Abstract:
An electrical condenser is disclosed for an automotive ignition system wherein the ignition system has first and second contact breaker points and first and second electrically conductive members electrically connected to the first and second contact breaker points, respectively, the first and second electrically conductive members having first and second faces, respectively, facing each other but not directly electrically connected to each other. The condenser comprises an attachment portion arranged to be interposed between the first and second faces of the electrically conductive members of the ignition system. The attachment portion has first and second oppositely facing electrically conductive faces for making electrical contact with the first and second faces, respectively, of the electrically conductive members of the ignition system. The first and second conductive faces of the attachment portion are not directly electrically connected to each other, but at least one capacitor is electrically connected between them.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to electrical condenser assemblies. 
         [0002]    The invention was initially conceived as a condenser assembly to deal with problems arising with some types of rotating-coil magneto for spark-ignition engines. However, it also has other applications. 
         [0003]    Referring to  FIG. 1  of the accompanying drawings, in many designs of rotating-coil magneto of the type fitted to the engines of British and other motorcycles (and other spark-ignition engines) around the middle of the twentieth century, such as the Lucas K1F, K2F and KVF models of magneto and various models by Magneto France, the magneto  10  has a main housing  12  containing a static permanent magnet and a rotating armature  14 . The armature  14  has a drive-end casting  16  and a tail-end casting  18  which are mounted in respective ball bearings  20 , 22  in the housing  12 , and the drive-end casting  16  has a drive shaft  24  which projects from the housing  12  and is, in use, driven by the engine on which the main housing  12  is mounted. A laminated core  26  is held between the drive-end and tail-end castings by long screws and spigots and is wound with a pair of windings  28 , namely with a low-tension (“LT”) winding, which in turn is over-wound with a high-tension (“HT”) winding. The LT and HT windings have a common end which is grounded to the drive-end casting  16 . A contact-breaker (“CB”) assembly  30  is secured to the tail-end casting  18  outside of the main housing  12  and inside a CB housing  32 . The CB assembly  30  includes a back plate  34  on which are mounted a fixed live contact point electrically connected to the live end of the LT winding and a movable earth contact point grounded to the CB back plate and, via the tail-end casting and the core and screws, to the drive-end casting. The movable earth contact point is actuated by a ring cam  36  mounted in the CB housing  32 . When the CB points are closed, the LT circuit therefore includes an electrical loop through the LT winding and the CB. An HT slip ring  38  is mounted on the tail-end casting  18  and electrically connected to a live end of the HT winding. One or two HT pick-ups  40  engage the slip ring  38 . In single or twin cylinder applications, the or each HT pick-up is connected via an HT cable  41  and plug cap to a live terminal of a sparking plug, and the sparking plug is grounded to the engine and thus to the magneto main housing  12  mounted on the engine. In multi-cylinder applications, the HT pick-up(s) is/are connected via a distributor to the engine&#39;s sparking plugs. To complete the HT circuit, an earth brush  42  is usually fitted between the main housing  12  and the drive-end casting  16  of the armature  14 , and the bearings are electrically insulated from the main housing, so that HT current does not pass through and cause erosion of the bearing races and balls. 
         [0004]    In normal operation, the armature  14  is rotated by the engine. During part of each rotation, energy is built up as electrical current in the LT winding (due to the dynamo effect between the permanent magnet and the rotating LT winding). The CB points are then opened by the ring cam  36 , whereupon a high voltage is induced across the LT winding (due to the inductance of the LT winding and the reluctance of the LT winding to allow changes of current through it) and an even higher voltage is induced across the HT winding (due to the transformer action between the LT and HT windings) sufficient to cause a spark between the electrodes of the sparking plug for initiating combustion in the engine. 
         [0005]    With a magneto as described so far, problems arising when the CB points open, which can cause damage to the magneto and unreliable spark production, are:
   (i) The voltages induced across the LT and HT windings may be sufficiently high to cause permanent breakdown of the electrical insulation of the windings. Absent any other effects, when a current flowing through a perfect inductor is instantaneously interrupted by a perfect switch, the voltage self-induced across the inductor is infinite, whatever the inductance value of the inductor (V=L·di/dt). Of course, the windings are not a perfect inductor and CB assembly  30  is not a perfect switch. Nevertheless, the voltages induced can be extremely high. If a breakdown of insulation does occur, it can be permanent, for example due to perforation of an insulator or the formation of a carbon track.   (ii) The voltage induced across the LT winding may be sufficient to cause substantial arcing between the CB points. One effect of this is to slow down the rate of decrease of current through the LT winding (because current continues to flow through the arc) and therefore to reduce the voltages across the LT and HT windings. Another effect is to cause pitting and pimpling of the CB points, and a resultant effect of that is to make the striking of an arc between the CB points even easier. In the event of severe arcing, the heat produced at the CB points may be sufficient to overheat and for example melt surrounding parts of the magneto and/or to weld the CB points together.   
 
         [0008]    To solve these problems, a condenser  44  is connected in parallel with the LT winding and the CB points. (A “condenser” is nowadays more commonly referred to as a “capacitor” in scientific and electrical engineering circles. Nevertheless, the original term “condenser” continues to be more commonly used in automotive engineering circles.) When the CB points are closed, the voltage across the CB points and the condenser  44  is substantially zero. When the CB points open, charging of the condenser  44  by current from the LT winding commences at a finite rate dependent, among other things, on the capacitance of the condenser  44 . Also, the voltage at which an arc will strike between the CB points is dependent, among other things, on the distance between the CB points. The capacitance of the condenser  44  is chosen to be sufficiently large that, within the range of running conditions of the magneto  10 , when the CB points are open the voltage across the condenser  44  (and therefore between the CB points) is always less than the voltage at which an arc will strike between the CB points (whatever the distance between the CB points). The condenser  44  therefore prevents any substantial arcing between the points, and so the rate of decrease of the current through the LT winding upon opening of the CB points is unaffected by an arc current. However, rate of decrease of the current through the LT winding is now affected by the condenser  44  and is dependent on, among other things, the capacitance of the condenser  44 . The condenser  44  acts to reduce the peak LT and HT voltages to levels at which insulation breakdown does not occur and arcing at the CB points is minimal. The condenser  44  also acts with the windings to form a resonant circuit so that the HT voltage oscillates and decays over a significant period of time. 
