Brightspark Magnetos

Reconfiguring the MO1, MN2 and N1

Special tools/materials required

• Soldering iron (50W preferably) and solder

• Adhesive-lined heat-shrink sleeving, and a heat gun or other suitable heat source for shrinking the sleeving

• Possibly a small solder tag

Procedure - overview

Having partly separated the drive-end end-piece from the bobbin, you will be able to see whether the armature is fitted with an original Lucas condenser (picture top right) or with a more modern device which is usually potted in resin. The procedures to be adopted are somewhat different.

Before proceeding any further, it is important that you mark the bobbin and the armature end-piece at the CB end so that you will put them back together the same way round. Also, work out and make a note of which of the two wires coming from the bobbin is earth (i.e. going to the bracket/tag screwed to the end piece) and which is live (i.e. going to the tag on the condenser which is not earthed).

Procedure - if fitted with original Lucas condenser

The thumbnail of the 'before-and-after' schematic on the right illustrates what is required. Click on the picture to see a clearer printable Acrobat file.

In detail, you need to unsolder the bobbin wires from the condenser so as to free the end-piece and condenser from the remainder of the armature.

Then remove the condenser from the end-piece by undoing the two small screws, noting how the insulating clamp plate and the insulating strip fit, and which way round the condenser was fitted.

Then tear off the earth bracket and grind or file down the stud to which the earth bracket was attached, as shown in the picture to the left.

Fit the old condenser back into the end-piece, the same way round as before, and check that the remains of the earth stud will not touch the end-piece, as shown in the picture to the right.

If the integral nut for the centre screw looks rusty (as in the photo at the top of the page), clean it up with a 3BA tap, if available.

Solder a solder tag to the end of the bobbin's earth wire.

Re-solder the bobbin's live wire back onto the live terminal of the condenser, making sure that nothing on the live side can short to the brass of the end-piece or the adjacent condenser fixing screw.

Now screw the solder tag on the earth wire into the position where the old earth bracket used to be, as shown in the picture to the left.

Then reassemble the end-piece with the bobbin, and refit the two long through-screws. Tighten them fully and then release them slightly.

You can now measure the LT and HT resistances again and check whether they are unchanged from the figures you obtained earlier. Make a note of the HT resistance.

Next: Reassembling armature

Procedure - if fitted with a non-original condenser

The original Lucas condenser also provides the nut for the contact-breaker centre screw. When a non-Lucas condenser was subsequently fitted, some method would have been adopted to provide a nut for the centre screw. Also, the new condenser may well have been potted in resin. It is impossible to provide any detailed instructions without knowing what is there at the moment.

However, the thumbnail of the 'before-and-after' schematic on the right illustrates generally what is required. Click on the picture to see a clearer printable Acrobat file.

In summary, you need to:

1. retain the original method of providing a nut for the CB centre screw, or provide some alternative which is insulated from the brass end-piece;

2. identify which is the earth wire from the bobbin and ensure that it remains connected to the brass end-piece;

3. identify which is the live wire from the bobbin and ensure that it remains connected to the nut for the centre screw without being able to short to the brass end-piece;

4. disconnect at least one of the wires to the previous condenser (and possibly remove it altogether) without affecting connections "2" and "3" above.

Once that has been done, reassemble the end-piece with the bobbin, and refit the two long through-screws. Tighten them fully and then release them slightly.

You can now measure the LT and HT resistances again and check whether they are unchanged from the figures you obtained earlier. Make a note of the HT resistance.

Next: Reassembling armature