 10 mils have been obtained, the travel of the armature is considerably less than 35 mils. In these circumstances, the residuals may be decreased slightly.  Decrease of residual (i.e. increase of armature travel) will increase the loudness of the bell up to a certain point, after which the ringing will become erratic.  If, in any particular case, the increase of the armature travel does not produce the required loudness, the bell should be changed.

10. Bell No. 59A (Adjustment for 16.66 c/s operation)

1. Remove the bias spring, if fitted.

2. The striker should hit the gong as near as possible to the lower edge, for the best sound.  This adjustment may be obtained, if necessary, by giving a slight cant to the shaft; the shaft should be bent at the armature end, using 'Pliers, Wiring, No. 2'

3. Check that the striker shaft is straight, and that it does not foul the base plate and wiring.

4. Loosen the bell gongs with a suitable screwdriver and move them well clear of the striker.

5. Check the armature travel as follows:- Hold the armature in an operated position so that one of the residual studs is pressed against the core face opposite to it.  Then, check the gap between the other core face and the residual stud, using gauges as follows:- a 20 mil gauge should be loose, but if this condition is not met, re-adjust as follows:-

   1. Loosen the cheese-headed screw in the upper face of the yoke sufficiently to free the cylindrical magnet and armature assembly.

   2. Insert a 10 mil and a 15 mil gauge, together, between one core face and the residual Stud.

   3. Set the armature squarely with the base of the bell so that the residual studs will strike centrally on the core faces, and press the armature firmly towards the core faces.

   4. Tighten the magnet fixing screw.

6. Adjust the gongs so that, when the armature is held over by hand, first in one direction and then in the other, the striker is just clear of the gong.

7. Refit the bias spring (if any), and check that ringing is satisfactory.

If the ringing is too loud, rotate the gongs so as to bring their edges closer together; the striking ,points on the gongs should still be equidistant from the central position of the striker.

If the ringing is not reliable, and the bell is part of a special circuit or operating under extreme conditions, the armature travel specified in (e) may be reduced as necessary, but not to a value less than 15 mils.

11. Bells No. 59A (Adjustment for 50 c/s. operation)

1. Proceed as in par. 10 up to, and including, sub-par. (e), using a 10 mil gauge for checking, and re-adjusting, if necessary.

2. The armature travel, however, should be such that a l0 mil gauge can be inserted without binding or without any observable play.

3. Adjust the gongs so that there is just perceptible clearance between the gongs and the striker, when the armature is held over by hand, first in one direction, and then in the other.

4. Check that a satisfactory ring is received.

If ringing is not satisfactory, the best gong adjustment may be obtained, by ear, as follows.  Rock the armature to and fro, by hand, by moving the armature itself, and not by the striker arm. Rotate the gongs so that a clear and sustained tone is given out when the gongs are struck.

12. Adjustment of Bells No. 67A

(a) Loosen the gongs with a 'Spanner, Box, No.16' and move them well clear of the striker.

(b) Check that the striker shaft is straight, and that the armature moves freely on its bearings.

(c) Check the armature travel as follows:-  Hold the armature in an operated position so that one of the residual studs is pressed against the core face opposite to it.  Then, check the gap between the other core face and the residual stud, by inserting gauges, as follows:- 

A 20 mil gauge must be loose, but

A 20 mil and 10 mil gauge, together, must either not go in, or do so with a tight fit.

If these conditions are not met, adjust the armature to a 25 mil nominal gap as follows:-

(i) Loosen the cheese-headed screw in the yoke sufficiently to free the cylindrical magnet and armature assembly

(ii) Insert a 10 mil and 15 mil gauge, together, between one core face and the residual stud

(iii) Set the armature squarely with the base of the bell so that the residual studs will strike centrally on the core faces and press the armature firmly towards the core faces

(iv) Tighten the magnet fixing screw.

(d) Adjust the gongs so that the gap between the striker and the appropriate gong, when the armature is held over by hand, is such that:-

A 10 mil gauge is a loose fit, and a 15 mil gauge is a tight fit.

(e) Check that a satisfactory ring is received.

13. Replacements
Individual items that can be changed are:-

1. For all types of bell 'Bell Gongs No. ..'

2. For bells etc. using a 'Bells No. 59A' movement ..... Armature Assembly Part No.1/SBE/33

3. Other cases will be met by changing the bell complete, but

(i) For replacement of obsolete types, see A 5105

(ii) For stock of 300 type telephone parts for replacement purposes, see A 5201.

14. Bells with "matched" gongs
Care should be taken when replacing faulty bells or bell sets fitted with "matched" gongs, i.e. with 'Bell Gongs No. 2 and No. 24' forming a pair, to ensure that the replacing bell or bell set also has "matched" gongs.  The stock of gongs referred to in A 3113, should be utilized for this purpose; if necessary, 'Bell Gongs Nos. 2 and 24, nickelled' should be used on 'Bells No. 594' and 'Bell, Gongs Nos. 2 and 24, oxidized' on Bells No. 1A.

15. Tools and their uses
The tools listed in Table 2 are required for adjusting magneto bells.

TABLE 2
TOOLS REQUIRED FOR MAINTENANCE OF MAGNETO BELLS

The tools listed in Table 2 are to be used only for the purposes for which they are intended.  A tool which is in such a condition that apparatus would be damaged by its use, should be changed.

Last revised: November 06, 2023