         [0009]    For the models of magneto  10  discussed above, the capacitance value chosen for the condenser  44  is preferably in the range of about 80 to 500 nF (and more preferably in the range of about 100 to 150 nF), and the breakdown voltage of the condenser  44  must be sufficiently high that the condenser  44  can withstand the voltages applied to it. At the time of design of the models of magneto  10  discussed above and with the technology available, the designers chose to use condensers  44  having a paper dielectric, and suitable original condensers  44  had a considerable size (for example 35 mm×30 mm×8 mm) and mass (for example 26 g). It will be noted that, with the LT winding and CB assembly  30  rotating, it is desirable that the condenser  44  is mounted on the armature  14 , so as to avoid the need for a slip ring connection (and therefore unwanted electrical resistance and a source of wear) between the condenser  44  and the remainder of the LT circuit. Furthermore, due to the mass of the original condensers  44 , it is desirable to mount the condenser  44  with its centre of gravity on the axis of the armature  14 . The designers of the magneto models described above chose to mount the condenser  44  centrally in a cavity provided in the drive-end casting  16 , the casing of the condenser being soldered to a bracket attached by a pair of screws to the casting  16 . The portion of the drive end casting  16  surrounding the cavity usually provides the track  46  for the HT earth brush  42 , and therefore such a location for the condenser  44  takes up space which would otherwise be wasted at least to some extent. 
         [0010]    Ignition system condensers occasionally break down. Also, condensers having a paper dielectric can be adversely affected by moisture and humidity. Often an engine having a faulty ignition condenser will still run, albeit not particularly well, and it may not be immediately apparent to the operator that the fault lies with the condenser. Continued running of the engine can then cause the problems discussed above, i.e. permanent breakdown of the electrical insulation of the windings and pitting and pimpling of the CB points. With the models of magneto discussed above, the CB points are accessible via an end cover  48  of the CB housing  32  and can therefore be readily inspected and replaced if eroded. However, if a permanent breakdown of the winding insulation occurs, it is necessary for the armature  14  to be rewound, which is a specialist and very expensive operation. 
         [0011]    With the location of the condenser  44  in the magneto models discussed above, the condenser  44  is far from being readily accessible or replaceable if the need arises. Removal of the condenser  44  involves: removing the magneto  10  from the engine; removing the end cover  48 , the CB assembly  30  and a pair of spark safety screws  50  from the magneto  10 ; removing the CB housing  32 , any shims  52  therefor and the armature  14  from the main housing  12 ; removing the inner race of the tail-end bearing  22  from the tail-end casting  18  using a special puller; removing some shims  54 , a grease flinger  56  and the slip ring  38  in order to obtain access to the heads of the long screws; removing those long screws; partially easing the drive end casting  16  away from the core  26 ; unsoldering the connections to the condenser  44 ; completing removal of the drive end casting  16  from the core  26 ; and then removing the condenser  44  from the drive end casting  16 . Replacement involves the opposite steps in reverse order, and additionally the steps of: truing the armature core  26  and end castings  16 , 18  upon assembly; checking and if necessary adjusting the shimming of the bearings  20 , 22  on the armature  14  and the shimming between the CB housing  32  and the main housing  12 ; checking and if necessary adjusting the CB points gap; and re-timing the ignition. Even when the above operations are carried out by a specialist using special tools, it is a time consuming and therefore expensive job, and it is all too easy to cause damage such as cracking of the delicate slip ring  38 , breaking of the live end of the HT winding where it is connected to the slip ring  38 , and deformation of the drive-end casting  16  and/or armature core  26  when attempting to separate them. 
         [0012]    The original types of condenser for the models of magneto discussed above are no longer available, other than occasionally as ‘new old stock’, in which case the condenser may already be faulty before it has even been used depending on the humidity in which it has been stored over the years. When a condenser needs replacing, typically a modern condenser having a paper, plastic film or ceramic dielectric is chosen. Such condensers typically have wire terminations and do not have a casing which can be soldered to the original condenser bracket. One wire of the condenser and the common connection of the LT and HT windings are therefore soldered to a solder tag which is screwed to the drive-end casting, while the other wire of the condenser is soldered to the live end of the LT winding and to a wire which extends through a passageway along the core to the tail end, where it is connected to the live point of the CB assembly. To prevent fracture of the condenser&#39;s wires during use, for example due to vibration and/or centrifugal forces, the condenser is preferably potted in resin in the cavity in the drive-end casting. However, should the need ever arise to replace the condenser again, removing the old condenser and its potting resin complicates the replacement procedure. 
         [0013]    Wire-ended condensers of the required specification are nowadays available in sizes sufficiently small that the condenser could be disposed at the tail-end of the magneto in an off-centre space on the CB back-plate, with the two wires of the condenser being connected to the live and grounded points. Unless the original condenser at the drive end had become completely out of circuit, it would be necessary to remove the old condenser, or at least disconnect it. However, condenser replacement thereafter might be relatively straightforward and could be carried out as a routine service procedure. However, it would be necessary to secure the body of the condenser to the CB back-plate to prevent the condenser wires breaking due to centrifugal force and/or vibration. This could be done with a special bracket, but there is little space available and it is expected that such a bracket would prove fiddly. Alternatively, the condenser could be glued to the back-plate, but then removal of the condenser might prove difficult, especially if it were desired to re-use the condenser. Depending on where the condenser were sited, it may be important to be able to remove the condenser temporarily, for example to permit adjustment of the CB points gap and/or replacement of the CB points, before replacing the condenser. 
         [0014]    In addition to potentially causing permanent damage to the magneto, another effect of running an engine with a faulty ignition condenser is that the efficiency of the engine is decreased so that fuel is wasted and the engine becomes less environmentally friendly. 
       SUMMARY OF THE INVENTION 
       [0015]    An aim of the present invention, or at least of specific embodiments of it, is to provide a magneto condenser assembly of simple and inexpensive manufacture and which can be readily fitted, removed and replaced. As a result, it is expected that the condenser assembly will be replaced as soon as a potential problem becomes apparent and/or as a regular service item, so that the magneto can function properly and fuel is not wasted. 
