1 Miller 3 brush dynamos are rare, so where could I get a replacement for this broken casting? Ebay was the answer – Paul Dunn (01782 856839) sells these excellent repros, CNC machined from solid aluminium. It’s better than original for a pretty reasonable £50.
4 But clean-up first. It’s amazing it worked at all with all this oil and gunge inside. I’ll put some heat-shrink on the field coil wires to prevent a short circuit.
7 To convert, remove the unnecessary stuff and connect one brush and one field wire to earth. The second field wire and other brush go to the output terminals.
5 Check the armature with a meter set to resistance (Ohms). There should be no continuity between any of the copper commutator segments and the main spindle.
8 Good – it works connected to a battery. If the direction is wrong, swap the two field wire connections over which will reverse the rotation.
2 This ‘third’ brush picks up current and feeds it to the field coil. By moving its position, current can be reduced, providing primitive regulation.
3 The cut-out prevents discharge when static. It’s neat and self-contained but we’ll convert to simpler 2 brush type.
6 But there should be continuity between each segment and the all the others. You can check the field coil by attaching a battery and seeing if it magnetises.
9 All that remains is to polarise the field for the bike’s electrical system. Take a live wire from the battery and touch it to the terminal connected to the field wire.