With the motor on the bench, remove the four retaining screws and two covers. Inside there is a copper strip whose ends rest on a copper stud, each of which are connected to the positive
end of a diode via an eyelet. If one end of the copper strip, is sticking up then rotate the knurled knob at the base of the motor until it lies flat. A continuity check can then be made between
each of the positive diode leads and the green wire eyelet connector in the centre of the copper strip. This should be zero or very close to zero resistance, but one or both will probably be
found to be high resistance. It is very difficult to clean the connections properly without bending the copper strip out of shape, so what may appear to be a drastic solution is needed.
Remove the screw holding this unit to the motor and carefully lift away the switch assembly, revealing the cog/cam underneath.
Warning: DO NOT rotate the knurled nut at the base of the motor at this stage as it will upset the timing.
Turn the switch assembly over and fit it vertically in a vice with the reverse side facing you; do not over-tighten as this may break the plastic. Now identify the centre rivet with the green wire eyelet and drill out the end of that rivet, facing you with a 3mm drill bit. This is a tricky operation as you need to break the rivet without drilling the plastic beneath. When this is done, remove the assembly from the vice and lay it on the bench with the copper strip facing you. It should now be possible to grab the head of the rivet and remove the copper strip and eyelet from the unit. Having done this it is now easy to clean the copper studs and the copper strip; first with paper towel to remove the gunge and then fine emery cloth to clean the contacts. Below where the copper strip sat, there are two round white pegs sitting in stepped holes in the plastic. These move the copper strip up and down when the cam rotates. It is a good idea at this stage to remove these and clean them, as any gunge will impede the movement of the pegs. Make sure the copper strip is flat, lightly tap with a small hammer on a flat surface if necessary.
Now the parts can be re-assembled. The hole for the rivet is 2.8mm in diameter and the author could not find a 2.5mm rivet readily available. Also hammering the end of the rivet over could damage the plastic, so it was decided to fit a 2.5mm bolt and nut instead. Making sure that the plastic pegs are sitting properly in their holes, fit a plain washer to the bolt and pass it through the hole from the reverse side. Next place the copper strip over the bolt thread followed by the eyelet with the green wire attached, a crinkle washer and nut. Tighten the nut and bolt, making sure that the ends of the copper strip are lying on top of the copper studs. To make absolutely sure that the nut did not come undone a spot of super glue was added to the end of the bolt. The switch assembly and covers can now be fitted back in place to the motor.