         [0016]    In accordance with a first aspect of the present invention, there is provided an electrical condenser assembly for an automotive ignition system wherein the ignition system has first and second contact breaker points and first and second electrically conductive members electrically connected to the first and second contact breaker points, respectively, the first and second electrically conductive members having first and second faces, respectively, facing each other but not directly electrically connected to each other. The condenser assembly comprises an attachment portion arranged to be interposed between such first and second faces of such electrically conductive members of such an ignition system, the attachment portion having first and second oppositely facing electrically conductive faces for making electrical contact with the first and second faces, respectively, of the electrically conductive members of the ignition system, the first and second conductive faces of the attachment portion not being directly electrically connected to each other. The condenser assembly furthermore comprises at least one capacitor electrically connected between the first and second conductive faces of the attachment portion. 
         [0017]    The attachment portion of the condenser assembly can therefore simply be clamped between two existing parts of the ignition system so as to hold the condenser assembly in place and electrically connect it to the reminder of the ignition system. 
         [0018]    The attachment portion preferably has a hole extending therethrough so that said first and second members of the ignition system can be mechanically connected through the hole, or so that one of said first and second members of the ignition system can pass through the hole, for example using an existing component such as a securing screw of the ignition system. 
         [0019]    In one embodiment of the invention, the first and second faces of the of the attachment portion are provided by first and second substantially-coaxial, spaced-apart, annular members (for example brass or copper washers) each having a respective hole therethrough, the or each capacitor being disposed between the annular members, but with a passageway extending through the condenser assembly including through the holes of the annular members. 
         [0020]    In the original CB assemblies of the Lucas K1F, K2F and KVF models of magneto described above, a mounting block for the fixed CB point is secured to the CB back plate by a 6BA screw extending through the fixed point mounting block into the back plate. The mounting block needs to be electrically insulated from the back plate, and therefore an insulating washer is placed under the head of the screw, and an insulating sleeve lines the hole through the mounting block. That insulating washer can therefore simply be replaced with the annular embodiment of condenser assembly (using a slightly longer screw if necessary). In this case, the first and second faces of the first and second members of the ignition system (as defined in the main statement of the first aspect of the invention) are therefore provided by the top face of the fixed point mounting block and the underside of the head of the 6BA screw (or a washer under the head of the screw). 
         [0021]    There may be a plurality of such capacitors arranged around the axis of the annular members, so that, especially if each capacitor has a generally cuboidal package, effective use can be made of the space available, and furthermore so that the compression force applied to the capacitors by the screw can be more evenly distributed. Interstices between the capacitors and around the passageway may be filled with an electrically insulating material, which can assist in holding the capacitors in place and also bear some of the compression force applied to the condenser assembly by the screw. 
         [0022]    The or each capacitor preferably has a generally cuboidal package with terminals at a pair of opposed ends of the package, and with each terminal facing and being electrically connected to a respective one of the annular members. Modern surface-mount capacitors may therefore be employed. Such capacitors of the multi-layer ceramic variety are available with capacitances (alone or in parallel combination) and voltage ratings which are suitable for use in ignition systems. They are inexpensive and are incredibly small in size when compared to the sizes of the condensers originally fitted to the models of magneto discussed above. 
         [0023]    In another embodiment of the invention, the attachment portion is provided by a region of board having a substrate of electrically insulating material sandwiched between first and second outer layers of electrically conductive material, and the first and second layers of electrically conductive material provide the first and second electrically conductive faces, respectively, of the attachment portion. For example, the attachment portion can be provided by a piece of copper clad board of the type used in the manufacture of printed circuit boards (“PCBs”). 
         [0024]    In the original CB assemblies of the Lucas K1F, K2F and KVF models of magneto described above, a small insulating wafer is disposed between the mounting block for the fixed CB point and the CB back plate. That insulating wafer can therefore simply be replaced with the board embodiment of condenser assembly as described above. In this case, the first and second faces of the first and second members of the ignition system (as defined in the main statement of the first aspect of the invention) are therefore provided by the outer face of the CB back plate and the underside of the fixed point mounting block. 
         [0025]    Alternatively, the insulating washer under the head of the 6BA securing screw for the fixed-point mounting block may simply be replaced with the board embodiment of condenser assembly. In this case, the first and second faces of the first and second members of the ignition system (as defined in the main statement of the first aspect of the invention) are therefore provided by the top face of the fixed point mounting block and the underside of the head of the 6BA screw (or a washer under the head of the screw). 
         [0026]    In later CB assemblies for those models of magneto, two stamped and pressed steel members are secured together by a screw with insulators preventing electrical contact between the two members, including an insulating washer under the head of the screw and an insulating block between the two members. The insulating washer can therefore simply be replaced with the board embodiment of condenser assembly. In this case, the first and second faces of the first and second members of the ignition system (as defined in the main statement of the first aspect of the invention) are the underside of the head of the screw (or a washer under the head of the screw) and the facing portion of the adjacent stamped and pressed steel member. Alternatively, the insulating block may simply be replaced with the board embodiment of condenser assembly. In this case, the first and second faces of the first and second members of the ignition system (as defined in the main statement of the first aspect of the invention) are the facing portions of the two stamped and pressed steel members. 
         [0027]    The board preferably also provides at least one capacitor mounting region in which the capacitor(s) is/are mounted. The capacitor or at least one of the capacitors may be electrically connected between first and second portions of one of the layers in that capacitor&#39;s mounting region. The first portion of that layer may be contiguous with the portion of that layer in the attachment region, and the second portion of that layer may be electrically connected to the portion of the other layer in the attachment region (for example using conventional PCB viaing techniques), without the first and second portions being directly electrically connected to each other. The first and second areas of the first layer may be separated by a furrow in the board (for example made using conventional PCB etching techniques), with the or each capacitor bridging the furrow. Again, multi-layer ceramic surface-mount capacitor(s) may be used, and may be soldered to the board in a conventional manner. 
         [0028]    A second aspect of the invention extends to a magneto having: a low tension coil; a contact breaker assembly having a pair of contact breaker points electrically connected in parallel with the low tension coil; and a condenser assembly according to the first aspect of the invention electrically connected in parallel with the low tension coil and the contact breaker points. In the case where the low tension coil and contact breaker assembly are mounted for rotation with a rotatable armature of the magneto, the condenser assembly is preferably mounted on the contact breaker assembly. As in the known magnetos described above, the armature may be mounted for rotation by a pair of spaced-apart bearings; the low tension coil may be disposed between the bearings; and the contact breaker assembly may mounted at one end of the armature beyond the bearings. 
         [0029]    In accordance with a third aspect of the present invention, there is provided a method of modification of a magneto having an armature with a low-tension winding and a condenser electrically connected in parallel with each other and disposed between a drive-end end-piece and a tail-end end-piece of the armature, and a contact breaker assembly mounted on the tail-end end-piece and having first and second contact breaker points electrically connected in parallel with the low-tension winding and first and second electrically conductive members electrically connected to the first and second contact breaker points, respectively, the first and second electrically conductive members having first and second faces, respectively, facing each other but not directly electrically connected to each other. The method comprises the steps of: rendering the condenser substantially ineffective on the low-tension winding; providing a condenser assembly according to the first aspect of the invention; and fitting the condenser assembly to the contact breaker assembly so that the attachment portion is interposed between the first and second faces of the electrically conductive members of the contact breaker assembly, and the first and second faces of the attachment portion make electrical contact with the first and second faces, respectively, of the electrically conductive members of the contact breaker assembly, whereby the capacitor(s) of the condenser assembly is/are electrically connected in parallel with the low-tension winding. 
         [0000]    With the embodiments of the invention that have been outlined above and that will now be described, purely by way of example, with reference to the accompanying drawings, once an original condenser that is disposed between the end pieces of the armature has been removed or disconnected, a condenser assembly according to the invention may be simply fitted to the CB assembly at the tail end of the armature without requiring an specialist skills or tools, and thereafter the condenser assembly may be simply replaced as a routine service item, again without requiring an specialist skills or tools. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0030]      FIG. 1  is an isometric view of a partially dismantled Lucas model K2F magneto taken from “Lucas Quality Equipment, Volume 2, Workshop Instructions, Motor Cycle Magnetos, Models N1, KN1, K1F, K2F and KVF”, Section L-5 Part A, Issue 1, published by Joseph Lucas Limited in January 1953; 
           [0031]      FIGS. 2A-C  are a plan view, side view and sectioned side view, respectively, of a back plate of a first known form of CB assembly for the magneto of  FIG. 1 , the sectioned view of  FIG. 2C  being taken on the section line  2 C- 2 C shown in  FIG. 2A ; 
           [0032]      FIGS. 3A-C  are a plan view, side view and sectioned side view, respectively, of a CB mounting block of the first form of CB assembly, the sectioned view of  FIG. 3C  being taken on the section line  3 C- 3 C shown in  FIG. 3A ; 
           [0033]      FIGS. 4A-B  are a dismantled isometric view and an assembled isometric view, respectively, of the first form of CB assembly (some parts being omitted); 
           [0034]      FIGS. 5A-B  are a plan view and an underplan view, respectively, of a circuit board for a first embodiment of condenser assembly for use with the first form of CB assembly; 
           [0035]      FIGS. 6A-C  are a plan view and two sectioned views, respectively and on an enlarged scale, of the first embodiment of condenser assembly, the sectioned views of  FIGS. 6B and 6C  being taken on the section lines  6 B- 6 B and  6 C- 6 C, respectively, shown in  FIG. 6A ; 
           [0036]      FIGS. 7A-C  are a dismantled isometric view, an assembled isometric view and a plan view, respectively, of the first form of CB assembly with the first embodiment of condenser assembly; 
           [0037]      FIGS. 8A-C  are an exploded isometric view, an unexploded isometric view and a modification thereto, respectively and on an enlarged scale, of a second embodiment of condenser assembly for use with the first form of CB assembly; 
           [0038]      FIGS. 9A-B  are a partly dismantled isometric view and an assembled isometric view, respectively, of the first form of CB assembly with the second embodiment of condenser assembly; 
           [0039]      FIGS. 10A-C  sectioned plan views, on an enlarged scale, of the second embodiment of condenser assembly and two further modifications thereto; 
           [0040]      FIGS. 11A-B  are exploded isometric views, on an enlarged scale and upside down with respect to each other, of a third embodiment of condenser assembly for use with the first form of CB assembly; 
           [0041]      FIG. 12  is an isometric view on the enlarged scale of the third embodiment of condenser assembly; 
           [0042]      FIGS. 13A-C  are a partly dismantled isometric view, an assembled isometric view and a plan view, respectively, of the first form of CB assembly with the third embodiment of condenser assembly; 
           [0043]      FIGS. 14A-C  are a side view, sectioned side view and a plan view, respectively, of a back plate of a second known form of a CB assembly for the magneto of  FIG. 1 ; 
           [0044]      FIGS. 15A-C  are a side view, sectioned side view and a plan view, respectively, of a fixed CB point assembly of the second form of CB assembly; 
           [0045]      FIGS. 16A-C  are a side view, sectioned side view and a plan view, respectively, of a spring anchor bracket of the second form of CB assembly; 
           [0046]      FIGS. 17A-B  are isometric views showing the top side and the under side, respectively, of an insulating block of the second form of CB assembly; 
           [0047]      FIG. 18  is a side view of an assembly screw of the second form of CB assembly; 
           [0048]      FIGS. 19A-B  are a sectioned side view and a plan view, respectively, of an insulating washer of the second form of CB assembly; 
           [0049]      FIGS. 20A-B  are a sectioned side view and a plan view, respectively, of an assembly washer of the second form of CB assembly; 
           [0050]      FIG. 21  is a side view of a centre screw of the second form of CB assembly; 
           [0051]      FIGS. 22A-C  are a side view, a sectioned side view and a plan view, respectively, of the second form of CB assembly, with the fixed CB point assembly shown in a centre adjustment position; 
           [0052]      FIG. 23  is similar to  FIG. 22C , but with the fixed CB point assembly shown at one end of its range of adjustment; 
           [0053]      FIGS. 24A-B  are a plan view and an underplan view, respectively, of circuit board for a fourth embodiment of condenser assembly for use with the second form of CB assembly; 
           [0054]      FIG. 25  is an isometric view of the fourth embodiment of condenser assembly; 
           [0055]      FIG. 26A-C  are similar to  FIGS. 22A-C , respectively, but fitted with the fourth embodiment of condenser assembly; and 
           [0056]      FIG. 27  is similar to  FIG. 23 , but fitted with the fourth embodiment of condenser assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0057]    CB Assembly with Brass Back Plate 
         [0058]    Referring to  FIGS. 2A-4B  of the drawings, a known early form of CB assembly  30  for the K1F, K2F and KVF models of magneto comprises:
       a brass back plate  34 .   a brass fixed-point mounting block  58 .   a steel screw  60 , a brass or steel washer  62 , an insulating washer  64 , insulating sleeves  66 , 68  and an insulating wafer  70  for mounting the fixed point mounting block  58  on the back plate  34 . The sleeve  66  is disposed in a hole  72  in the fixed-point mounting block  58 , and the screw  60  passes through the washers  62 , 64 , the sleeve  66  and a hole  74  in the insulating wafer  70  into a screw-threaded hole  76  in the back plate  34 . The sleeve  68  is shouldered externally so as to have a larger diameter portion which is accommodated in a rabbet  78  at the lower end of a further hole  80  in the fixed-point mounting block  58 , and a smaller diameter portion which passes through a further hole  82  in the insulating wafer  70  into a central hole  84  through the back plate  34 .   a fixed point  86  having a screw-threaded stem  88  which is adjustably secured in a screw-threaded hole  90  in the fixed-point mounting block  58  and locked with a lock nut  92  and spring washer (not shown).   a moving point assembly (not shown) which has a body pivotally mounted on a pivot post  94  of the back plate  34  beneath a retaining spring (not shown) cantilevered from a retaining post  96  of the back plate  34 . The moving point assembly has a moving point for making electrical contact with the fixed point  86 , a follower heel for engaging with the ring cam  36 , and a return leaf spring which extends part way around the periphery of the back plate  34  from the body of the moving point assembly to a spring anchor post  98  of the back plate  38  and serves to urge the moving point towards the fixed point  86  and also to make an electrical connection between the moving point and the back plate  34 .   a steel centre screw  100  which passes through the hole  80  in the fixed-point mounting block  58 , the insulating sleeve  68  and the central hole  84  in the back plate  34  into engagement with a centre nut (not shown) in the armature so to secure the CB assembly  38  to the tail-end casting  18  of the magneto  10  with a keyed and tapered boss  102  on the underside of the back-plate  34  engaging in a keywayed and tapered hole in the tail-end casting  18  so that the CB assembly  30  rotates with the armature  14  of the magneto  10 .       
 
         [0065]    The centre nut for the centre screw  100  is electrically connected to the ‘live’ end of the LT winding, and the tail-end casting  18  is electrically connected to the ‘earth’ end of the LT winding. The fixed point  86  is therefore connected to the live end of the LT winding via the fixed-point mounting block  58  and the centre screw  100 , but is electrically insulated from the earth end of the LT winding by the insulating washer  64 , the insulating sleeve  66  and the insulating wafer  70 . On the other hand, the moving point is connected to the earth end of the LT winding via the return spring and the back plate  34 . 
         [0066]    The ring cam  36  ( FIG. 1 ) has two lobes. For each turn of the armature  14 , the cam  36  and the heel of the moving point assembly open and close the moving point and the fixed point  86  twice. While the CB points are closed, current builds up in the LT winding due to the dynamo effect between the permanent magnet in the magneto body  12  and the rotating LT winding. When the CB points are then opened, a high voltage is induced across the LT winding. 
         [0067]    In the original configuration of the magneto  10 , the condenser  44  at the drive end of the armature  14  is connected between the ends of the LT winding. When the magneto  10  is converted for use with the first embodiment of the invention, the original condenser  44  is removed from the magneto  10  or electrically disconnected from the LT winding, and the insulating wafer  70  is replaced by a condenser assembly  104  of the first embodiment of the invention, as will now be described with reference to  FIGS. 5A-7B . 
       First Embodiment of Condenser Assembly 
       [0068]    As shown in  FIGS. 5A-B  and  6 B-C, the condenser assembly  104  comprises an L-shaped piece of PCB  106  having a substrate  108  of insulating material sandwiched between first and second outer copper layers  110 , 112 . The longer limb of the L-shape has holes  74 , 82  corresponding in size and spacing to the holes  74 , 82  of the insulating wafer  70  described above and shown in  FIG. 7A . The first copper layer  110  is etched to form a furrow  114  between first and second portions  116 , 118  of the copper layer  110  on the shorter limb of the L-shape. The first portion  116  is contiguous with a third portion  120  of the first copper layer  110  on the longer limb of the L-shape. However, the second portion  118  of the first layer  110  is electrically insulated from the remainder of the first layer  110  but is electrically connected to the second copper layer  112  by one or more plated through-holes or vias  121 . The PCB  106  may be manufactured using conventional PCB manufacturing techniques. 
         [0069]    As shown in FIGS.  6 A and  7 A-C, the condenser assembly  104  also comprises a capacitor  122  disposed over the shorter limb of the PCB  106  and connected between the first and second portions  116 , 118  of the first copper layer  110  of the PCB  106 . The capacitor  122  is of the multi-layer ceramic type in a surface mount package which is substantially cuboidal and has terminals  124 , 126  at a pair of opposite faces of the package. The terminals  124 , 126  are soldered to the PCB  106  using any conventional PCB soldering technique. 
         [0070]    As shown in  FIGS. 7A-C , the condenser assembly  104  is mounted in the CB assembly  30  in place of the insulating wafer  70  with the second copper layer  112  contacting the back plate  34 , with the third portion of the  120  of the first copper layer  110  contacting the fixed-point mounting block  58 , and with the shorter limb of the PCB  106  and the capacitor  122  being disposed to one side of the fixed-point mounting block  58  in the space between the retaining post  96  and the anchor post  98 . The capacitor  122  therefore becomes electrically connected between the fixed-point mounting block  58  and the back plate  34 , and therefore between the fixed and moving points, and, when the CB assembly  30  is assembled with the remainder of the magneto  10 , between the ends of the LT winding. 
         [0071]    In a prototype of the condenser assembly  104 , the insulating substrate  108  of the PCB  106  had a thickness of about 1.00 mm and each copper layer  110 , 112  had a “weight” of about 2.0 oz/ft 2  (equivalent to about 0.07 mm per layer), so that the overall thickness of the PCB  106  was about 1.14 mm. The thickness of the original insulating wafer  70  is typically 19 swg (1.016 mm), and so the replacement of the original insulating wafer  70  by the condenser assembly  104  did not cause any substantial misalignment (or substantially better alignment) of the fixed and moving CB points. In the prototype, the capacitor  122  had a nominal capacitance of 150 nF, a voltage rating of 630 V DC, and a package size of 5.7×5.0×2.0 mm, a suitable capacitor  122  being available at the time of writing this specification under the part number GRM55DR72J154 KW01L from Murata Manufacturing Co Ltd of Japan. 
       Second Embodiment of Condenser Assembly 
       [0072]    When the magneto  10  is converted for use with the second embodiment of the invention, the original condenser  44  is, as with the first embodiment, removed from the magneto  10  or electrically disconnected from the LT winding, but, instead of replacing the insulating wafer  70  by a condenser assembly  104  of the first embodiment, the insulating washer  64  and screw  60  are replaced by a condenser assembly  128  of the second embodiment of the invention and a modified screw  60 ′, as will now be described with reference to  FIGS. 8A-10C . 
         [0073]    As shown in  FIGS. 8A-B  and  10 A, the condenser assembly  128  comprises a close-packed, regular circular array of seven substantially identical capacitors  122  soldered between a pair of standard M3 brass washers  130 , 132  so that the capacitors  122  are electrically connected in parallel. The capacitors  122  are arranged around a passageway  134  coaxial with the holes through the washers  130 , 132 . Each capacitor  122  is of the multi-layer ceramic type in a surface mount package which is substantially cuboidal and has terminals  124 , 126  at its upper and lower ends. Each terminal  124 , 126  faces a respective one of the washers  130 , 132  and is soldered to it. During manufacture, the abutting surfaces of the washers  130 , 132  and terminals  124 , 126  are coated with solder paste. The washers  130 , 132  and capacitors  122  are then placed in a jig, and heat and slight pressure are applied to the washers sufficient to melt the solder paste and join the washers  130 , 132  to the respective terminals  124 , 126 . 
         [0074]    As shown in  FIGS. 9A-B  the condenser assembly  128  is mounted in the CB assembly  30  in place of the insulating washer  64  with the screw  60 ′ passing through the holes in the washers  130 , 132  and through the passageway  134  around which the capacitors  122  are arranged. The top side of the upper washer  130  contacts the underside of the steel or brass washer  62  beneath the head of the screw  60 ′, and the underside of the lower washer  132  contacts the fixed-point mounting block  58 . As shown in  FIG. 9A , the modified screw  60 ′ is longer than the original screw  60  to take account of the increased thickness of the condenser assembly  128  compared with the original insulating washer  64 . Also, the screw  60 ′ has a waisted portion  136  disposed so that the screw  60 ′ does not touch the lower washer  132  or the terminals  126  at the lower ends of the capacitors  122 . The condenser assembly  128  becomes electrically connected between the fixed-point mounting block  58  and the screw  60 ′, and therefore between the fixed and moving points, and, when the CB assembly  30  is assembled with the remainder of the magneto  10 , between the ends of the LT winding. 
         [0075]    In a prototype of the condenser assembly  128  of  FIGS. 8A-B  and  9 A- 10 A, each washer  130 , 132  had outside and inside diameters of about 6.8 mm and 3.3 mm respectively. The capacitors  122  each had a nominal capacitance of 15 nF, a voltage rating of 630 V DC, and a package size of 3.2×1.6×1.6 mm, a suitable capacitor  122  being available at the time of writing this specification under the part number GRM31CR72J153 KW03L from Murata Manufacturing Co Ltd of Japan. The nominal overall capacitance of the condenser assembly  128  was therefore 105 nF. 
         [0076]    Different numbers and sizes of capacitors  122  may be employed. For example,  FIG. 10B  shows five capacitors  122  each having a package size of 3.2×2.5×1.5 mm and a nominal capacitance of 22 nF (Murata part number GRM32QR72J223 KW01L) so that the nominal overall capacitance of the condenser assembly  128  is 110 nF.  FIG. 10C  shows three capacitors  122  each having a package size of 3.2×2.5×2.0 mm and a nominal capacitance of 33 nF (Murata part number GRM32DR72J333 KW01L) so that the nominal overall capacitance of the condenser assembly  128  is 99 nF. In this case, the washers  130 , 132  have a reduced inside diameter of 2.83 mm, which still provides sufficient clearance for the 6BA screw  60  which has a major diameter of 2.786 mm. 
         [0077]    As shown in  FIGS. 8C and 10C , the space between the washers  130 , 132  that is not occupied by the capacitors  122  and the passageway  134  may be filled with an electrically insulating filler material  138 , for example of plastics resin, to as to protect the capacitors  122  from their environment and to stiffen the condenser assembly  128  against the compressive force applied by the screw  60 ′. 
       Third Embodiment of Condenser Assembly 
       [0078]    When the magneto  10  is converted for use with the third embodiment of the invention, the original condenser  44  is, as with the first embodiment, removed from the magneto  10  or electrically disconnected from the LT winding, and the insulating washer  64  is replaced by a condenser assembly  140  of the third embodiment of the invention, as will now be described with reference to  FIGS. 11A-13C . 
         [0079]    As shown in  FIGS. 11A-12 , the condenser assembly  140  comprises a piece of PCB  106  having a substrate  108  of insulating material sandwiched between first and second outer copper layers. The PCB  106  has a central portion  142  and a pair of wing portions  144 , and the central portion has a hole  146  to receive the screw  60 . As shown in  FIG. 11A , the first copper layer is etched to form furrows  114  between first and second portions  116 , 118  of the copper layer on each wing portion  144  of the PCB  106 . Each first portion  116  is contiguous with a third portion  120  of the first copper layer in the central portion  142  of the PCB  106 , but each second portion  118  is isolated therefrom. As shown in  FIG. 11B , the second copper layer is etched to form furrows  115  between first and second portions  117 , 119  of the copper layer on the wing portions  144  of the PCB  106 . Each first portion  117  is contiguous with a third portion  121  of the second copper layer in the central portion  142  of the PCB  106 , but each second portion  119  is isolated therefrom. The second portions  119  of the second copper layer are beneath the first portions  116  of the first copper layer and are electrically connected thereto by one or more plated through-holes or vias  121 . Also, the first portions  117  of the second copper layer are beneath the second portions  118  of the first copper layer and are electrically connected thereto by one or more plated through-holes or vias  123 . Therefore, the third portion  120  of the first copper layer is electrically connected to the first portions  116  of the first copper layer and the second portions  119  of the second copper layer, and the third portion  121  of the second copper layer is electrically connected to the first portions  117  of the second copper layer and the second portions  118  of the first copper layer. The PCB  106  may be manufactured using conventional PCB manufacturing techniques. 
         [0080]    The condenser assembly  140  also comprises four capacitors  122  disposed over or under the wing portions  144  of the PCB  106  and connected between the first and second portions  116 , 118 ;  117 , 119  of the respective copper layer of the PCB  106 . Each capacitor  122  is of the multi-layer ceramic type in a surface mount package which is substantially cuboidal and has terminals  124 , 126  at a pair of opposite faces of the package. The terminals  124 , 126  are soldered to the PCB  106  using any conventional PCB soldering technique. The four capacitors  122  therefore become connected in parallel between the third portions  120 , 121  of the copper layers. 
         [0081]    As shown in  FIGS. 13A-C , the condenser assembly  140  is mounted in the CB assembly  30  in place of the insulating washer  64  with the third portion  120  of the first copper layer contacting the brass or steel washer  62  under the head of the screw  60  and with the third portion  121  of the second copper layer contacting the fixed-point mounting block  58 . The condenser assembly  140  therefore becomes electrically connected between the fixed-point mounting block  58  and the back plate  34 , and therefore between the fixed and moving points, and, when the CB assembly  30  is assembled with the remainder of the magneto  10 , between the ends of the LT winding. 
         [0082]    As can be seen in  FIG. 13A , the fixed-point mounting block  58  has a reduced height around the hole  72 . The reason for this was possibly so that the screw  60  could have a standard length of ½ inch (12.7 mm). The outline of the PCB  106  is shaped so that it is a snug fit against the riser  142  of the step in the upper surface of the fixed-point mounting block  58 .  FIG. 13C  shows the return leaf spring  144  which extends part way around the periphery of the back plate  34  from the body (not shown) of the moving point assembly to the spring anchor post  98 . The outline of the PCB  106  is shaped so that it does not interfere with the return spring  144 . As shown in  FIGS. 11A-12 , the copper layers of the PCB  106  are etched so that they stop short of the outer periphery of the PCB  106  and also stop short of the hole  146  so as to prevent unintentional electrical contact and short-circuiting of the edges of the copper layers with the return spring  144 , the fixed-point mounting block  58  or the screw  60 . It will be appreciated that the condenser assembly  140  may be fitted either way up. 
         [0083]    Each capacitor  122  of the condenser assembly  140  may have a package size of 3.2×2.5× mm and a nominal capacitance of 33 nF (Murata part number GRM32DR72J333 KW01L) so that the nominal overall capacitance of the condenser assembly  140  is 132 nF. 
         [0084]    CB Assembly with Steel Back Plate 
         [0085]    Referring now to  FIGS. 14A to 21 , a known later form of CB assembly for the K1F, K2F and KVF models of magneto, sometimes referred to as the “low inertia” CB assembly, comprises:
       a steel back plate  150  ( FIGS. 14A-C ) with a tapered and keyed boss  102  on its underside, a central hole  84 , a pivot post  94  and a screw-threaded hole  76  for receiving an assembly screw  152  ( FIG. 18 ) provided with an insulating washer  64  ( FIG. 19A-B ) and a steel or brass washer  62  ( FIGS. 20A-B ).   a fixed CB point assembly  154  ( FIGS. 15A-C ) formed from two pieces  156 , 158  of stamped and bent steel sheet riveted together by a pair of rivets which also form a pair of fixed CB points  160 . One of the pieces  156  has a hole  162  to fit the pivot post  94  of the back plate  150 , and the other piece  158  has an arcuate slot  164  to receive the assembly screw  152 . A central irregularly shaped hole  166  is formed between the two pieces  156 , 158 .   a spring anchor bracket  168  ( FIGS. 16A-C ) formed from a piece of stamped and bent steel sheet. The anchor bracket  168  has a base portion  170  with a hole  172  to receive a centre screw  100  ( FIG. 21 ) and a further hole  174  to receive the assembly screw  152 . The anchor bracket  168  also has a pair of upturned ears  176  with screw threaded holes  178  therethrough.   an insulating block  180  ( FIGS. 17A-B ) of moulded nylon which will be described in more detail later.   moving point assembly (not shown) which has a body arranged to be pivotally mounted on the pivot post  94  of the back plate  34 . The moving point assembly has a moving point for making electrical contact with one of the fixed points  160  (depending on which way round the moving point assembly is fitted to the pivot post  94 ), a follower heel for engaging with the ring cam  36 , and a return leaf spring which is arranged to extend part way around the periphery of the back plate  150  from the body of the moving point assembly to one of the ears  176  of the anchor bracket  168  (depending on which way round the moving point assembly is fitted to the pivot post  94 ) where the spring can anchored by a screw engaging in the hole  178 . The return spring serves to urge the moving point towards the fixed point  86  and also to make an electrical connection between the moving point and the anchor bracket  168 .       
 
         [0091]    The assembled CB assembly  182  (apart from the moving point assembly) is shown in  FIGS. 22A-C . The pieces  156 , 158  of the fixed point assembly  154  rest on the back plate  150 , in electrical contact therewith, with the pivot post  94  engaged in the hole  162  in the piece  156  of the fixed point assembly  154 . The insulating block  180  has a thin portion  184  which rests on the piece  158  of the fixed point assembly around the slot  164 , and a thick portion  186  which passes through the central hole  166  in the fixed point assembly  154  and rests on the back plate  150 . The thick portion  186  of the insulating block  180  has a boss  188  which extends into the central hole  84  in the back plate  150 , and a hole  190  which passes centrally through the boss  188 . The base  170  of the anchor bracket  168  rests on the insulating block  180 . The block  180  has a further boss  192  which extends into the hole  174  in the anchor bracket  168 , and a further hole  194  which passes centrally through the further boss  192 . The insulating block  180  also has an upstanding lip  196  which partly surrounds the inner end of the base  170  of the anchor bracket  168  so as to centralise the hole  172  in the anchor bracket  168  with the holes  190 , 84  in the insulating block  180  and the back plate  150 , respectively. The assembly screw  152  passes through the conductive washer  62 , the insulating washer  64 , the hole  194  in the insulating block  180  and the slot  164  in the piece  158  of the fixed CB point assembly  154  into engagement with the screw-threaded hole  76  in the back plate  150 . The centre screw  100  passes through the hole  172  in the base  170  of the anchor bracket  168  and the hole  190  in the insulating block  180 . When the CB assembly  182  is assembled with the magneto  10 , the centre screw  100  engages with the centre nut (not shown, but previously described in connection with the early form of CB assembly  30 ) which is connected to the live end of the LT winding. 
         [0092]    It will therefore be appreciated that, when the CB assembly  182  is assembled with the magneto  10 , the live end of the LT winding is connected via the centre nut (not shown), the centre screw  100 , the spring anchor bracket  168  and the return spring for the moving point assembly (not shown) to the moving point (not shown) which is therefore live. By contrast, the grounded end of the LT winding is connected via the body of the armature  14 , the back plate  150  and the fixed CB point assembly  154  to whichever fixed point  160  is in use (depending on which way round the moving point assembly is fitted to the pivot post  94 ). It should also be noted that the assembly screw  152  is grounded due to its engagement in the screw-threaded hole  76  in the back plate  150 , and is isolated from the live side of the CB assembly  182  by the boss  192  of the insulating block  180  and by the insulating washer  64 . The upper face of the insulating washer  64  is in contact with the grounded brass or steel washer  62 , whereas the lower face of the insulating washer  64  is in contact with the base  170  of the live spring anchor plate  168 . 
         [0093]    The points gap between the moving point (not shown) and whichever of the fixed points  160  is in use can be adjusted by slightly loosening the assembly screw  152 , pivoting the fixed point assembly  154  about the pivot post  94  as necessary, and then re-tightening the assembly screw  152 .  FIG. 23  shows the limit in one direction of the adjustment of the fixed point assembly  154 . In practice, the limit is not defined by the angular extent of the arcuate slot  164  in the fixed point assembly  154 , but instead by collision of the fixed point assembly with the insulating block at the point referenced  198  in  FIG. 23 . 
         [0094]    In the original configuration of the magneto  10  with the second form of CB assembly  182 , the condenser  44  at the drive end of the armature  14  is connected between the ends of the LT winding. When the magneto  10  is converted for use with the fourth embodiment of the invention, the original condenser  44  is removed from the magneto  10  or electrically disconnected from the LT winding, and the insulating washer  64  is replaced by a condenser assembly  200  of the fourth embodiment of the invention, as will now be described with reference to  FIGS. 24A-27 . 
       Fourth Embodiment of Condenser Assembly 
       [0095]    As shown in  FIGS. 24A-25 , the condenser assembly  200  comprises a piece of PCB  202  having a substrate  108  of insulating material sandwiched between first and second outer copper layers. The PCB  202  has a central portion  204  and a pair of wing portions  206 . The central portion  204  has a hole  146  to receive the screw  60 . As shown in  FIG. 24A , the first copper layer is etched to form furrows  114  between a central portion  208  and two tip portions  210  of the first copper layer so that each tip portion  210  is isolated from the central portion  208 . Also, the central portion  208  of the first copper layer is etched so that it stops short of an arcuate side edge  211  of the PCB  202 . As shown in  FIG. 24B , the second copper layer  212  is etched so that it stops short of the central hole  146  through the PCB  202 . The second copper layer  212  is electrically connected to each tip portion  210  of the first copper layer by one or more plated through-holes or vias  121 . The PCB  202  may be manufactured using conventional PCB manufacturing techniques. 
         [0096]    As shown in  FIG. 25 , the condenser assembly  200  also comprises two capacitors  122 , each disposed so as to bridge a respective furrow  114  and connected between the central portion  208  and a respective tip portion  210  of the first copper layer of the PCB  202 . Each capacitor  122  is of the multi-layer ceramic type in a surface mount package which is substantially cuboidal and has terminals  124 , 126  at a pair of opposite faces of the package. The terminals  124 , 126  are soldered to the PCB  202  using any conventional PCB soldering technique. The two capacitors  122  therefore become connected in parallel between the second copper layer  212  and the central portion  208  of the first copper layer. 
         [0097]    As shown in  FIGS. 26A-C , the condenser assembly  200  is mounted in the CB assembly  182  in place of the insulating washer  64 , with the central portion  208  of the first copper layer contacting the brass or steel washer  62  under the head of the screw  152  and with the second copper layer  212  contacting the base  170  of the anchor bracket  168 . The condenser assembly  200  therefore becomes electrically connected between the anchor bracket  168  and the back plate  102 , and therefore between the fixed and moving points, and, when the CB assembly  182  is assembled with the remainder of the magneto  10 , between the ends of the LT winding. 
         [0098]    As shown in  FIG. 26C , the arcuate side edge  211  of the PCB  202  abuts or stops slightly short of the upturned ears  176  of the live anchor bracket  168 , so that the ears  176  can assist in correctly orienting the condenser assembly  200  in the CB assembly  182 . However, the grounded central portion  208  of the first copper layer of the PCB  202  cannot make contact and short with the ears  176  because, as mentioned above, it is etched to stop short of the arcuate side edge  211 . Also, the live second copper layer  212  cannot make contact and short with the grounded assembly screw  152  because, as mentioned above, it is etched to stop short of the central hole  146  through the PCB  202 . As shown in  FIG. 27 , the wing portions  206  of the PCB  202  are sufficiently narrow that, when the fixed point assembly  154  is at its limits of adjustment, a gap  214  remains between the wing portions  206  of the PCB and the portions  216  of the fixed point assembly  154  on which the fixed points  160  are mounted. 
         [0099]    Each capacitor  122  of the condenser assembly  200  may have a package size of 3.2×4.5×1.5 mm and a nominal capacitance of 68 nF (Murata part number GRM43QR72J683 KW01L) so that the nominal overall capacitance of the condenser assembly  200  is 136 nF. 
         [0100]    It will be appreciated that many modifications and developments may be made to the embodiments of the invention described above. For example, the condenser assembly may be arranged to be clamped between other parts of the CB assemblies  30 , 182 . In particular, the insulating block  180  ( FIGS. 17A-B ) used in the second form of CB assembly  182  may be replaced with a condenser assembly having portions of copper layer which make electrical contact with the fixed point assembly  154  and the anchor bracket  168 . 
         [0101]    It should be noted that the embodiments of the invention have been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.